Ni-Based Coatings for High Temperature Corrosion Protection

Ni-Based Coatings for High Temperature Corrosion ProtectionCritical load-bearing components in biomass-/waste-fired boilers, e.g., water-wall and superheater tubes are severely deteriorated due to Cl-induced high temperature corrosion. Deposition of a dense and adherent Ni-based coating containing protective scale-form-ing elements such as Al or/and Cr using high velocity air-fuel (HVAF) thermal spray tech-nique is a promising approach to extend the component’s lifetime, hence increase the thermal/electrical efficiency of the boilers.

While the HVAF-sprayed coatings exhibit unique microstructural characteristics, there are a few technical challenges in producing extremely high-temperature corrosion resis-tant coatings including the emergence of pores and splat boundaries. Complex interac-tion of in-flight particles with supersonic flame in the HVAF process, and formation of a complex deposit structure comprising of pores, as well as splat boundaries encumber understanding the inter-relation of process, microstructure, properties, and corrosion per-formance. An additional challenge is development of an in-depth understanding on high temperature corrosion mechanisms of coatings due to their intricate microstructures. This dissertation aims to establish a correlation between the feedstock composition, mi-crostructure, and high temperature corrosion performance of coatings.

Esmaeil SadeghimereshtEsmaeil received his Bachelor’s degree in Material Science & Engineering from Amirkabir University of Technology - Tehran Polytechnic, Iran in 2008. He did his Master’s program in Corrosion Science & Engineering at Tarbiat Modares University, Iran in 2010. His research interests stand broadly in the area of corrosion, coating and additive manufacturing.

ISBN 978-91-87531-70-5

PhD ThesisProduction Technology2018 No. 23

Ni-Based Coatings for High Temperature Corrosion Protection

Esmaeil Sadeghimeresht

NI-BASED COATIN

GS FOR H

IGH

TEMPERATU

RE CORRO

SION

PROTECTION

ESMAEIL SADEGHIM

ERESHT2018 NO.23

Tryck: BrandFactory, april 2018.

University West SE-46186 Trollhättan Sweden +46 52022 30 00 www.hv.se © Esmaeil Sadeghimeresht 2018 ISBN 978-91-87531-70-5 (Printed version) ISBN 978-91-87531-69-9 (Electronic version)

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Dedication

I dedicate this PhD thesis to my family, and firstly to my beloved wife, Paria, who has enthusiastically supported me with her love and willingly sacrificed many things in her life for me to get this far.

Secondly, to my father, the world’s greatest teacher, who taught me the high value of science and education and handed me a powerful tool called “vision”. And to my mother, who raised and taught me to love others by giving her love and care.

Thirdly, to my brothers and sister, who have strongly encouraged me to pursue my studies to the highest level.

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Acknowledgements

First and foremost, I would like to express my sincere appreciation to my wonderful supervisors at the Production Technology Center (PTC), University West, Professor Shrikant Joshi, and Professor Nicolaie Markocsan for giving me the chance to work in this exciting field of engineering under their supervision. I will never forget your endless support during my PhD studies and thanks for always letting me find my own way back to the main track of research!

I wish to sincerely thank Professor Per Nylén, who is a great boss with brilliant ideas in the field of thermal spraying (though he never pronounced my second name in public)! At least, I will never forget your statement in my life, “there is no down and we are always going up!”, really unforgettable!

I would like to show my deepest gratitude towards our technical engineers, Stefan Björklund, Jonas Olsson, Mats Högström, Kjell Hurtig, and Kenneth Andersson for all the great times we had together discussing the thermal spraying, additive manufacturing, and also microscopies! Thanks for your all support.

I am appreciative to the exceptionally wonderful people at the High Temperature Corrosion Center (HTC), Chalmers University, Professor Lars-Gunnar Johansson (who was my opponent in the Licentiate Seminar), Professor Jan-Eric Svensson, Professor Jesper Liske, Dr. Torbjörn Jonsson, and PhD students Johan Eklund and Julien Phother Simon, and also to the Department of Management and Engineering, Division of Engineering Materials, Linköping University, Professor Ru Peng, and PhD student Pimin Zhang for the profound collaboration that we already established together in the field of corrosion which will be hopefully pursued in the future.

Dr. Tanvir Hussain is greatly acknowledged for hosting me at The Faculty of Engineering, The University of Nottingham during my PhD internship. That was an amazing experience working with your PhD students Liam Reddy, Saira Bano, and Alexander Sabard as well as Dr. Mingwen Bai who enthusiastically supported me so much during my stay there.

Dr. Senad Dizdar from Höganas AB, Sweden, and Dr. Christoph Lyphout, from Swerea IVF, Sweden are highly appreciated for their great scientific and technical support during my first two years of PhD studies when I was studying the low temperature corrosion behavior of Fe-based coatings.

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I am grateful to my great team of master students, Elie Guerin (University of Limoges, France), Sudarshan Raman (Nanyang Technological University, Singapore), Celine Clement (University of Limoges, France), Johan Wennberg (University West, Sweden), Omkar Aranke (University West, Sweden) and Reza Jafari (Tarbiat Modares University, Iran) who all did a great job during their periods of study.

I would like to take the chance and acknowledge those people who inspired me in the field of thermal spraying, Prof. Lech Pawlowski, Prof. Christopher Berndt, Prof. Sanjay Sampath, Prof. Robert Vaßen, Prof. Luca Lusvarghi (who was my opponent in the End Seminar before the final dissertation), Dr. Steve Matthews, Dr. Filofteia-Laura Toma, and Dr. Nicholas Curry. Prof. Petri Vuoristo (Tampere University) as the opponent in the final dissertation, and Prof. Melanie Montgomery (Danish Technical University, Denmark), Assoc. Prof. Rikard Norling (Swerea Kimab), as well as Assoc. Prof. Fang Liu (Chalmers University), who will be the members in the examination board are particularly acknowledged. It was my great pleasure to discuss with you my subject of study and receive invaluable feedbacks from you all.

Eva Bränneby and Victoria Sjöstedt are warmly acknowledged for their administrative support. I also extend thanks to all my delightful colleagues at the Division of welding and Additive Manufacturing for their friendship and the pleasant times that we have had together.

The Västra Götalandsregionen (VGR) and The Knowledge Foundation are gratefully thanked for funding this research.

I express my gratitude to my friends at PTC for their kind companionship and providing a friendly environment for research. Satyapal, Ashish, Sneha, Wellington, Tayo, Chamara, Tahira, Vahid, Sukhdeep, Fabian, Ebrahim, Josefine, Asun, Ana, Anna, Nages, Arun, Andreas, Edvard, Emile, and Xiaoxiao; I wish you all the best in your future career!

At the end, I devote my ultimate appreciation to my wife, Paria, and my parents. Without their care, love, and encouragement, it would have been impossible to reach this stage of life.


31st of May 2018, Trollhättan

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Populärvetenskaplig Sammanfattning

Nyckelord: Termisk Sprutad skikt; HVAF; Högtemperatur korrosionsskydd; Kemisk sammansättning; mikrostruktur; Dispergerad oxid

Titel: Ni-baserade beläggningar för högtemperatur korrosionsskydd

Bärande komponenter, t.ex. vattenvägs- och överhettningsrör, i biomassa/avfallseldade pannor utsätts för allvarlig högtemperaturkorrosion, på grund av en krävande miljö innehållande klor. Deponering av ett tät nickelbaserad beläggning genom High Velocity Air Fuel (HVAF) sprutning är en intressant teknik för att förlänga pannornas livslängd som samtidigt kan möjliggöra en högre förbränningstemperatur vilket är åtråvärt ur emissionssynpunkt. I den här studien har olika högtemperaturkorrosionsbeständiga nickelbaserade HVAF beläggningar (Ni21Cr, Ni21Cr7AlY, Ni5Al, Ni21Cr9Mo, Ni21Cr9Mo-SiO2) utvärderats genom laboratoriestudier i luft, fukt- och HCl-innehållande miljöer.

Korrosionstesterna utfördes vid 600 °C i upp till 168 timmar med och utan KCl tillsats. Alla beläggningar visade sig ge ett bra skydd i alla miljöer som inte innehöll KCl. En huvudanledning till det goda skyddet var bildandet av en skyddande beläggning på beläggningsytan av antigen aluminiumoxid eller kromoxid beroende på tillsatsmaterialets kemiska sammansättning. När KCl tillkom i miljön nedbröts de kromoxidbildande beläggningarna genom en tvåstegsmekanism; 1) bildning av K2CrO4 och Cl- följt av diffusion av Cl- vid oxidkorngränserna, vilket ledde till bildning av Cl2 och en icke skyddande oxid, och 2) diffusion av den bildade Cl2 via beläggningsdefekter i den icke-skyddande oxiden, vilket ledde till förångning av metallkloriden och avbrytning av önskad oxidation.

Korrosionsbeständigheten hos den krombildande Ni21Cr-beläggningen förbättrades genom tillsats av legeringselement, såsom Al eller Mo. Det visades också att tillsättning av dispergerad SiO2 ytterligare kan öka beläggningarnas korrosionsbeständighet. Oxiden som bildades i närvaro av SiO2 dämpade effektivt Cl- inträde och sänkte därmed korrosionshastigheten, på grund av att den bildade oxiden var kompakt (utan sprickor och defekter) och rik på Cr. I den krävande miljön innehållande Cl- kunde de olika beläggningarnas beständighet rankas som (från högsta till lägsta korrosionsbeständighet); Ni21Cr9Mo-SiO2> Ni21Cr7AlY> Ni5Al> Ni21Cr9Mo> Ni21Cr. Resultatet bekräftar hypotesen att ett förbättrat korrosionsskydd hos krombildande beläggningar kan åstadkommas genom närvaro av legeringselement och dispergerad SiO2.

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Abstract

Title: Ni-based coatings for high temperature corrosion protection

Keywords: Thermal Spray Coating; HVAF; High Temperature Corrosion Protection; Composition; Microstructure; Dispersed Oxide

ISBN: Printed: 978-91-87531-70-5 Electronic: 978-91-87531-69-9

Biomass/waste-fired boilers severely suffer from high temperature corrosion of critical load-bearing components, e.g. water-wall and superheater tubes, due to presence of Cl-containing corrosive species. Deposition of a dense and adherent Ni-based coating by high velocity air-fuel (HVAF) thermal spray technique is a promising approach to extend the component’s lifetime and, hence, increase the thermal/electrical efficiency of the boilers.

In this research, high temperature corrosion of candidate Ni-based coatings – Ni21Cr, Ni21Cr7AlY, Ni5Al, Ni21Cr9Mo, Ni21Cr9Mo-SiO2 – sprayed by HVAF has been investigated through detailed laboratory studies in ambient air, moisture and HCl-laden environments. The exposures were conducted at 600 °C for up to 168 h with and without presence of KCl salt. All coatings were highly protective in all environments in the absence of KCl due to formation of corresponding protective scales of alumina or chromia on the coating surface. When KCl was introduced, chromia-forming coatings degraded through a two-stage mechanism; 1) formation of K2CrO4 and Cl- followed by diffusion of Cl- through oxide grain boundaries, leading to formation of Cl2, metal chlorides as well as a non-protective oxide, and 2) inward diffusion of the formed Cl2 through defects in the non-protective oxide, leading to metal chloride evaporation and breakaway oxidation.

The corrosion behavior of the chromia-forming Ni21Cr coating was improved by addition of alloying elements such as Al and Mo. It was also shown that adding dispersed SiO2 further increased the corrosion resistance of the coatings. The oxide scale formed in the presence of SiO2 effectively suppressed Cl- ingress and lowered the corrosion rate, since the formed oxide was continuous, adherent and rich in Cr. The performance of the coatings in the complex Cl-containing environment was ranked as (from highest to lowest corrosion resistance); Ni21Cr9Mo-SiO2 > Ni21Cr7AlY > Ni5Al > Ni21Cr9Mo > Ni21Cr, confirming the enhanced corrosion protection of chromia-forming coatings in the presence of alloying elements and dispersed SiO2.

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Appended Publications

The research described in this thesis has led to the following appended publications:

Paper A. A comparative study on Ni-based coatings prepared by HVAF, HVOF, and APS methods for corrosion protection applications Esmaeil Sadeghimeresht, Nicolaie Markocsan, Per Nylén Journal of Thermal Spray Technology, vol. 25, pp. 1604-1616, 2016; https://doi.org/10.1007/s11666-016-0474-9

Paper B. Isothermal oxidation of HVAF-sprayed Ni-based chromia, alumina and mixed-oxide scale forming coatings in ambient air Esmaeil Sadeghimeresht, Nicolaie Markocsan, Matti Huhtakangas, Shrikant Joshi Journal of Surface and Coatings Technology, vol. 316, pp. 10-21, 2017; https://doi.org/10.1016/j.surfcoat.2017.03.017

Paper C. KCl-induced high temperature corrosion behavior of HVAF-sprayed Ni-based coatings in ambient air Reza Jafari, Esmaeil Sadeghimeresht, Taghi Shahrabi Farahani, Matti Huhtakangas, Nicolaie Markocsan, Shrikant Joshi Journal of Thermal Spray Technology, vol. 27, pp. 500-511, 2018; https://doi.org/10.1007/s11666-017-0684-9

PPaper D. Effect of water vapor on the oxidation behavior of HVAF-sprayed NiCr and NiCrAlY coatings Esmaeil Sadeghimeresht, Johan Eklund, Julien Phother Simon, Jesper Liske, Nicolaie Markocsan, Shrikant Joshi Journal of Materials and Corrosion, accepted, 2018;

Paper E. Chlorine-induced high temperature corrosion of HVAF-sprayed Ni-based alumina and chromia forming coatings Esmaeil Sadeghimeresht, Liam Reddy, Tanvir Hussain, Matti Huhtakangas, Nicolaie Markocsan, Shrikant Joshi Journal of Corrosion Science, vol. 132, pp. 170-184, 2018; https://doi.org/10.1016/j.corsci.2017.12.033.

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Paper F. Influence of KCl and HCl on high temperature corrosion of HVAF-sprayed NiCrAlY and NiCrMo coatings Esmaeil Sadeghimeresht, Liam Reddy, Tanvir Hussain, Nicolaie Markocsan, Shrikant Joshi Journal of Material & Design, vol. 148, pp.17-29, 2018; https://doi.org/10.1016/j.matdes.2018.03.048

Paper G. Effect of SiO2 dispersion on high temperature chlorine corrosion of HVAF-sprayed NiCrMo coating Esmaeil Sadeghimeresht, Tanvir Hussain, Nicolaie Markocsan, Matti Huhtakangas, Shrikant Joshi Journal of CORROSION, accepted, 2018.

As the main Author (except for Paper C), Esmaeil Sadeghimeresht has performed all the experimental characterization, analyzed all the results, designed the structure of all articles, and had the main responsibility in writing the articles. Co-authors contributed in formulating concepts and ideas, planning the project, spraying and article editing. In Paper C, Esmaeil Sadeghimeresht supervised the first author in doing the experiments and writing the article.

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Related work

The following papers are not appended but relevant to the work presented in this thesis.

Publications in peer-reviewed international scientific journals

Esmaeil Sadeghimeresht, Shrikant Joshi; Chlorine-induced High-TemperatureCorrosion and Erosion-Corrosion of HVAF- and HVOF-Sprayed FeCrNiMoBSiCAmorphous Coatings. 2018; under review. Esmaeil Sadeghimeresht, Hamed Hooshyar, Nicolaie Markocsan, ShrikantJoshi, Per Nylén: Oxidation Behavior of HVAF-Sprayed NiCoCrAlY Coating inH2–H2O Environment. Oxidation of Metals 2016; 86(299-314).E. Sadeghimeresht, N. Markocsan, S. Joshi: Isothermal oxidation behavior ofHVAF-sprayed Ni and NiCr coatings in H2-H2O environment Surface and Coatings.Surface & Coatings Technology 2017; 317(17-25).Pimin Zhang, Esmaeil Sadeghimeresht, et al., A multi-approach study of transient

alumina scale formed on HVAF-sprayed NiCoCrAlYcoatings and the effect of surface treatment, Corrosion Science, under review, 2018.Esmaeil Sadeghimeresht, Nicolaie Markocsan, Electrochemical Behavior ofBilayer Thermal-Spray Coatings in Low-Temperature Corrosion Protection, Coatings,7, 165, 2017.Esmaeil Sadeghimeresht, Nicolaie Markocsan, Per Nylén: Microstructuralcharacteristics and corrosion behavior of HVAF- and HVOF-sprayed Fe-based coatings.Surface and Coatings Technology 2017; 318(365-373).Esmaeil Sadeghimeresht, Nicolaie Markocsan, Per Nylén: Microstructural andelectrochemical characterization of Ni-based bi-layer coatings produced by the HVAFprocess. Surface and Coatings Technology 10/2016; 304(606-619).Esmaeil Sadeghimeresht, Nicolaie Markocsan, Per Nylén: Microstructure effectof intermediate coat layer on corrosion behavior of HVAF-sprayed bi-layer coatings.Journal of Thermal Spray Technology 08/2016; 26(1-2).Esmaeil Sadeghimeresht, Nicolaie Markocsan, Per Nylén: A ComparativeStudy of Corrosion Resistance for HVAF-Sprayed Fe- and Co-Based Coatings.Journal of Coatings Technology and Research 03/2016; 6(1-15).Esmaeil Sadeghimeresht, Nicolaie Markocsan, Per Nylén, Stefan Björklund:Corrosion performance of bi-layer Ni/Cr2C3-NiCr HVAF thermal spray coating.Applied Surface Science 02/2016; 369 (470-481).

Conference Proceedings

R. Jafari, E. Sadeghimeresht, T. Shahrabi Farahani, N. Markocsan, S. Joshi,KCl-induced corrosion behavior of HVAF-sprayed Ni-based coatings in ambient air,ITSC17; 7-9 June 2017, Dusseldorf, Germany.

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Pimin Zhang, Esmaeil Sadeghimeresht, Ru Lin Peng, Xin-Hai Li, Nicolaie Markocsan, Shrikant Joshi, Sten Johansson, Isothermal oxidation behavior of HVAF-sprayed NiCoCrAlY coatings: Effect of surface treatment, ITSC17; 7-9 June 2017, Dusseldorf, Germany.

E. Sadeghimeresht, J. Eklund, J. Phother Simon, J. Liske, N. Markocsan, S. Joshi, Oxidation behavior of HVAF-sprayed NiCr coating in moisture-laden environment, ITSC17; 7-9 June 2017, Dusseldorf, Germany.

Esmaeil Sadeghimeresht, Nicolaie Markocsan, Per Nylén, Corrosion Behavior of HVAF-Sprayed Bi-Layer Coatings; Effect of Intermediate Layer’s Microstructure and Chemical Composition, ITSC2017; June 2017, Dusseldorf, Germany.

J. Eklund, J. Phother Simon, E. Sadeghimeresht, L-G Johansson, T. Jonsson, S. Joshi, J. Liske, High temperature corrosion of Ni-base coatings for boiler applications - A microstructural study, International Conference on Metallurgical Coatings and Thin Films (ICMCTF44); April 2017, San Diego, USA.

Esmaeil Sadeghimeresht, Nicolaie Markocsan, Per Nylén: Corrosion behavior of bi-layer coatings. ITSC16; May 2015; Shanghai, China.

Esmaeil Sadeghimeresht, Nicolaie Markocsan, Per Nylén: Corrosion behavior of high-chromium Fe-based coatings produced by HVAF thermal spraying technique. 7RIPT; Dec 2015; Limoges, France.

C. Clement, E. Sadeghimeresht, Nicolaie Markocsan, Per Nylén: Corrosion behavior of HVAF-and HVOF-sprayed high- chromium Fe-based coatings. 7RIPT; Dec 2015; Limoges, France.

Esmaeil Sadeghimeresht, Nicolaie Markocsan, Per Nylén: HVAF thermal spray Fe-based coating: An environmentally acceptable alternative to cobalt-based coating. EUROCORR15; Sep 2015; Graz, Austria.

E Guerin, E Sadeghimeresht, N Markocsan, S Joshi, Role of Chemistry on Oxidation Behavior of Various Ni-based HVAF-Sprayed Coatings in Simulated Boiler Environments, 8RIPT, Dec 2017; Limoges, France.

Esmaeil Sadeghimeresht, Sudharshan Raman, Nicolaie Markocsan, Shrikant Joshi, High Temperature Erosion-Corrosion of HVAF-Sprayed Fe-based Coatings, 8RIPT, Dec 2017; Limoges, France.

Esmaeil Sadeghimeresht, Reza Jafari, Taghi Shahrabi Farahani, Nicolaie Markocsan, Shrikant Joshia, High Temperature Corrosion of HVAF-Sprayed NiCrAlY Coatings Exposed to Various Corrosive Environments, 8RIPT, Dec 2017; Limoges, France.

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Abbreviations / Nomenclature

APS: Atmospheric Plasma Spraying

BIB: Broad Ion Beam Milling

CTE: Coefficient of Thermal Expansion

Dwell Time: The total time that a particle spends in a flame/plasma jet

EDS: Energy Dispersive Spectroscopy

HAZ: Heat Affected Zone

HCl: Hydrochloric Acid

HVAF: High-Velocity Air Fuel

HVOF: High-Velocity Oxy Fuel

KCl: Potassium Chloride

K2CrO4: Potassium Chromate

Splat boundaries: Boundaries between the splats formed due to particle flattening on the substrate

SEM: Scanning Electron Microscope

SEM-BSE: Scanning Electron Microscope in Backscattered Electron Mode

SEM-SE: Scanning Electron Microscope in Secondary Electron Mode

XRD: X-ray Diffraction

The power generation industry has been rapidly shifting towards utilization of more ecologically compliant biomass and waste fuels, e.g., straw and municipal solid wastes (MSW) to reduce the usage of fossil fuels, and accordingly the emissions of CO2 [1]. Unlike the fossil fuels, biomass and waste fuels are firstly considered CO2-neutral, i.e. the carbon released during combustion in the form of CO2 emission can be re-used for new plant growth. Thus, no net increase in atmospheric greenhouse gases is yielded. Secondly, burning such inexpensive fuels to produce electricity/heat diminishes waste landfill. Furthermore, the environmental legislation implemented since 2002 forces Sweden to ensure that 50% of the total energy consumed by it is sourced from renewable energies by 2020, which has dramatically accelerated interest in combustion of biomass and waste-derived fuels [2]. Although there are several incentives to utilize such fuels, these fuels contain high concentrations of corrosive species such as chlorine (Cl2), hydrogen chloride (HCl) and alkali chlorides (e.g., KCl, or NaCl), which are released along with the hot fuel gas and accelerate corrosion of boiler components, in particular water-wall and superheater tubes [3], see Figure 1-1. During operation, these corrosive compounds are deposited on the boiler parts, thereby not only curtailing the heat transfer but also triggering corrosion mechanisms. Corrosion reduces the thermal/electrical efficiency of the boiler and causes high maintenance, replacement, and outage costs [4].

To control the corrosion caused by the corrosive species and enhance the lifetime of boiler components, two practical and widely accepted solutions are either a) using highly alloyed materials, or b) alleviating the environment (Figure 1-2).

Boiler environment: H2O, O2, HCl, CO2, SO2, NOX, and erosive particlesOOOOOOOOOOO22222222222,, , , ,, NONONONONONONONONONNNONONOOOOOOOOOOOOOOXXXXXXXXXX, ,, , ,, ,, , anananananananananananannnd d d d d dd dd dd ererererererererrererrosososososososososososo ivivivivivivivviviivivivivi eeeeeeeeeee pppppppppppppapapapapapaapapaapaapapartrtrtrtrtrtrtrttrtrrtrticicicicicicicicicicicicicccclelelelleleleleleleleessssssssss

Steam

Steel tube alloyed with:Fe, Cr, Ni, Mn, and Mo, etc.

Deposit:NaCl, KCl, CaSO4, K2SO4, K2CO3, and SiO2, etc.

Corrosion products 2: Oxides(Fe,Cr)2O3, Fe2O3, (Fe,Cr,Ni)3O4, etc.(usually found at chromates/chlorides interface)

Corrosion products 1: ChromatesK2CrO4, and CaCrO4, etc.(usually found at deposit/oxides interface)

Corrosion products 3: ChloridesFeCl2, and CrCl3, etc.(usually found at oxides/steel tube) interface

2

While in the latter solution adding S-containing additives or reducing the operating temperature are commonly proposed [5], austenitic stainless steels or Ni-based materials are frequently suggested as the former solution [6], [7]. Reducing the working temperature leads to a significant decrease in the boiler’s thermal/electrical efficiency, whereas the highly alloyed materials are complex, time consuming to develop and extremely expensive. Hence, the above solutions are not always alluring both economically and technically. As an alternative to the above solutions, corrosion has been increasingly controlled in recent years by deposition of dense and adherent coatings using two extensively used techniques of thermal spraying, and weld overlay [8], [9]–[12].

Figure 1-2: Commonly proposed approaches to control high temperature corrosion in boilers.

Although the high temperature corrosion resistance of components increases using coatings, there are a few technical challenges in producing coatings that can meet high-performance requirements. Weld overlays are metallic coatings welded directly onto the substrate. The high-heat welding process forms a metallurgical bond with the base metal, essentially alloying the coating to the substrate at the interface. Laser cladding (or laser deposition) is a processing technique used for adding one material to the surface of another in a controlled manner. A stream of a desired powder is fed into a focused laser beam as it is scanned across the target surface, leaving behind a deposited coating of the chosen material. In both weld overlay and laser cladding, the result is a durable, and almost nonporous coating with good resistance to corrosion [12], [13]. Both coatings are typically applied in high thicknesses. As such, substantial amounts of material may be applied in a

Corrosion control approaches in boilers

Material-wise Environment-wise

Advanced highly alloyed materials

Coatings

Adding S-containing additives

Reducing the operating temperature

Thermal spray Weld overlay Laser Cladding

3

INTRODUCTION

comparatively short time. There would be high set up costs for equipment and installation for the two processes. The size of equipment is rather large which limits the systems to be portable. High build rate, incorrect setup and power can cause stress cracking in the cladding. In Chapter 2, the two processes would be further discussed.

The coatings manufactured by conventional thermal spray methods, e.g. high velocity oxy-fuel (HVOF) [11], [14], or atmospheric plasma spraying (APS) [15] have their own unique characteristics, such as lamellar structures, in flight oxidation of the sprayed particles and certain process-dependent pore formation which make the coatings inefficient for highly corrosive environments [16]. The coatings produced by the high velocity air-fuel (HVAF) method can be characterized by a relatively dense microstructure with low amount of oxides [12], [13], [17]. Although the HVAF coatings are not entirely free from pores either [18], [19], among the above-mentioned thermal spray processes, HVAF has the lowest flame temperature and highest flame velocity (T<1800 °C, V= 700-1500 m/s) [20]. Spraying temperature and velocity significantly affect the microstructural features, in particular in situ oxide formation, splat morphology and level of porosity that alter the corrosion behavior of the coatings [21].

Along with the microstructure, the coating composition also plays a critical role in corrosion protection mechanisms [8], [11]. Ni-based coatings with addition of protective scale-forming elements, e.g., Cr or/and Al, have demonstrated reasonable high temperature corrosion protection and have been studied both in commercial boilers and in laboratory furnaces with simulated corrosive environments [22], [23]. However, the corrosive agents have been found to diffuse through some of the coatings, accompanied by depletion of Cr or/and Al [24].

Several studies have also focused on chemical interaction between Cl (either ionic or molecular) and oxide scale formed on the alloy surface. Grabke et al. [25] have appointed molecular chlorine (Cl2), formed from HCl in the flue gas and alkali chlorides in the deposit, as the main reason for oxide breakdown and alloy failure. Other studies have pointed towards oxide dissolution in molten chlorides as a primary source for breakaway oxidation [26], [27]. Pettersson et al. [28], [29] have found that, on high-alloyed steels, oxide breakdown takes place when KCl reacts with chromia and forms potassium chromate (K2CrO4). These studies have provided important information regarding breakdown of metallic materials in the presence of Cl-containing compounds. However, there is still disagreement regarding the underlying failure mechanisms, which demands further investigations. While the prevailing corrosion mechanisms are a subject of debate in case of alloys, the situation is even more complicated in case of coatings, in

4

which both the microstructures and compositions are relatively more heterogeneous. Little attention has been paid to coating’s corrosion performance and methods for reducing the corrosion rate in the coatings. It is still unclear how Cl is able to diffuse (whether as Cl2 through the oxide pores and cracks, or as Cl- through the oxide grain boundaries) and how the presence of metal chlorides accelerates corrosion in the complex chemistry and microstructure of the thermally sprayed coatings. Understanding corrosion mechanisms in such heterogeneous coatings and identifying pathways to control it is of crucial importance for the development of coatings that are highly protective in aggressive working environments.

In this study, the aim was to investigate the ability of the HVAF coatings prepared from Ni-based materials with different scale-forming ability to protect boiler components in ambient air, moisture- and HCl-laden environments with and without KCl salt. A key objective of the study was to understand the degradation mechanism of the Ni-based chromia and alumina forming coatings in Cl-containing environments.

The approach adopted in this research work was structured as shown in Figure 1-3. In the first step, the corrosion behavior of Ni-based coatings deposited with different thermal spray processes was studied to understand the effect of coating microstructure on its behavior in a corrosive environment. The process yielding a dense and least-defective coating was selected for further investigation. The focus in the first step was to identify the coating characteristics that give the best corrosion resistance. The gained knowledge was then used in the second step for spraying the different Ni-based coatings using the selected process. Varying compositions of coatings containing Cr and Al yielding chromia and alumina scales respectively, were evaluated in a mild high temperature corrosive environment of ambient air and water vapor. In the third step, a harsh corrosive environment containing KCl and HCl was used to understand the corrosion performance of the Ni-based chromia- and alumina-forming coatings in an aggressive condition. Altering the coating architecture as a potential route to further enhance the high temperature corrosion resistance of the thermal spray coatings, the influence of incorporating a dispersed oxide into the coating was evaluated in the final fourth step.

INTRODUCTION

5

Figure 1-3: Schematic of the methodological approach adopted to evaluate corrosion behavior of thermal spray coatings.

1.1. Thesis outline

The thesis consists of eight chapters, including this Introduction that briefly describes the frame of this research work and the outline of the thesis. The scope and limitations of the work, as well as the research questions addressed, are discussed at the end of this chapter. Chapter 2 provides background information, to confer a better understanding of the main motivation behind this study and why this research work is relevant to the scientific society, specifically to the thermal spray community. This chapter begins with a review of power plant boilers utilizing biomass and waste fuels, and the encountered high temperature corrosion problems. A literature review on previous works performed to reduce corrosion using coatings in power plants is presented, however the main focus is on coatings produced with various thermal spray processes. Requirements for thermal spray coatings in boiler coatings are also formulated. Chapter 3 explains the main characteristics of the coatings with focus on microstructure and composition, in connection with high temperature corrosion behavior. Chapter 4 describes the experimental methods utilized produce the coatings and investigate them before and after high temperature corrosion exposures. Details of the corrosion test methods and equipment are also described. In Chapter 5, the effect of coating composition, microstructure, and architecture on high temperature

Effect of a

dispersed oxide

Selection of an appropriate thermal spraying process

Corrosion performance of the coatings in a

very aggressive environment

Corrosion performance of chromia- & alumina-forming coatings in

mild corrosive environments

corrosion behavior is discussed. Chapters 6 and 7 give conclusions and recommendations for future work, respectively. Finally, Chapter 8 summarizes the research publications appended in this thesis.

The objective of this study was to gain a better understanding of how high temperature corrosion behavior of a thermal spray coating is influenced by spray process, feedstock composition, and microstructure in a given environment. Additionally, there were also two sub-objectives. FFirst, to investigate the underlying corrosion mechanisms responsible for degradation in a given environment, and thereby contribute to the implementation of more reliable coatings that may further increase the corrosion resistance. Second, to improve the corrosion resistance of a coating in a simulated boiler environment, and hence increase the lifetime of critical parts in the boilers, e.g. heat exchangers. The objective and sub-objectives were achieved by addressing the research questions (RQs), formulated as follows:

RQ1: What process-dependent microstructural features can lead to superior corrosion resistance in a thermal spray coating?

RQ2: How can coating composition influence corrosion resistance in a given environment?

RQ3: How can corrosion be further decelerated by incorporation of a dispersed oxide in the coating?

The present research was an experimental study aiming to investigate thermal spray coatings against high temperature corrosion for waste and biomass-fired boiler applications.

Certain limitations should be taken into close consideration if the results of this study should be applied to other spraying techniques or coating applications. The limitations of the current study are summarized as follows:

INTRODUCTION

7

i. Materials

Only Ni-based coating materials (Ni5Al, Ni21Cr, Ni21Cr7AlY, Ni21Cr9Mo, and Ni21Cr9Mo-SiO2) were investigated in the present study. For substrate material, a commercially available low carbon steel (16Mo3) which is commonly used in boilers was examined.

ii. Coating applications

The work was focused on Ni-based thermal spray coatings for biomass and waste-fired boilers. However, the screening study showed coating microstructures and compositions might be protective in other high temperature applications (~ 600 °C).

iii. High temperature corrosion environments

During combustion in boilers, Cl contained in biomass and waste fuels mainly forms gaseous HCl, or alkali chlorides like KCl. Water vapor and oxygen are also present in the boiler’s corrosive environment. The current study mainly focused on understanding the influence of such environments (ambient air, 5 vol.% O2 + 500 vppm HCl + N2, 5 vol.% O2 + 20 vol.% H2O + N2 with and without KCl) on corrosion behavior of the above-mentioned Ni-based thermal spray coatings.

iv. Spraying techniques

The spraying of coatings was mainly performed using high-velocity air fuel (HVAF) technique. Although high-velocity oxy fuel (HVOF) and atmospheric plasma spraying (APS) were briefly explored in this research, the high temperature corrosion tests were mainly conducted on the coatings sprayed using HVAF.

v. Coating characterization techniques

Experimental coating characterization performed in this study was limited by the characterization techniques (porosity, hardness, and surface roughness measurement) used. Microstructure analysis was performed using a scanning electron microscope (SEM) equipped with X-ray energy dispersive spectroscopy (EDS). The phases formed before and after the high temperature corrosion exposures were evaluated using an X-ray diffractometer (XRD).

9

2. Background and motivation

2.1. Biomass/Waste-fired boilers

The aim of reducing CO2 emissions from fossil fuels in power (heat and electricity) generation sectors has significantly increased the use of biomass and waste as partial substitution for coal in co-combustion boilers [30]. Biomass boilers can utilize various types of fuels, including wood, forest residues, recycled wood, municipal solid waste (MSW) and agricultural crops, e.g., straw, see Figure 2-1. The operating conditions of different boilers (temperature, flue gas composition, and fly ash, etc.) depend on the fuels and the operating process parameters, and may differ inside the boiler [31]. The burning of such complex fuels leads to very harsh conditions and may cause fouling, slagging, and high temperature corrosion of the metallic heat exchangers, see Figure 2-2. Corrosion can lead to severe material damage and tube leakage, resulting in serious operational efficiency problems and expensive consequences such as unplanned shutdowns of the plant, high maintenance costs, reduced availability of the plant for power generation, and shortened lifetime of the boiler tubes. Additionally, the demand for higher electricity/heat efficiency of the boilers requires an increase in steam temperature, which in turn increases the temperature of boiler tubes, leading to even more severe corrosion problems, especially for superheater components [32], [33].

10

Figure 2-1: Examples of biomass-fired boilers, a) MSW-fired reciprocating-grate boiler, and b) Straw-fired vibrating-grate boiler [34].

Figure 2-2: Tube failures resulting from a highly corrosive biomass fuel (from a circulating fluidized bed unit in Germany) [35].

2.2. High temperature corrosion in boilers

Biomass contains carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulphur (S), and chlorine (Cl), as well as major ash-forming elements (Al, Ca, Fe, K, Mg, Na, P, Si, Ti) and minor ash-forming elements (As, Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Tl, V, Zn). The content of each species depends on the type of biomass fuel used in a boiler [34]. During the combustion in boilers, Cl contained in the biomass mainly forms gaseous HCl, or alkali chlorides (e.g., KCl and NaCl) [36]. Due to the subsequent cooling of the flue gas in the boiler, a large part of Cl condenses as salts on the heat exchanger surfaces or on fly ash particles in the

a b

flue gas. The main consequences of Cl are the corrosive effect of chloride salts and HCl on metal surfaces in the boiler [37], and acidic pollutant emissions (e.g., HCl) [38]. In short, the release of the relevant inorganic elements (e.g., Cl, and K) not only leads to direct pollutant emissions (e.g., HCl), but also causes corrosion problems when they are deposited on the metal surfaces.

There are several high temperature corrosion mechanisms, which can concurrently occur in boilers operating with Cl-containing fuels, e.g., biomass and waste. In the superheater areas, corrosion triggers from reactions between fly ash, gaseous components and metal surfaces, where a deposit of corrosion products is precipitated. The corrosion reactions may result from combustion gas containing HCl or deposited ash containing low-melting-point salts such as alkali chlorides (KCl, or/and NaCl) [3], [39], [40]. Cl-induced corrosion is detrimental to many elements, including Ni, Cr, Al, and Mo, etc. and the corrosion starts almost instantly after introduction of Cl-containing species into the environment. Cl is able to penetrate and destroy the metal oxide layer through cracks (in form of Cl2) and grain boundaries (in form of Cl-) of the oxide scale. At the metal-scale interface, the Cl2/Cl- reacts with the present alloying elements to form volatile metal chlorides [41], [42]. Corrosion of the boiler tubes can also occur due to presence of molten phases [27]. Indeed, one of the extremely damaging corrosion mechanisms in biomass and waste-to-energy boilers is molten salt attack due to low-melting-point eutectic compounds formed between alkali chlorides (NaCl, and KCl) and metal chlorides (FeCl2, NiCl2, and CrCl2, etc.) [26].

Due to a large number of gaseous, solid, and even liquid compounds present in boilers, various competing reactions can occur. Hence, prediction of corrosion rates or the type of corrosion attack is challenging. However, with a combination of simplified and controlled laboratory exposures, significant features associated with fireside corrosion of superheater tubes in full-scale power plants can be highlighted. Although SO2-containing compounds [43] may also have an impact on alloy performance, the main mechanisms associated with oxide breakdown are associated with the presence of Cl-containing compounds present in the gaseous, liquid, or solid state, which are briefly explained below:

The corrosion problems related to biomass and waste-fired boilers have been reported to be mainly caused by Cl. Several corrosion mechanisms proposed concerning the effect of Cl, however only two of them are addressed in the present study including; 1) chlorine-induced active corrosion [25], and 2) electrochemistry [29].

22.2.1.1. Chlorine-induced active corrosion According to Nielsen [3], the level of HCl when firing biomass fuels usually ranges between 100 and 1000 ppm and, during waste incineration, it is even higher [44]. In atmospheres containing both HCl and O2, accelerated corrosion has been observed. A common opinion is that Cl2 forms according to the Deacon reaction (Eq. 1) [25]. This is normally a very slow reaction but could be catalyzed by the oxidized metal surface. During combustion of waste, alkali chlorides are deposited on the superheater tubes and formation of Cl2 may take place based on:

(1)

It was also proposed by Lee and McNallan [45] and further investigated by Grabke and Zahs [41] that the Cl-containing compounds, e.g., KCl, react with the alloying elements present in the boiler components, and trigger accelerated corrosion through the “chlorine-induced active corrosion” mechanism, schematically illustrated in Figure 2-3. Cl2 sourced from the Deacon process (Eq. 1) or alkali chlorides (Eq. 2) diffuses through the defects in the oxide scale (e.g., cracks and pores) towards the metal/scale interface. Formation of metal chlorides can be accelerated whenever less oxygen (or lower oxygen partial pressure - pO2) is present. At temperatures above ~400 °C, evaporation of the metal chlorides (those with higher vapor pressures) and their subsequent outward diffusion towards the gas phase can occur. The gaseous metal chlorides convert to oxides wherever pO2) is available. The new oxides form within the cracks and pores of the existing oxide scale, leading to subsequent cracking and spallation of the oxide [41].

p

p

BACKGROUND AND MOTIVATION

13

( ) + 4 ( ) + ( ) = 2 ( ) + 2 ( ) (2) ( 2 4) 73.8 / 600

Chemical analyses of corrosion products formed in oxidizing-chloridizing environments have confirmed that metal chlorides are frequently observed at the metal/deposit interface (low pO2), while the outer part of the deposit (high pO2) contains mainly metal oxides [46]. In other words, there is an inward diffusion of Cl2 through the scale and a subsequent accumulation of Cl2 at the metal/deposit interface. In this case, the molecular Cl2 must diffuse through the oxide layer, presumably through cracks and pores. In an oxidizing-chloridizing environment, the stability of metal oxides and chlorides depends on the partial pressure of O2 and Cl2. At the metal/deposit interface, pO2 is low and metal chlorides are stable. If the system is assumed to consist of only Ni and Cr, the metallic chlorides are formed according to Eqs. 3 to 5: ( ) + ( ) = ( ) ( ) 173.9 / 600 (3) ( ) + ( ) = ( ) ( ) 287.0 / 600 (4) ( ) + ( ) = ( ) ( ) 185.8 / 600 (5)

At elevated temperatures, the vapor pressure of metal chlorides can be significant. Therefore, volatile chlorides form according to Eqs. 6 and 7 and diffuse towards the oxygen-rich outer part of the scale (high pO2) where they are destabilized according to Eqs. 8 and 9 [25]: ( ) = ( ) (6) ( ) = ( ) (7) 2 ( ) + 2 ( ) = ( ) + 2 ( ) ( ) 255.6 / 600 (8) ( ) + ( ) = ( ) + ( ) ( ) 50.9 / 600 (9)

Although the chlorine cycle is frequently used for describing corrosion in oxidizing-chlorinating environments, there are concerns with this proposed mechanism. Folkeson et al, [47] argued that it is unlikely that a low pO2 can be established at the metal/oxide interface if molecular transport is taking place. This would assume that the oxide is permeable for the bulky Cl2 molecule while being impermeable for the smaller O2. The initiation of corrosion, i.e. the breakdown of the Cr-rich protective oxide, which is initially present on Cr2O3-forming alloys, cannot be explained by this mechanism. Moreover, Cl2 and K2CrO4 formed through the proposed mechanism suggested in Eqs. 1 and 2 are not thermodynamically favored, and are hence non-spontaneous [28].

14

22.2.1.2. Electrochemical mechanism In another proposed theory, known as the “electrochemical” mechanism [48], the transport of Cl through the oxide scale is considered to be ionic rather than gaseous, especially during the initial stages. The corresponding anodic process at the metal/oxide interface creates outward diffusing cations (like Cr3+). The alkali chlorides react with Cr in the protective oxide scale to form K2CrO4 at early stages, leading to Cr-depleted oxides and breakaway oxidation. The mechanism would then be explained by HCl dissociating at the scale/gas interface according to the cathodic process according to Eqs. 10 and 11: = ( ) + 2 (10)

Scale surface: + 4 ( ) + ( ) + 2 = ( ) + 2 (11)

After this initial step, Cl- diffuses through the oxide grain boundaries towards the metal/oxide interface via electrochemical reactions [29].

At locations where Cl- and Cr2+ meet each other within the coating based on Eqs.

12 and 13: + = ( ) (12) ( ) = ( ) (13)

The sum reaction becomes (for the case of Cr oxidation), Eq. 14: + 2 ( ) + ( ) + = ( ) + ( ) (14)

( ) 77.38 / 600

A summary of the two proposed mechansims in Cl-containing environments is shown in Figure 2-4.

Ni-based coatings alloyed with ~20 wt% Cr [49] are widely used for protecting components against oxidation in dry environments as an oxide scale of Cr2O3 [50] can potentially protect the coating and substrate from further oxidation. However, water vapor is present in many environments of industrial relevance (e.g., boilers). Oxidation in a water vapor-laden environment is particularly severe in the presence of water vapor (pH2O=0.02-0.04 atm [51] reported to be sufficient to trigger oxidation) and oxygen, where formation of a volatile, unstable and non-protective CrO2(OH)2(g) (see Eq. 15) depletes the oxide in Cr and suppresses the inherent protective nature of the oxide [52], [53]. It is worth noting that sizeable literature is available on the role of H2O (g); however, the exact effect of water vapor on the extent of degradation is still a controversial issue [54].

(15)

Several mechanisms responsible for corrosion in moisture-laden environments have been proposed [55], [56], [57], [58], [59], [60], [61] such as: dissociation mechanism [55], H+/OH- incorporation mechanism in the oxide [56], as well as vaporization mechanism and formation of volatile Fe(OH)3 or CrO2(OH)2 [57], etc.

Chlorine-induced active corrosion

Triggered by formation of gaseous Cl2 in the Deacon or K2CrO4 formation reactionsDiffusion of Cl2 and volatile metallic chlorides through the oxide defects, e.g., cracks and poresMultilayered corrosion products including chlorides at the bottom (with low pO2) and oxide at the top (with high pO2) of corrosion products

Cons- Thermodynamically unfavorable reactions- Diffusion of big Cl2 but not small O2 or volatile metallicchlorides-Initiation: No explanation for breakaway of dense oxide

Electrochemistry

Triggered by formation ofionic Cl- and K2CrO4Formation of Cr2+/Cr3+ in ananodic process at themetal/oxide interfaceDiffusion of Cl- and

Cr2+/Cr3+ through oxide grainboundariesK2CrO4 acts as sink for Cr

Although different corrosion mechanisms are possible depending upon the complexity of environment prevailing during combustion and although the details of protective oxide breakdown are still of debate, it is evident that the alkali chlorides are the main concern in biomass and waste-fired boilers. Studies have thus been performed to avoid the formation and accumulation of alkali chlorides on metallic surfaces. Sulfur compounds are often added during the combustion of biomass in order to convert alkali chlorides to less corrosive alkali sulfates. Spiegel and Grabke [62] studied the effect of S-containing gases (e.g., SO2) on corrosion under alkali chloride deposits and found that the corrosion rates decreased in the presence of SO2. They found that the chlorides are transformed into sulphate at high concentrations of SO2, which are less aggressive than the corresponding chlorides. Such alkali sulphates are considered rather stable in waste-to-energy plants and are not expected to cause severe corrosion. Salmenoja et al, [63] obtained a similar positive effect of S after investigating the corrosiveness of different fuel blends. A S:Cl (sulphur/chlorine) ratio of 4:1 in the fuel was proposed as an optimum condition to prevent severe corrosion problems. During combustion, the S:Cl ratios are usually significantly lower. Recently, actual boiler tests have also been performed with S-containing additives to the fuel, which have been highly effective in reducing corrosion rates [64]–[67].

A number of different materials are commonly used for superheater tubes, e.g. low-alloyed ferritic steels, 9-12% Cr ferritic-martensitic steels, austenitic stainless steels, and Ni-base alloys [68]. These materials combine good thermal conductivity with good mechanical properties, as well as machinability and weldability. Such advanced materials are complex, time consuming to develop and extremely expensive; Hence, use of highly alloyed materials is not always attractive from both economic and technical perspectives.

At present, the most economically and technically-favored method of protection for boiler’s components from corrosion is by deploying a protective coating. Coatings provide a way of extending the limits of use of materials at the upper end of their performance capabilities by allowing the mechanical properties of the substrate materials to be maintained while protecting them against high temperature corrosion [69], [70]. A large number of coating processes are available to provide surface protection as already discussed in Chapter 1. High-

alloyed alloys can be applied on low-alloy steels as weld overlays or laser claddings. Weld overlays are thick, typically 2 mm, and dense. Problems associated with the weld overlay coatings include varying corrosion resistance of beads due to heat affected zone (HAZ), increased risk of stress corrosion cracking and mismatch in coefficients of thermal expansion. In laser cladding, a laser beam fuses a consumable onto the substrate surface producing a dense, uniform and crack-free coating with low porosity. During deposition, the top layer of the substrate is melted to give metallurgical bonding with some dilution of the coating to the substrate. Compared to weld overlays, laser cladding gives lower dilution and lower component heating and the laser beam can more rapidly treat larger surface areas. One of the main drawbacks in claddings, both weld overlays and laser cladding, is the metallurgical bonding mechanism of coating to the substrate material. While this could ensure high adherence of the coating to substrate, it may also have a negative influence on the mechanical properties of boiler tubes under high pressure. Therefore, the main focus in the present work is on thermal spray coatings due to their minimal thermal degradation of substrate during coating deposition, as well as lower processing and equipment costs.

In a thermal spray process, a metallic or non-metallic material of the desired properties is deposited on the substrate [71]. The feedstock material typically in form of powder, wire or solution/suspension is heated up to a molten or a semi-molten state and accelerated in a flame and then propelled onto the substrate. The fully or partially melted particles deform when they impact the substrate and form so-called “splats”. The coating material bonds to the substrate by mechanical interlocking upon impact and/or solidification. Not only a wide variety of materials can be deposited, but also worn parts can be re-coated easily by thermal spray techniques [71].

Although all thermally sprayed coatings typically contain splats, there are huge differences among the coatings in terms of; the nature of splat boundaries (like oxide pick-up usually evident at such boundaries), level of porosity and residual stress. In particular, splat boundaries, and interconnected pores can act as short-circuit diffusion paths for corrosive agents [14]. Therefore, the composition and microstructure of the coatings should be tailored to endure harsh corrosive conditions in order to protect the underlying steel tubes. The coefficient of thermal expansion (CTE) of the coating should match the substrate material in order to endure the thermal variations during start-ups, shutdowns, and the boiler process itself without cracking on account of CTE mismatch. The CTE (from 20 to 600 °C) for NiCr is ~14×10-6/°C, for NiCrAlY is ~ 12×10-6/°C, for Alloy 625 is ~ 13-16×10-6/°C and for Fe19Cr is ~ 10×10-6/°C [11] compared to 10-14×10-

6/°C for ferritic steels as typical boiler materials. Dense coating structure and high adherence to the substrate are of great importance for coatings operating in high temperature conditions, where corrosive gases and molten phases may be present.

As seen in Figure 2-5, the particle temperature and velocity may vary in different thermal spray processes, as each process has its own inherent characteristics in terms of gas temperature and gas velocity that govern particle heat-up and acceleration. The difference in velocity and temperature of particles in various methods is a major factor that explains the diversity in resulting coating microstructures.

[72]

22.3.4.1. High-Velocity Air Fuel (HVAF) spraying The HVAF process is a relatively new technology in the family of thermal spray processes. In the HVAF process, compressed air and a fuel gas (propane, propylene, or natural gas) are pre-mixed before entering the combustion chamber where ignition occurs with the help of an electric spark plug (Figure 2-6). When the catalytic ceramic wall of the chamber is heated above the auto-ignition temperature of the mixture (shortly after the process starts), it provides further ignition and combustion taking thus over the spark plug’s role [73]. The powder is injected into the process axially using an injector through the hot back wall with N2 as a carrier gas. The operating cost is low, due to use of air in combustion. Fuel (such as propane) is also added between the first and second nozzle of the chamber to further increase the particle velocity and control the temperature of particles [74]. Type of fuels, feature of nozzles (length, diameter), amount of gas injected into both the primary (input chamber) and

3000

2500

2000

1500

1000

500

0200 400 600 800 1000 1200 1400

In-fl

ight

par

ticle

tem

pera

ture

, C

APS

HVOF

HVAF

In-flight particle velocity, m/s

secondary (in-die) process are of high importance to control the combustion process. The temperature and velocity of particles is also influenced by feedstock material characteristics (chemical composition, particle size/distribution, and morphology) which in-turn will influence the coating properties and performance. The HVAF process creates a jet stream with a velocity of about 1400 m/s which accelerates injected powder particles to a velocity over 1100-1200 m/s [75]. This high velocity of particles establishes a good adhesion strength between the coating and substrate. The temperature of the HVAF flame is reported to be less than 1950 C, which heats up the in-flight particles to a maximum temperature of around 1500 C [75], depending upon the thermos-physical properties of the material being sprayed. The comparatively low process temperature enables spraying of materials that are inherently sensitive to high temperature and oxidation such as materials comprising Cr or/and Al [71]. The low heat input in the process also leads to a negligible level of oxide content in the coating. Under typical spraying conditions, the total oxygen content in HVAF coatings can be kept below 1 wt% for most materials particularly for Ni-based coatings [76]. The coatings have been shown to yield high bond strengths, sometimes well higher than what can be determined by the most common adhesion test method ASTM C633 [77]. The deposition efficiency (DE) of the HVAF process has been reported to be in the range of 58-65% for metal powders and 50-75% for carbide powders [71]. The feedstock material characteristics (such as optimal particle size with narrow distribution) combined with optimized process parameters increase the DE.

The coating produced by the HVAF process consists of a typical thermal spray structure comprising a lamellar structure embedded with a few solid particles, porosity, and almost no oxides or inclusions [18]. The high velocity of particles within the flame in addition to plastic deformation of particles produce compact/dense coatings with minimal formation of voids/pores at the surface and splat boundaries [78]. A typical microstructure of the coating produced by HVAF can be seen in Section 5.1.

20

22.3.4.2. High-Velocity Oxy Fuel (HVOF) spraying Figure 2-7 shows a schematic view of the HVOF process. In the HVOF process, the fuel (kerosene, acetylene, propylene, or hydrogen) is mixed with oxygen and burned in the combustion chamber. The energy produced by combustion is channeled through a nozzle to produce a jet stream with a temperature of around 3000 °C and velocity of about 550 m/s [79]. The feedstock powder is radially or axially injected into the jet, heated up, and accelerated towards the substrate. The high velocity of the fully or partially melted particles produces compact/dense coatings [80]. The coatings produced by this process show a slightly higher amount of porosity (<3 vol.%) and usually higher amount of oxides compared to HVAF-sprayed coatings [81] due to slower and hotter in-flight particles propelling on to the substrate [82]. The typical coating microstructure produced by this process includes splat boundaries, some unmelted particles, pores, oxides and inclusions [83]. A typical microstructure of the coating produced by HVOF can be seen in Section 5.1.

Figure 2-7: Schematic of the HVOF spray process.

2.3.4.3. Atmospheric Plasma Spraying (APS) Figure 2-8 shows a schematic view of the APS process. In APS, a plasma jet is used in atmospheric conditions to spray the feedstock material. Due to the high flame temperature in the plasma (15000 °C) and thus the high thermal energy of the plasma jet, APS enables spraying of almost all types of feedstock materials including ceramics [84]. The plasma spraying process uses a direct current (DC) electric arc to generate a stream of high-temperature-ionized plasma from one or a mixture of inert gases (Ar, He, H2 or N2), which acts as the spraying heat source [71]. The feedstock material is conveyed by a carrier gas (typically Ar) into the plasma jet, where it is heated and propelled with a typical particle velocity of 200-300 m/s at impact with the substrate [85], [86]. A typical microstructure of an APS coating comprises of splat boundaries, a higher porosity, and oxide content than HVAF and HVOF produced coatings. A larger number of unmelted particles makes this process more unfavorable for corrosion protection

Air in

Air in

Fuel/oxygen

Fuel/oxygen

Convergent/divergent nozzle

Powder in

Combustion chamber

BACKGROUND AND MOTIVATION

21

applications [87]. A typical microstructure of the coating produced by APS can be seen in Section 5.1.

Figure 2-8: Schematic of the APS process.

2.4. High temperature corrosion of thermally sprayed coatings

The extent of corrosion could be greatly reduced by altering the composition, microstructure, and architecture of the coatings, see Figure 2-9. Major alloying elements like Ni and minor alloying elements, e.g., protective scale-forming elements (Al or/and Cr) can alter the corrosion mechanisms [88]. Microstructural features in the coating, such as pores and splat boundaries, can also significantly affect the corrosion performance [89]. The coating microstructure can be varied by selection of spraying process, spraying parameters, or post-spray treatment [90] [91]. Another potential approach would be altering the architecture of the coating either by creating a new coating design, such as what has already been achieved in multilayered or functionally graded coatings [92] or by incorporating certain oxygen-active elements into coatings to fabricate composite structures [93].

Inert gas(Ar, He, H2 or N2)

Cooling waterPlasma

Anode (+)

Cathode (-)

Arc

Powder in

22

Figure 2-9: Approaches to control corrosion with coatings. The methods used in the present study are underlined.

Several prior efforts have focused on the oxidation behavior of Ni-based coatings deposited by commonly used thermal spraying techniques such as APS and HVOF [11], [49], [94]–[98]. However, the coating features associated with the above techniques, such as splat boundaries, pores and in situ formed oxides [14], lead to the formation of a discontinuous oxide scale which provides motivation to seek even more efficient protective barriers. The high amount of interconnected pores as well as poor splat cohesion due to the oxides and/or voids formed at the splat boundaries in the coatings have an adverse effect on the oxidation behavior of the coatings [99]. Previous studies, while claiming that the HVOF-sprayed Ni-based coatings could provide oxidation protection [11], [15], showed that oxidation occurred through the splat boundaries and the pores of the coating. Cr or/and Al cannot be uniformly supplied to the surface to form a uniform protective oxide scale if such coating features are present within the diffusion paths. Therefore, a highly cohesive, pore-free and less in situ oxidized coating could provide superior performance in high-temperature corrosive environments.

The HVAF technique is being increasingly employed to address the above-mentioned challenges and produce coatings with better protective characteristics [100]. While the supersonic spray particle velocity helps in obtaining well-adherent coatings, the low particle temperature ensures that there is almost no in situ oxide pick-up or phase transformation during spraying [101]. Therefore, the protective scale-forming elements such as Cr or/and Al are not depleted during the spraying process but preserved for oxidation protection. The few studies performed so far on HVAF-sprayed Ni-based coatings show that the coatings possess excellent oxidation behavior in high-temperature environments [102]–[104]. However, the high temperature corrosion mechanisms in HVAF-sprayed Ni-based coatings in a given corrosive environment need to be further studied.

Corrosion control approaches using coatings

by altering Composition

by altering Microstructure

by altering Architecture

• Additional layer• Functionally graded

coatings• Preformed oxide on

the coating surface

Composite with a secondary phase, e.g., silica, yttria,

alumina, chromia,and ceria, etc.

• Spraying processes• Post treatment

• Ni-based, and Fe-based, etc.

• Alloying elements(to form chromia,alumina, and silicaetc.)

Composition, microstructure and architecture of the coatings need to be tailored in order to endure high temperature corrosion environment prevailing in boilers. In this chapter, the importance of coating composition (e.g., alloying elements), microstructure and architecture in corrosive environments, particularly in Cl-containing environments is briefly explained.

Ni-based coatings often display a good resistance to high temperature corrosion. The Ni matrix is preferred over other metal matrices like Fe in Cl-containing environments, as formation of NiCl2 is less thermodynamically favored (with higher Gibbs free energy) than the other detrimental metallic chlorides such as FeCl2, see Eqs. 1 and 2:

Figure 3-1 shows the predominance diagram of the Ni-O-Cl system taken from the HSC Chemistry 6.0 (Outotec, Espoo, Finland). The most aggressive test condition used in this study is marked with a star in the graph. It implies that the test condition (considering pCl2 and pO2) is at the border between formation of the metallic chloride (NiCl2) and the oxide (NiO). Addition of alloying elements or formation of a protective oxide scale on the coating surface, which are discussed in the next sections, may alter the amount of Cl2 and O2 available for the formation of NiO and NiCl2.

The most important elements for forming protective oxide layers at high temperature are Cr, Al, and Si. Other alloying elements, e.g. Mo, can also influence the high temperature corrosion behavior in Cl-containing environment.

33.1.2.1. Chromium (Cr) Cr is a very important and common alloying element in materials for high temperature applications and forms the stable oxide (Cr2O3) with the hexagonal corundum structure. Diffusion of corrosive species through Cr2O3 is rather slow, thus it is considered as a protective oxide. As long as the oxide scale is well attached to the underlying alloy, and growth rate of the scale is controlled by diffusion of ions through the scale, oxidation follows a parabolic rate [105]. Figure 3-2 shows the ternary phase diagram of the Cr-O-Cl system at 600 °C illustrating that various corrosion products that can form depending upon the availability of Cl and O. The most aggressive condition used in this study thermodynamically favors formation of Cr2O3.

p

p

p

p

CHARACTERISTICS OF COATINGS

25

33.1.2.2. Aluminum (Al) Al -Al2O3, which has the

-Al2O3 has excellent protective properties with a lower growth rate than the other oxides found on engineering alloys. However, during the transient oxidation of Al, -Al2O3 -Al2O3, and -Al2O3 may also form [106]. These transient oxides are less protective so that Al2O3-forming alloys should be pre-oxidized in controlled atmospheres to ensure formation of protective -Al2O3.

Al2O3 has no volatile form and is not sensitive to water-containing atmospheres. Mechanical properties may be impaired by a high Al content, but if Cr is added to an Al2O3-forming alloy, the level of Al needed for forming a protective oxide layer is lowered [107]. This is commonly referred to as the third element effect, but the exact mechanism is under debate. Figure 3-3 shows a ternary phase diagram of the Al-O-Cl system at 600 °C. The formation of Al2O3 is expected in the most corrosive conditions used in this study as shown in Figure 3-3; however, formation of AlCl3 could also be anticipated.

Figure 3-3: Predominance diagram for Al-O-Cl system produced using HSC 6.0

Chemistry.

3.1.2.3. Silicon (Si) Low concentrations of Si are often used in high temperature alloys. If Cr is also available in the alloy, the high oxygen affinity of Si leads to the formation of a SiO2 beneath the chromia scale. Different crystal structures are possible depending on the specific conditions that prevail [108]. For instance, amorphous SiO2 is frequently encountered, but crystalline phases may also form. SiO2 has excellent protective properties. However, the Si concentration should be kept below 2-3 wt% to avoid embrittlement of the material. Poor oxide adhesion has also been reported in case of SiO2-forming alloys during thermal cycling conditions [109]. Figure 3-4 shows a ternary phase diagram of the Si-O-Cl system at 600 °C, implying that formation of SiO2 is thermodynamically more favored

Al2O3

Log

pCl 2

Log pO2

50403020100

-10-20-30-40-50

-50 -30 -10 10 30 50

AlCl3

rather than SiCl4 in O-Cl-containing environments such as one shown in Figure 3-4.

33.1.2.4. Molybdenum (Mo)

Based on the ternary phase diagram of the Mo-O-Cl system at 600 °C showed in Figure 3-5, various corrosion products can form in oxidizing-chloridizing environments. While addition of Mo is frequently reported to be beneficial in such environments, formation of volatile MoClx is found to be highly detrimental to the corrosion behavior of the alloys [105]. Thermodynamically, formation of such metallic chloride is highly favored and spontaneous, see Eq. 3:

There have been several efforts to enhance the corrosion resistance of thermal spray coatings by incorporating certain oxygen-active elements (like SiO2, Al2O3

p

p

p

p

or Y2O3) into the coating to fabricate composite structures [110]–[112]. It has already been reported that the high-temperature corrosion performance of alloys can be improved by uniformly distributed oxide dispersoids, which act as nucleation sites and facilitate eventual formation of a continuous, dense and protective oxide scale on the surface [113]. However, this proposed mechanism is under debate and has been rejected by others [114], as no change can be seen between the oxidation behavior of a dispersoids-containing alloy and a free-dispersoids alloy at the early stages of oxidation exposure. Therefore, it is proposed that the dispersoids alter microstructure of the formed oxide by reducing the grain size which promotes inward diffusion of O2- rather than outward diffusion of Cr3+ through the oxide grain boundaries [114]. The dispersed oxides can lead to excellent scale adherence to the substrate material based on a mechanism known as “pegging effect” or “mechanical keying”, as frequently reported by various authors [115], [116]. The oxide scale protrusions into the underlying alloy was also found which pin the scale to the substrate. The dispersed oxides in the alloy are also reported to act as preferred sites for nucleation of vacancies which are, therefore, prevented from concentrating at the oxide scale/metal interface [117].

As mentioned earlier, “splat” is the term given to a single impacted particle, as seen in Figure 3-6. Many overlapped splats, solidified and adhered to one another form a thermal sprayed coating. Thus, the splat is the basic structural building block in thermal spray coatings. The splats are formed when the accelerated particles impact a surface. The arriving molten or semi-molten droplets are generally spherical, and on impact with the substrate surface, they flatten and spread in the form of disk-like structures (splats) and fill the underlying interstices (free spaces). The splat shape is indicative of the degree of particle melting achieved in flight as well as deformation under impact (if the particle do not melt completely). The degree of melting indicated by these splats, largely determines the adhesion, cohesion, porosity, and subsequent corrosion properties of the coating [88].

The observation of high temperature corrosion damage in the thermal spray coatings indicates that the attack at intersplat boundaries is the major corrosion mechanism, most likely due to the fact that they represent short-circuit diffusion paths for O2 or Cl2, see Figure 3-6. Indeed, the most severe corrosion failure takes place along the boundaries of rounded, unmelted particles and, more generally, along splat boundaries [118]. These areas are important in corrosion protection

due to several reasons. Firstly, poor splat cohesion (lack of intimate contact between splats) causes the splat boundaries to act as corrosion sites: thus, passivation is prevented and corrosion processes can be triggered [119], [120]. Secondly, in situ oxidation of the feedstock particles (occurring either in the gas jet or just after particle impact) leads to presence of oxides around the splats. This oxidation depletes the protective scale-forming elements like Cr or Al from the chemical composition of the coating, which may favor the onset of breakaway oxidation [121], [122]. The presence of elements like Cr and Al is crucial for the coating to provide corrosion protection [123]–[125]. However, due to their in situ oxidation, their effective content in the coating is reduced and the protection is limited. In most HVAF coatings, cohesion among the splats is excellent which makes it very difficult for the corrosive species to diffuse easily along splat boundaries and find direct paths to the substrate [126].

The most common source of coating porosity is trapped, or unmelted particles. Depending on the particle temperature, the arriving droplets may cover the full range from a fully molten liquid to an entirely unmolten solid state. The liquid-state particles flow easily and fill most voids. On the other hand, solid or partially melted particles are plastically deformed on impact. Thus, it can be seen that higher particle velocities lead to greater particle deformation and therefore better closing of intersplat voids. Solid particles, some of which may re-bound from the solid surface or become trapped in the coatings by the next arriving particles. These undeformed particles are not well bonded, nor are they in intimate contact with the underlying splat, which creates voids. It is well known that the corrosion resistance of the coatings strongly depends on the pore content [83], [127]–[133]. If coatings are porous, the corrosive species can easily penetrate the coating through the pores and reach the substrate. In such cases, the coating delaminates in a short time and its ability to impart protection is lost. Porosity can be categorized as through-porosity (interconnected) and non-through (closed) porosity, and each of these plays roles of varying importance in

SplatsPorositySplat boundary

Diffusion of corrosive species

CHARACTERISTICS OF COATINGS

29

determining the corrosion behavior of the coatings, as seen in Figure 3-7. If the pores are interconnected, it leads to a rapid corrosion attack of the substrate. Compared to closed porosity, interconnected porosity is much more sensitive to corrosion as it forms direct channels between the corrosive environment and the substrate [134].

a) closed porosity

b) interconnected porosity Figure 3-7: Effect of different types of porosity on corrosion behavior of the

coatings. Long corrosion exposure leads the close pores to be interconnected.

If closed pores exist in the coating, the role of a) porosity itself, and b) chemistry around porosity on the corrosion resistance of a coating should be taken into close consideration [133]. During a long-term corrosion reaction, the chemical/electrochemical reaction in the region surrounding the pores makes the isolated pores grow and eventually interconnect with nearby pores. In this way, open channels can be also formed which facilitate the corrosive medium in reaching the coating/substrate interface [135]. This effect can be estimated by the average porosity ratio of the coatings: the more the porosity ratio, the easier the interconnection of isolated pores.

The second role is that porosity alters the local chemistry of the coating and thereby promotes corrosion [133]. It was already shown that the surface film in zones of higher porosity has higher Cr concentration, mainly rich in hexavalent chromium (Cr6+) and relatively lacking in trivalent chromium (Cr3+). This high-valence species constitutes the outer layer of the passive films, which is generally more defective. More importantly, the highly inhomogeneous distribution of Cr on the coating surface is responsible for the poor corrosion resistance of coatings with high porosity.

SplatPorositySplat boundary


SplatPorositySplat boundary


31

4. Experimental procedure

4.1. Coating materials

The Ni-based powders employed in this work are summarized in Table 4-1. These alloys were selected through a systematic study, starting from simple binary alloys to more complex ones containing both alumina and chromia formers. All these materials are promising candidates for high temperature corrosion protection especially in harsh boiler conditions.

Table 4-1: Chemical compositions (wt%) of powders used for coating deposition

Composition Powder cut size (μm)

Trade name Supplier

Ni5Al -45+5 281.003 H.C.Starck, Germany Ni21Cr -45+22 251.001 H.C.Starck, Germany Ni21Cr7AlY -45+22 413.001 H.C.Starck, Germany Ni21Cr9Mo -45+15 380.074 H.C.Starck, Germany Ni21Cr9Mo-SiO2 -45+22 CorEr (proprietary) M.H.Engineering, Sweden

4.2. Substrate material

The substrate specimens used were round coupons (diameter= 16 mm and thickness= 5 mm) cut from a commercially available low carbon steel 16Mo3 material (nominal composition in wt%; 0.01Cr-0.3Mo-0.5 Mn0.3Si-0.15C- Fe bal.) procured in the form of a rod. The substrates were grit blasted with alumina particles (63±10 μm) in order to roughen and clean the surfaces before coating.

4.3. HVAF spraying

All specimens were sprayed with a M3TM-HVAF spraying system (Uniquecoat, Oilville, VA, USA). The 16Mo3 rod was fixed in a horizontal rotating mandrel to first coat on the cylindrical surface, see Figure 4-1. The rod was then sliced into buttons of 5 mm thickness (step A in Figure 4-1) and both flat surfaces of the buttons were firstly grit blasted (step B in Figure 4-1) and then HVAF sprayed (step C in Figure 4-1) to ensure that the specimens were coated on all sides. The HVAF spray parameters given in Table 4-2 were chosen based on preliminary coating trials performed to obtain the least porous microstructure. The coatings

32

to achieve a surface with Ra roughness on both coatings.

Figure 4-1: Schematic of the procedure followed to produce the fully coated specimens. The 16Mo3 rod with the length of 500 mm and diameter of 16 mm

was firstly grit blasted and then coated using the HVAF process. Especial attention was paid to the corner of buttons in order to avoid any damage during

cutting, grit blasting and spraying.

Table 4-2: HVAF process parameters used to spray the coatings

Variables

Nozzle type* 3L2G

Air pressure, MPa 0.8

Fuel 1 pressure-Propane, MPa 0.7

Fuel 2 pressure-Propane, MPa 0.7

Carrier gas pressure-N2, MPa 0.4

Feed rate, g/min 150

Pass velocity, m/min 50

Pass spacing, mm/rev. 5

Spray distance, mm 300 * 4L2G was only used for the NiCrMo-SiO2 coating

As-received 16Mo3 rod

Grit-blasted 16Mo3 rod

Grit blasting

HVAF spraying

As-coated 16Mo3 rod

Cutting to small buttons

Grit blasting the top and bottom sides

Coating the top and bottom sides

16 mm

500

mm

5 m

mEnlarged cross-section of the buttons in

different steps

A B C

CBA

Grit blasted surface

Coating

Isothermal oxidation and high temperature corrosion exposures were performed at 600 °C for up to 168 h. The highest temperature in typical biomass and waste boilers on superheater tubes is reported as ~450 and 550 °C respectively, during the course of operation (>10,000 h) [136], [137]. The reason for choosing a higher test temperature (600 °C) was to accelerate the dominant corrosion mechanisms in a shorter period of exposure (168 h). Three distinct environments were selected for the corrosion studies, i.e., ambient air, H2O, and HCl-laden environments. The aim of selecting such corrosive environments with and without KCl(s) was to study the corrosion behavior of the coating in three distinct environments, corresponding to predominant gas phase and solid salt rich in Cl.

A KCl suspension was prepared with ethanol, and applied on the surface of the samples using a paintbrush to ensure a KCl deposit of ~0.1 mg/cm2. The samples were then placed in individual alumina crucibles. Just before the test, each sample and each crucible was individually weighed using a SartoriusTM balance (Cubis MSA3.6P0TRDM, Sartorius, Germany) with microgram resolution. The crucible and sample with KCl deposit were also weighed together to calculate the exact amount of deposit placed on the sample. After the test, the samples were removed from the furnace, weighed in their individual crucibles together with any oxide scale spalled from the exposed material. The individual samples were also weighed alone without the crucibles.

Isothermal oxidation tests were performed on the as-sprayed coatings and reference materials for exposure times of 1, 5, 12, 24, 48, 96, and 168 h using a box furnace (Nabertherm, Germany) in ambient environment. The as-received reference materials and as-suspension to a roughness of Ra exposure to unify the surface conditions (in particular, surface roughness) for all the samples. The polished coatings were only exposed to two time intervals of 96 and 168 h at 600 °C in order to study the effect of the coating’s surface roughness on the formation of the oxide scales. At the end of the test, the samples were cooled to room temperature in the furnace. Before exposure, the samples were ultrasonically degreased, rinsed in high-purity water followed by ethanol, and finally air-dried.

The corrosion tests were performed in 5 vol.% O2 + 500 vppm HCl + N2 at time steps of 24, 96, and 168 h. The set-up comprised a horizontal tube furnace with

a stainless steel vessel, with the inside of the chamber entirely lined with high purity alumina. A mass flow controller was used to introduce 35 cm3/min of gas composed of 5% O2-500 vppm HCl-N2 through the chamber during the test. The samples were under the flow of N2 throughout the exposure to avoid any corrosion during cooling.

The exposure was performed in a tube furnace fitted with a 44 mm diameter silica-glass tube. The experiments were carried out in a 5%O2+20%H2O+N2 environment, at ambient pressure and flow rate of 3 cm3/s. The water-vapor content was regulated by setting the dew point of the gas to 60.4 °C using a reflux condenser. The flow rate of the gas was measured by a BIOS DC2 Flow Calibrator. A flow of N2 over the specimens was maintained throughout the exposure to avoid any corrosion during cooling.

Many characterization techniques are available to investigate the as-sprayed and exposed thermal spray coatings [138]–[141]. However, only those which are within the scope of this research are presented in this chapter.

Prior to carrying out a detailed microstructure study, it is critical to prepare samples by proper metallographic techniques. Typically, sample preparation of as-sprayed coatings comprises cutting, mounting in a low viscosity epoxy resin, grinding and polishing the samples manually or using a semi-automated machine. The standard procedure is presented in detail elsewhere [142]. It should be noted that metallographic preparation significantly influences interpretation of coating quality. The heterogeneous nature of most coatings makes polishing them very challenging in order to avoid artefacts. Pull-out or fracturing of poorly adhered splat sections is a common source of error in porosity measurement.

To observe the surface morphology of the as-sprayed and exposed coatings, the specimens were placed onto carbon stubs and examined using a scanning electron microscope (SEM). To analyze the cross sections, the as-sprayed coatings were cut using a diamond tipped precision saw and then cold mounted in a low shrinkage resin to prevent spallation of the formed oxide scale. The mounted

-section and surface morphology of all as-sprayed and exposed coatings was characterized using a QUANTA-200 FEG (FEI, Oregon, USA) SEM equipped

EXPERIMENTAL PROCEDURE

35

with X-ray energy dispersive spectroscopy (EDS). The surface morphologies of the polished coatings were analyzed using a secondary electron (SE) detector, whereas the cross-sections of the exposed coatings were studied using backscattered electron (BSE) signals. An accelerating voltage of 20 kV in BSE and 10 kV in SE mode was used for the SEM analysis in order to improve the spatial resolution.

For a few specimens (those exposed to water vapor), a Leica EM TIC 3X Triple Ion-Beam Cutter (Leica Microsystems, Helsinki, Finland) equipped with an argon ion gun and operated at 6 kV for sputtering was used to prepare broad and smooth cross sections in the millimeter range. The board ion beam (BIB) cross sectioning is an advanced technique to produce accurate cross sections with limited artifacts and distortion through the corrosion product in order to study the fragile corrosion layers.

The phases present in the coatings before and after the oxidation tests were identified by an X-ray diffractometer (XRD) (D5000, Siemens, Germany), equipped for grazing incidence analysis with CuK

and 80°. Before XRD measurements, a calibration sample (LaB6) was used to avoid peak broadening due to instrumental parameters, e.g. collimator size, detector resolution and beam divergence. The surface roughness (Ra) of the coatings was measured using a stylus-based profilometer (Surftest 301, Mitutoyo, Japan). Three measurements were performed and the measured values were averaged. Hardness measurements were performed on the polished cross-sections of the coatings according to the ASTM E384 standard with a Vickers indenter (Shimadzu, HMV-2, Tokyo, Japan) using a load of 0.3 kg and dwell time of 15 s.

Image analysis method is widely used for assessment of coating porosity due to its accessibility, simplicity, and the ability to measure the porosity given by closed pores which most of the other techniques cannot perform [143], [144]. This method quantifies porosity by analysis of microstructure pictures of the coated sample obtained by an optical microscope (OM) or a scanning electron microscope (SEM). In this work, an image analyzer was used with the open access ImageJ software based on ASTM standard B276. The porosity was measured by converting the micrographs (taken from Olympus BX60M optical microscope) into binary images and quantifying the percentages based on grey scale contrast [18], as seen in Figure 4-2.

a) a typical SEM micrograph ofthe cross-section of the coating

b) a binary image

Stability diagrams of the coating elements were plotted using the HSC 6.0 Chemistry software [145] to correlate experimental observations and thermodynamic calculations at different O2 and Cl2 partial pressures at 600 °C.

37

5. Effect of coating microstructure,composition and architecture on the hightemperature corrosion behavior

As discussed in the previous chapters, the high temperature corrosion behavior of coatings is affected by (a) microstructure, (b) composition, and (c) architecture. In this study, the microstructure and corrosion performance of different thermal spray processes (HVAF, HVOF, and APS) were firstly evaluated to understand the link between process, coating microstructure, and corrosion behavior. In the second step, the applicability of the Ni-based coatings with varying scale-forming ability (chromia and alumina) for improved protection of boiler components in corrosive environments was investigated. Finally, the potential benefits of adding dispersed SiO2 in coatings on the high temperature corrosion behavior was studied.

5.1. Coating microstructure: role of pores and splat boundaries

Since a dense and homogenous structure is essential for thermal spray coatings to provide adequate protection against corrosion, particular attention needs to be paid to the selection of spraying technique and optimization of spraying process parameters. Dense and well-adherent coating microstructures required for corrosion protection can be developed by altering the spraying processes and spray parameters. The microstructure and properties of the coatings can then be analyzed to find interdependencies between process parameters, coating microstructure, and corrosion behavior. In the present work, the least and most porous microstructures were obtained using the HVAF and APS processes respectively (Figure 5-1, Figure 5-2, and Figure 5-3). The highest density of the HVAF coatings was attributed to the high velocity of in-flight particles impacting the substrate, see Paper A.

Coatings sprayed by APS showed a distinct lamellar structure with a high content of porosity. As the particle temperature increases during spraying (for instance, in APS), in flight particles become more ductile and more prone to oxidation. The splats are strongly deformed at higher particle temperature, which generally leads to less porous coating. Particle melting state and oxidation is affected by the dwell time, heat transfer, and interactions of the particles upstream with the flame and downstream with ambient air [146]. The oxidation levels and inter-splat bonding have an influence on coating performance [147]. As already mentioned, the

HVAF spray gun can generate very high velocities of jet stream, with particle velocities up to 1200 m/s, so high density can be achieved.

a

a

a

b

b

b

The coatings sprayed by HVAF were selected for high temperature corrosion tests in controlled laboratory conditions. The selection of candidate coatings for

high temperature corrosion testing was based on the least pore content obtained in the coatings. While the role of pores and splat boundaries on the corrosion behavior was studied using the Ni21Cr, and Ni5Al coatings (Paper A), other Ni-based compositions including Ni21Cr7AlY, Ni21Cr9Mo, and Ni21Cr9Mo-SiO2 were also added to the test matrix in order to investigate the effect of coating composition and architecture (Papers B to G). As shown in Figure 5-4 and given in Table 5-1, all coatings sprayed with HVAF had a low content of porosity.

No. Coatings Pore content (vol.%) 1 Ni5Al 0.2 ± 0.072 Ni21Cr 0.1 ± 0.023 Ni21Cr7AlY 0.4 ± 0.104 Ni21Cr9Mo 1.5 ± 0.305 Ni21Cr9Mo-SiO2 0.2 ± 0.06

The high temperature corrosion of four different coatings (Ni21Cr, Ni5Al, Ni21Cr9Mo-SiO2 and Ni21Cr7AlY) was studied in an ambient air environment (see Paper B). Figure 5-5 shows the weight changes of the oxidized coatings after 168 h of isothermal exposure at 600 °C.

While the weight gain of the exposed Ni21Cr, Ni5Al, and Ni21Cr7AlY coatings was almost similar in ambient air without KCl, the Ni21Cr9Mo-SiO2 coating showed the highest value. When KCl was introduced to the environment, the weight gain significantly increased in all coatings, highlighting the major contribution of KCl in hastening coating failures. While the corrosion performance of the chromia-forming Ni21Cr and Ni21Cr7AlY coatings was better than the Ni5Al coating in the absence of KCl, the results showed slightly better corrosion behavior of Ni5Al in the presence of KCl. Similar to the exposure without KCl, Ni21Cr9Mo-SiO2 exposed to KCl illustrated the highest weight change, indicating the worst corrosion performance.

It is pertinent to mention that the weight change for the coated samples especially those exposed to KCl can be sourced from different factors as follows;

a) Formation of oxides such as Cr2O3 or NiCr2O4 [148],

EFFECT OF COATING MICROSTRUCTURE, COMPOSITION AND ARCHITECTURE ON THE HIGH TEMPERATURE CORROSION BEHAVIOR

41

b) Formation of metallic chlorides, mainly NiCl2, MoCl3, CrCl3, or CrCl2 withinthe coating [41],

c) Evaporation of metallic chlorides, e.g., NiCl2, MoCl3, or/and CrCl2 which canpossibly form in such Cl-containing environment [149].

d) Spallation of corrosion products [150],

e) Deterioration of the substrate (16Mo3) by Cl and formation of new phases(either oxides or chlorides) at the coating/substrate interface [151], and

f) Sintering of KCl on the surface during the exposure [42],

while the problem with the spallation was coped using the crucibles to collect corrosion products in case of possible spallation, the total weight change reported for each time step in Figure 5-5 was a combination of the above sources. Once a weight gain was recorded, it implies that the factors leading to weight gain (such as formation of an oxide) outweigh the factors leading to weight loss (such as vaporization) and vice versa.

Figure 5-5: Weight change in HVAF-sprayed Ni21Cr, Ni5Al, Ni21Cr7AlY, and Ni21Cr9Mo-SiO2 coatings in ambient air after 168 h at 600 °C exposed with and

without KCl.

When a coating is exposed to a simple oxidizing environment such as ambient air used in this study, an oxide film of chromia or alumina (depending on coating

0.01

1.97

0.02

1.65

0.01

3.48

0.13

4.12

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Ambient air Ambient air with KCl

Weig

ht c

hang

e (m

g/cm

2 )

Ni21Cr Ni5Al Ni21Cr7AlY Ni21Cr9Mo-SiO2

0.014 0.023 0.008

0.128

0.0

0.1

0.2

0.3

Ambient air

composition) thickens on the coating surface and progressively isolates the coating from its environment. If the oxide remains crack-free and stable, the rate of oxidation reaction is often inversely proportional to the oxide thickness, known as “parabolic” rate law. The parabolic rate law arises when the rate of an oxidation reaction is limited by ionic diffusion through a thickening oxide film [162]. Although electrons must also move across the film to sustain the oxidation process, most metal oxides (with the notable exception of Al2O3) are good electronic conductors, leaving ionic diffusion through the semiconductor oxide “electrolyte” between the coating and its environment as the rate-limiting step.

Figure 5-6 shows a typical microstructure of the HVAF coatings exposed to the ambient air environment at 600 °C after 168 h. The figure confirms that the coating’s splat boundaries allow much more rapid diffusion of corrosive species than what can occur through the splats. For this reason, the initial oxidation at splat boundaries is typically higher than in the core of splats. The protectiveness of the oxide formed at the splat boundaries depends on the composition of the oxide and the extent to which it inhibits the easy inward diffusion of corrosive species down the splat boundaries.

When a coating with a complex chemical composition is exposed to an oxidizing environment, all the metal atoms on its surface tend to be oxidized. The XRD results showed the constituents of the oxide scales formed on different coatings exposed to ambient air (see Table 5-2). To the extent that these oxides form a barrier to further oxidation, subsequent reactions oxidize only the most reactive elements in the alloy, such as Cr and Al. The minor constituents in the surface

Thin surface oxide

Oxidation withinthe splat boundaries

20 m


43

oxide alter the corrosion rate by acting as dopants in the semiconductor oxide and changing the diffusion rates of anions and cations across this oxide [152].

After the exposure in ambient air without KCl, a layer of NiO with a small content of Al2O3 was detected on the Ni5Al coating, whereas NiO and Cr2O3 were the main oxidation product on the surface of the Ni21Cr and Ni21Cr9Mo-SiO2 coatings see Table 5-2). The mixed-oxide scale forming Ni21Cr7AlY coating showed the best oxidation resistance due to the formation of a thin and slow-growing Al2O3 scale along with Cr2O3. In general, the oxidation resistance of the coatings only based on the kinetics studies had the following ranking (in the descending order from the largest to the least damage): Ni21Cr7AlY Ni21Cr > Ni5Al > Ni21Cr9Mo-SiO2. As it was already mentioned, one should also consider the present phases on the oxidized coatings and integrity of the oxide scale along with the weight change results in order to shortlist the candidate coatings for such high temperature applications.

Table 5-2: Corrosion products formed on the surface of coatings exposed to ambient air for 168 h at 600 °C

Coatings Corrosion products

Without KCl With KCl Ni5Al NiO, Al2O3 NiO, Al2O3 Ni21Cr NiO, Cr2O3 NiO, NiCr2O4, Cr2O3 Ni21Cr7AlY NiO, Cr2O3, Al2O3, Ni(Al,Cr)2O4 NiO, Cr2O3, Al2O3, NiCr2O4, NiCl2 Ni21Cr9Mo-SiO2 NiO, Cr2O3, MoO3 NiO, Cr2O3, NiCr2O4, MoO3, K2CrO4

When KCl was introduced in the ambient air environment (Paper C), Ni5Al showed slightly lower weight change than Ni21Cr, see Figure 5-5. The corrosion resistance of the coatings only based on the kinetics had the following ranking (from the best to worst): Ni5Al >Ni21Cr> Ni21Cr7AlY> Ni21Cr9Mo-SiO2.

The chromia forming coatings were found to be prone to degradation by an “electrochemical” mechanism, in addition to the “chlorine-induced active corrosion” mechanism. This corrosion mechanisms will be further discussed in the next sections. The exposed coatings remained attached to the substrate after the test and slowed down the corrosive agents to access the coating/substrate interface.

The isothermal oxidation behavior of Ni21Cr, Ni5Al and Ni21Cr7AlY coating deposited by HVAF was investigated in 5% O2 + 20% H2O + N2 at 600 °C for 168 h (Paper D). As shown in Figure 5-7, alike the other two coatings, Ni5Al showed weight loss when KCl was added. All coatings succeeded in maintaining their integrity with the substrate during the exposure.

A thin Cr-rich oxide scale (mixed layer of NiCr2O4 and Cr2O3) formed on the Ni21Cr coating, whereas a thin scale of Al2O3 mixed with NiAl2O4 and NiCr2O4 formed on the Ni21Cr7AlY coating, see Table 5-3. While a sign of internal oxidation in the Ni21Cr7AlY coating was observed, no breakaway oxidation occurred in the three exposed coatings in the presence of water vapor without KCl, see Figure 5-8. The water vapor effect was insignificant in such a low pO2 environment (5 vol.%) due to the large Cr or Al reservoir, and the continued growth of Cr-rich oxide or Al2O3. Once KCl was introduced, formation of K2CrO4 was detected on both Ni21Cr and Ni21Cr7AlY coatings; however, formation of volatile species such as KCl(g) or NiCl2(g) formed on Ni5Al. As the microstructure of the Ni5Al coating was less affected by KCl compared to the other two coatings where K2CrO4 formed, formation of KCl(g) could be assumed as the main reason for the weight loss. This assumption needs to be further investigated and confirmed. The splat boundaries were found to be the main diffusion paths for the corrosive species, see Figure 5-8.

0.16

2.42

0.11-0.290.10

0.38

-0.5

0

0.5

1

1.5

2

2.5

3

Weig

ht c

hang

e (m

g/cm

2 )

Ni21Cr Ni5Al Ni21Cr7AlY

5% O2 + 20% H2O + N2 5% O2 + 20% H2O + N2 +KCl


Without KCl With KCl Ni5Al NiO, Al2O3, NiAl2O4 Al2O3, NiAl2O4 Ni21Cr Cr2O3, NiCr2O4 CrCl3, NiCr2O4, K2CrO4 Ni21Cr7AlY Al2O3, Ni(Cr,Al)2O4 Ni(Cr,Al)2O4, K2CrO4

Cl-induced high temperature corrosion of the four HVAF-sprayed Ni21Cr, Ni5Al, Ni21Cr7AlY and Ni21Cr9Mo-SiO2 coatings was investigated in 5 vol.% O2 + 500 vppm HCl + N2 with and without KCl at 600 °C up to 168 h (Papers E and F). As shown in Figure 5-9 and Figure 5-10, all coatings performed well in the absence of KCl in controlling the corrosion of the substrate and acting as a barrier against the corrosive chlorine-containing environment. The oxidation behavior was strongly dependent on the protective scale-forming elements such as Cr or/and Al in the coatings, which developed a protective Al2O3 or/and Cr2O3 scale (respectively) at the test temperature, see Table 5-4.

0.5

11.32

0.45

2.49

0.371.49

0.31 0.67

0

2

4

6

8

10

12

14

Weig

ht c

hang

e (m

g/cm

2 )

Ni21Cr Ni5Al Ni21Cr7AlY Ni21Cr9Mo-SiO2

5% O2 + 500 ppm HCl + N2 5% O2 + 500 ppm HCl + N2+KCl


Without KCl With KCl Ni5Al Al2O3 Al2O3, NiAl2O4 Ni21Cr NiO, Cr2O3, NiCr2O4 Cr2O3, NiCr2O4, K2CrO4 Ni21Cr7AlY Cr2O3 CrCl3, K2CrO4, NiCr2O4 Ni21Cr9Mo-SiO2 MoO3, NiCr2O4 MoO3, NiCr2O4, SiO2

It was proposed that under KCl, the chromia-forming coatings degraded through a two-stage mechanism, Figure 5-11. In the first stage, the corrosion was initiated by the formation of K2CrO4 and Cl- in a reaction between KCl and the protective Cr2O3 formed according to Eq. 1. Cl could form as suggested in Eqs 2 and 3. K2CrO4 depleted the oxide scale in Cr leading to a loss of protective properties of chromia. The metallic chlorides and Cl2 formed in this stage. In the second stage, the formed Cl2 diffused inward through the defects (cracks and pores) of the non-protective oxide scales formed in the previous stage. Cl- and Cl2 diffused through the oxide grain boundaries and oxide’s defects respectively to reach the coating/oxide interface.

Cl-

K2CrO4

CrCl3(s)

KCl

Cr3+

KCl

Stage 1: Early stages of corrosion exposure

Cr

K2CrO4

Sintered KCl

Cr3+

Stage 2: Last stages of corrosion exposure

Cl-

CrCl3(s)

CrCl3(g)

Cl2

Cr

CrCl3 (s)

Cr3+

Cl2

Cl-

CrCl3 (g)

Cr

Coating’s splat boundaries

K2CrO4

Porous chromia scale

Dense chromia with grain boundaries

Inner oxide

Gap

Cl2

Cl-

Cracks and pores

As-sprayed splats (rich in Cr)

Cr-depleted region


49

Degradation of the protective oxide layer attributed to formation of K2CrO4 at the beginning of exposure was observed by Israelsson et al. and Shu et al. [42], [153]. It was reported that instead of Cl2 (proposed in the “chlorine-induced active corrosion” mechanism), Cl- (formed as suggested in Eqs. 1 to 3) could penetrate the oxide scale through the grain boundaries, leading to failure (with time) of the protective oxide layer. As already mentioned, Cl2 penetration through the scale is not explained by “chlorine-induced active corrosion” mechanism. Moreover, the reaction proposed in the “chlorine-induced active corrosion” mechanism to produce Cl2 could thus be proposed that Cl- diffuses into the oxide scale and coating through the grain boundaries and splat boundaries, respectively where the oxide is available. As Cl- is smaller than Cl2, it has higher mobility in the grain boundary region of an oxide. Once Cl- reaches the scale/coating interface, it reacts with transition metal ions formed by the oxidation of coating [29]. It should be noted that the scale/coating interface could be either on top of the coating or inside the coating in the splat boundaries where the oxide can form.

Scale/coating interface: = ( ) + 2 (1)

Scale surface: + 4 ( ) + ( ) + 2 = ( ) + 2 (2)

HCl dissociates at the scale/gas interface according to a cathodic process: + + 2 = 2 + (3)

The sum reaction becomes (for the case of Cr oxidation) as shown in Eqs. 4 and 5: + 2 ( ) + ( ) + = ( ) + ( ) (4)

( 2 4) 97.9 / 600 + 2 ( ) + ( ) + = ( ) + ( ) (5)

( 2 4) 292.6 / 600

The formed solid chlorides have considerable equilibrium vapor pressures, evaporate readily, and diffuse upward towards the gas–oxide scale interface.

50

When sufficient pO2 is available, the gaseous chlorides react with the available oxygen to form solid oxides, releasing gaseous chlorine, see Eqs. 6 and 7 [41], e.g.: 2 ( ) + 2 ( ) = ( ) + 2 ( ) (6) ( ) 255.6 / 600 ( ) + ( ) = ( ) + ( ) (7) ( ) 50.9 / 600

The oxide scales formed through these reactions are rather porous, non-protective, and non-adherent to the coating, based on the results obtained in this study, and in agreement with the literature [107]. Independently of Eqs. 1 to 5, regarding formation of Cl- via an electrochemical mechanism and the subsequent Cl- diffusion through the oxide grain boundaries, Cl2 formed according to Eqs. 6 and 7, could also diffuse through defects (mainly pores and cracks) in the new formed oxide scale towards the scale-coating interface and even further towards the coating-substrate interface (where pO2 was still low). Depending upon the available alloying elements in the coating which were Ni, and Cr in the present study, solid metal chlorides which are thermodynamically stable could form after reacting with Cl2 (where pO2 was high), see Eqs. 8 to 10. These reactions occur along with the reactions proposed before in Eqs. 1 to 5. Cr seems to be more preferably attacked than Ni, as the formation of CrCl2 or CrCl3 has more negative Gibbs free energy than the formation of NiCl2 [154], see Eqs 8 to 10. ( ) + ( ) = ( ) ( ) 173.9 / 600 (8) ( ) + ( ) = ( ) ( ) 287.0 / 600 (9) ( ) + ( ) = ( ) ( ) 185.8 / 600 (10)

It can be hypothesized that while the Ni21Cr coating was initially degraded via the electrochemical mechanism in the first stage, the “chlorine-induced active corrosion” mechanism along with the electrochemical mechanism contributed in the second stage of the coating’s failure.

The alumina scale formed on the Ni5Al coating was rather protective under KCl, see Figure 5-10. The protective alumina impeded the diffusion of Cl- in the first stage. The corrosion problems might only arise if the oxide scale spalls-off or cracks forming wide openings for diffusion of Cl- to be present at the coating surface or oxide/coating interface and the subsequent Cl-/Cl2 diffusion.


51

The interconnected porosity and splat boundaries could act as paths for the Cl-/Cl2 diffusion as long as the oxide was present. Cl- could diffuse through the splat boundaries of the coatings forming metal chlorides and accelerating corrosion indicating that such regions were microstructural weak points to control chlorine-induced corrosion.

5.3. Coating architecture: role of dispersed SiO2

Coatings of both Ni21Cr9Mo with and without dispersed SiO2 were deposited in similar spraying conditions (HVAF) and investigated. As shown in Figure 5-12, in the absence of KCl, the SiO2-containing coating illustrated a lower weight change than the SiO2-free coating, which was most probably due to formation of a more protective and adherent Cr-rich oxide scale (see Paper G). Once KCl was introduced, the formed corrosion product on the SiO2-free coating was highly porous, non-adherent, and thick, see Figure 5-13. SiO2 decelerated the short-circuit diffusion of Cr3+ through scale’s defects, e.g., vacancies, and promoted a selective oxidation of Cr to form the protective Cr-rich oxide scale. Furthermore, the presence of SiO2 led to less subsurface depletion of Cr in the coating, and accordingly less corrosion of the substrate.

Experimental results also showed that the effect of SiO2 was more significant once KCl was introduced. The difference between the weight change of Ni21Cr9Mo-SiO2 and Ni21Cr9Mo was much higher with KCl. In the absence of the SiO2 dispersoids, formation of NiCl2 was promoted, see Table 5-5. The adhesion of the scale to the coating was greatly improved as no gap was observed at the interface. It was shown that the uniform supply of Cr to the formed oxide scale promoted the formation of a more continuous and denser Cr-rich scale that exhibited low growth kinetics and less spallation. Therefore, formation of undesirable K2CrO4, which depletes Cr2O3 in Cr was interrupted as KCl was locally consumed during the course of exposure. Moreover, fast formation of an external Cr-rich scale as a result of the presence of homogenous SiO2 resulted in notably enhanced corrosion resistance of the coating. The non-adherent scale on the SiO2-free coating underwent fast grain growth.

0.35

1.2

0.31

0.67

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6W

eight

cha

nge

(mg/

cm2 )

Ni21Cr9Mo Ni21Cr9Mo-SiO2

5% O2 + 500 ppm HCl + N2 5% O2 + 500 ppm HCl + N2+KCl


53

Table 5-5: Corrosion products formed on the surface of the SiO2-containing and SiO2-free coatings exposed to HCl-laden environment for 168 h at 600 °C


Without KCl With KCl Ni21Cr9Mo MoO3, NiCr2O4 NiCl2, NiCr2O4 Ni21Cr9Mo-SiO2 MoO3, NiCr2O4 NiCr2O4, SiO2

By adding SiO2 dispersoids to the NiCrMo coating, Cr3+ was continuously supplied to the formed protective Cr-rich oxide scale in step 1, Figure 5-14. Therefore, formation of Cl- and K2CrO4, which depletes the oxide scale in Cr, was slowed down (see Eq. 2 and stage 2 in Figure 5-11). Consequently, less volatile metallic chlorides and accordingly voids formed in the corrosion products on the coating. Several explanations have been proposed for the lower oxidation rate of the oxide-dispersed alloys [110], [115], [155], [156]. Stringer et al. [117] suggested that the dispersoids present on the surface act as oxide nucleation sites, and led to a faster formation of a continuous Cr2O3 scale. Giggins and Pettit [157], using a Pt marker, showed that the position of the markers depends on the relative diffusion rates of the components involved, i.e., outward diffusion of Cr3+ and inward diffusion of O2-. They suggested that the slow outward diffusion of Cr3+ might be caused by the blocking action of dispersoids in the Cr2O3 scale. However, all these studies were performed in ambient air (without KCl) and no focus was on the Cl- and K2CrO4 formation on the formed corrosion product. Based on the results obtained in the present study, while the SiO2-free NiCrMo coating was severely affected by the presence of KCl, the SiO2-containing NiCrMo coating showed improved corrosion resistance. There is a beneficial influence of the dispersoids on scale adhesion, in accordance with the findings in the literature [113]. This might be due to the thinner scale, which is more compliant with the coating.

(a)

Cr

O2-

(b)

O2-

Blocky (Cr,Ni)-rich oxideAs-sprayed splats (rich in Cr)

Dispersed SiO2 Dense Cr-rich oxide with grain boundaries

Cr3+

O2-

Cr

Cr-depleted region

Cr3+

O2 O2O2 O2

55

6. Summary of appended publications

Paper A- A comparative study on Ni-based coatings prepared by HVAF, HVOF, and APS methods for corrosion protection applications

Author contribution: As the main author, Esmaeil Sadeghimeresht has performed all the experimental characterization, analyzed all the results, designed the structure of the article, and had the main responsibility in writing it. The co-authors contributed in formulating concepts and ideas, planning the project, spraying and article editing. Connection to the research questions: The paper aimed to find some answers on how a Ni-based coating with varied microstructures obtained by using different spraying processes enhance the corrosion resistance, which relates to RQ1. Summary: The first paper was an initiative for the present thesis work in which formerly optimized HVOF- and plasma-sprayed Ni, Ni21Cr and Ni5Al coatings were compared with HVAF-sprayed coatings in terms of microstructure and corrosion resistance. The microstructure of the coatings was optimized by altering the HVAF spraying process parameters to produce dense and well-adherent coatings for corrosion protection. The microstructure and properties of the coating were analyzed to understand the link between process parameters, coating microstructure, and corrosion behavior.

Paper B- Isothermal oxidation of HVAF-sprayed Ni-based chromia, alumina and mixed-oxide scale forming coatings in ambient air

Author contribution: As the main author, Esmaeil Sadeghimeresht has performed all the experimental characterization, analyzed all the results, designed the structure of the article, and had the main responsibility in writing it. The co-authors contributed in formulating concepts and ideas, planning the project, spraying and article editing. Connection to the research questions: The paper aimed to investigate the applicability of the HVAF coatings of Ni-based materials with varying scale-forming ability for improved protection of boiler components in corrosive environments. This work relates to RQ2. Summary: The isothermal oxidation behavior of the Ni21Cr, Ni21Cr7AlY, Ni21Cr9Mo-SiO2 and Ni5Al coatings was studied in ambient air environment at 600 °C for different time intervals i.e. 1, 5, 10, 24, 48, 96, and 168 h. The oxidation

56

behavior of the as-sprayed and polished coatings was compared. The protective -Al2O3 was not detected on the exposed alumina-forming Ni5Al coating. On the

other hand, Cr2O3 along with a small amount of NiO were the main oxidation products on the surface of the Ni21Cr and Ni21Cr9Mo-SiO2 coatings, and were found to be relatively less protective. The mixed-oxide scale forming Ni21Cr7AlY coatings showed the best oxidation resistance due to the formation of a thin and slow-growing Al2O3 scale along with Ni(Al,Cr)2O4 and Cr2O3. The four coatings were selected to be tested in a more aggressive environment in the next study.

Paper C- KCl-induced high temperature corrosion behavior of HVAF-sprayed Ni-based coatings in ambient air

Author contribution: As the corresponding author along with the other co-authors, Esmaeil Sadeghimeresht has contributed in formulating concepts and ideas, planning the project, spraying and article editing. The first author performed all the experimental characterization, analyzed all the results, designed the structure of the article, and had the main responsibility in writing it. Connection to the research questions: The paper was a continuation on the previous paper with addition of KCl. The paper aimed to find some answers on how the coatings with varied protective scale-forming abilities could enhance the corrosion resistance in ambient air + KCl environment, which relates to RQ2. Summary: The results showed that the small addition of KCl significantly accelerated degradation to the four coatings. All coatings provided better corrosion resistance compared to the reference materials. The alumina-forming Ni5Al coating under KCl deposit was slightly better in forming a protective oxide scale compared to the chromia-forming coatings as penetration of Cl through diffusion paths was hindered. Both “chlorine-active corrosion” and “electrochemical” mechanisms were found to be responsible for the corrosion damages. The corrosion resistance of the coatings based on the microstructure analysis and kinetics had the following ranking (from the best to worst): Ni5Al >Ni21Cr> Ni21Cr7AlY> Ni21Cr9Mo-SiO2.

Paper D- Effect of water vapor on the oxidation behavior of HVAF-sprayed NiCr and NiCrAlY coatings

Author contribution: As the main author, Esmaeil Sadeghimeresht has performed all the experimental characterization, analyzed all the results, designed the structure of the article, and had the main responsibility in writing it. The co-authors contributed in formulating concepts and ideas, planning the project, spraying and article editing. Connection to the research questions: The paper aimed to find some answers on how a chromia and an alumina-forming coating could enhance the corrosion resistance of a coating in water vapor laden environment, which relates to RQ2.

SUMMARY OF APPENDED PUBLICATIONS

57

Summary: Whereas Ni21Cr7AlY showed lower weight gain compared to Ni21Cr, both coatings succeed in maintaining the integrity with the substrate during the exposure without any breakaway oxidation. A thin Cr2O3 scale mixed with NiCr2O4 formed on Ni21Cr, and a thin Al2O3 scale mixed with NiAl2O4 formed on Ni21Cr7AlY significantly increased the oxidation protection in the presence of water vapor. The splat boundaries was found as the main diffusion paths for the corrosive species.

Paper E- Chlorine-induced high temperature corrosion of HVAF-sprayed Ni-based alumina and chromia forming coatings

Author contribution: As the main author, Esmaeil Sadeghimeresht has performed all the experimental characterization, analyzed all the results, designed the structure of the article, and had the main responsibility in writing it. The co-authors contributed in formulating concepts and ideas, planning the project, spraying and article editing. Connection to the research questions: The paper aimed to investigate the ability of the HVAF coatings prepared from Ni-based materials with varying scale-forming ability to protect boiler components in chlorine-laden environments. A key objective of the study was also to understand the mechanism associated with Cl interacting at high temperature with two Ni-based chromia and alumina forming coatings - Ni21Cr and Ni5Al, which relates to RQ2. Summary: Chlorine-induced corrosion of HVAF-sprayed Ni21Cr and Ni5Al coatings was investigated in 5 vol.% O2 + 500 vppm HCl + N2 with and without KCl at 600 °C up to 168 h. Both coatings were protective in the absence of KCl. Under KCl, Ni21Cr degraded through a two-stage mechanism; 1) formation of K2CrO4 followed by diffusion of Cl- through oxide grain boundaries, formation of chlorine and a non-protective oxide, and 2) inward diffusion of the chlorine though defects of the non-protective oxide, which leads to breakaway oxidation. Formation of a protective alumina scale on Ni5Al impeded the first stage, hence increased the corrosion protection.

Paper F- Influence of KCl and HCl on high temperature corrosion of HVAF-sprayed NiCrAlY and NiCrMo coatings

Author contribution: As the main author, Esmaeil Sadeghimeresht has performed all the experimental characterization, analyzed all the results, designed the structure of the article, and had the main responsibility in writing it. The co-authors contributed in formulating concepts and ideas, planning the project, spraying and article editing.

58

Connection to the research questions: The present work was an extension of a previous study in which a chromia- and an alumina-forming coating were investigated in 5%O2 + 500 vppm HCl + N2 (bal.) The chromia-forming Ni21Cr7AlY and Ni21Cr9Mo-SiO2 coatings were selected in this study as formation of protective and continuous chromia were more favored rather than

-Al2O3 in such corrosive condition. Addition of alloying elements was already shown to improve the corrosion performance of the chromia-forming coatings. So, this works aims to evaluate the role of alloying elements such as Mo and Al on corrosion performance of the chromia-forming Ni21Cr coatings in the similar corrosive environment with and without KCl at 600 °C for 168 h. Special attention was paid to the effect of the alloying element and coatings’ microstructure on the high temperature chlorine-induced corrosion resistance of the coatings. This work relates to RQ2.

Summary: Oxidation of Ni21Cr7AlY and Ni21Cr9Mo-SiO2 coatings thermally sprayed by high velocity air-fuel (HVAF) technique has been investigated in a chloridizing-oxidizing environment at 600 °C for up to 168 h with and without KCl deposit. Both coatings protected the substrate in the absence of KCl due to formation of a dense Cr-rich oxide scale. With KCl, Cl-/Cl2 diffused through a non-protective and porous NiCr2O4 scale formed on Ni21Cr7AlY, leading to formation of volatile CrCl3. Addition of Mo in Ni21Cr9Mo-SiO2 stimulated the formation of a more protective Cr-rich oxide scale which increased the corrosion resistance by reducing the Cl-/Cl2 diffusion, and accordingly lowering vaporization of CrCl3.

Paper G- Effect of SiO2 dispersion on high temperature chlorine corrosion of HVAF-sprayed NiCrMo coating

Author contribution: As the main author, Esmaeil Sadeghimeresht has performed all the experimental characterization, analyzed all the results, designed the structure of the article, and had the main responsibility in writing it. The co-authors contributed in formulating concepts and ideas, planning the project, spraying and article editing. Connection to the research questions: The paper aimed to provide a better understanding of the potential consequences of the SiO2 dispersion whether it can improve the high temperature corrosion behavior of the coatings in a chlorine-laden environment. The different transport phenomena in the oxide scale and the differences in scale morphology were emphasized, and possible implications for scale growth were discussed. This work related to RQ3.

SUMMARY OF APPENDED PUBLICATIONS

59

Summary: Ni21Cr9Mo coatings with and without dispersed SiO2 were deposited by high velocity air-fuel (HVAF) technique. Thermogravimetric experiments conducted in 5% O2 + 500 vppm HCl + N2 at 600 °C with KCl deposit for up to 168 h showed lower weight change of SiO2-containing coating due to formation of a protective and adherent Cr-rich oxide scale. The formed oxide scale on SiO2-free coating was highly porous, and thick in the presence of KCl. SiO2 retarded short-circuit diffusion of Cr3+ through the scale and served as nucleation sites for formation of a continuous Cr-rich oxide scale. Furthermore, the presence of SiO2 led to less subsurface depletion of Cr in the coating, accordingly less corrosion of the substrate.

61

7. Conclusions

This study reported the high temperature corrosion performance of the HVAF-sprayed Ni-based coatings in simulated biomass power plant conditions. The coatings can be recommended for corrosion protection and, hence, for increasing the lifetime of carbon steel or low-alloy steel substrates in biomass-fired boilers where chlorine and low melting compounds can cause severe corrosion. The key challenges are thus correct material selection for specific conditions and application of the coatings at large scale with high coating quality. On the basis of the results presented in this thesis and attached publications, the following specific conclusions on the corrosion performance of the coatings can be drawn:

1) Dense Ni-based coatings with low oxygen content can be produced with thehigh velocity air-fuel (HVAF) process. Splat cohesion property is important inhigh temperature corrosion environments and hence sufficient melting state andoxidation level of powder particles together with high kinetic energy are requiredfor formation of a dense coating microstructure. Such dense microstructure isessential to the corrosion resistance of thermal spray coatings in order to preventthe transport of corrosive elements towards the substrate. Structure and qualityof the coating, such as adhesion, cohesion, and porosity can be influenced byoptimization of thermal spraying process parameters.

2) Once coatings were exposed to ambient air, all the coatings prepared by HVAFwere found to be successful in maintaining its integrity with its respectivesubstrates throughout the exposure to the environment of the study, without anysignificant spallation of its oxide scales. The oxidation behavior was stronglydependent on the amount of passive-forming elements such as Cr or/and Al inthe coating, which can develop a protective alumina or chromia scale at the testtemperature. In ambient air environment, the oxidation resistance of the coatingsbased on the kinetic studies had the following ranking (in the descending orderfrom the largest to the least damage): Ni21Cr7AlY Ni21Cr>Ni5Al>Ni21Cr9Mo-SiO2.

3) Once KCl was added to ambient air environment, the properties of a scalechange, allowing for inward chlorine transport. In all coatings, the oxide scalescovered the sample surfaces, which had different microstructure and density afterthe exposure to KCl. Protective layers formed on the Ni5Al and Ni21Cr coatingsin which less alloying element depletion and weight gain observed in comparisonwith Ni21Cr9Mo and Ni21Cr7AlY. The oxidation resistance of the coatingsbased on the kinetic studies had the following ranking (in the descending order

62

from the largest to the least damage): Ni5Al>Ni21Cr>Ni21Cr7AlY>Ni21Cr9Mo-SiO2.

4) In the presence of water vapor, a thin Cr-rich oxide scale (mixed layer ofNiCr2O4 and Cr2O3) formed on the Ni21Cr coating, a thin Al-rich layer (Al2O3,and NiAl2O4) formed on Ni5Al and a thin Al2O3 layer mixed with NiAl2O4 andNiCr2O4 formed on the Ni21Cr7AlY coating. While there was a sign of internaloxidation in the Ni21Cr7AlY coating, no breakaway oxidation occurred in thethree exposed coatings in the presence of water vapor. The water vapor effectwas insignificant in such a rather low pO2 environment (5 vol.%) due to the largeCr or Al reservoir, and the continued growth of Cr-rich oxide or Al2O3. All thecoatings found to be successful in maintaining the integrity with their respectivesubstrates throughout the exposure to the environment of the study, without anysignificant spalling of the oxide scales. Once KCl was introduced, K2CrO4 formedon the surface of the chromia-forming coatings (Ni21Cr and Ni21Cr7AlY),whereas the microstructure of Ni5Al was almost unaffected.

5) In the presence of HCl, all coatings performed well in the absence of KCl incontrolling the corrosion of the substrate and acting as a barrier against thecorrosive chlorine-containing environment. The oxidation behavior was stronglydependent on the protective scale-forming elements such as Cr or/and Al in thecoatings, which developed a protective Al2O3 or Cr2O3 scale at the testtemperature. A two-stage mechanism was proposed for degradation of theNi21Cr coating. In the first stages, the corrosion was initiated by the formationof K2CrO4 and Cl- in a reaction between KCl and protective Cr2O3. K2CrO4

depleted the oxide scale in Cr leading to a loss of the protective properties of thechromia. The metallic chlorides and Cl2 formed in this stage. In the second stage,the formed Cl2 diffused inward through the defects (cracks and pores) of the non-protectives oxide scale formed in the previous stage. Cl- and Cl2 diffused throughthe oxide grain boundaries and oxide’s defects respectively to reach thecoating/oxide interface. The alumina scale formed on the Ni5Al coating wasrather protective under KCl. The protective alumina impeded the diffusion of Cl- in the first stage. The corrosion problems might only arise if the oxide scale spalls-off or cracks forming wide openings for diffusion of Cl- to be present at thecoating surface or oxide/coating interface and the subsequent Cl-/Cl2 diffusion.The interconnected porosity and splat boundaries could act as paths for the Cl-/Cl2 diffusion as long as the oxide was present. Cl- could diffuse through the splatboundaries of the coatings forming metal chlorides and accelerating corrosionindicating that such regions were microstructural weak points to control chlorine-induced corrosion. In this environment both with and without KCl, the coatingswere similarly ranked from the highest to lowest corrosion resistance as;Ni21Cr9Mo-SiO2>Ni21Cr7AlY>Ni5Al>Ni21Cr. The coatings demonstrated

63

low corrosion attack in HCl alone, but they were subjected to an accelerated corrosion in the presence of KCl.

6) Experimental results showed that addition of SiO2 into the Ni21Cr9Mo coatingincreased the high-temperature corrosion resistance by gaining low weight changeand low scale spallation. The selective oxidation of Cr to form a continuous Cr2O3

scale was promoted. The adhesion of scale–coating was greatly improved. Thegrowth rate of Cr2O3 was reduced. The establishment of an inner SiO2 layerpromoted the formation of a thin, flat, and dense Cr2O3 scale that exhibitedreduced growth kinetics and less spallation. The adherent scales on the SiO2-containing coating had a stable, and dense structure, whereas the non-adherentscale on the SiO2-free coating underwent fast grain growth. The oxide formed onNi21Cr9Mo-SiO2 tended to maintain stable. Therefore, the presence of thedispersed oxide could notably enhance the corrosion resistance.

65

8. Future works

The present study mainly aimed to understand the effect of the coating microstructure, composition and architecture on high temperature corrosion behavior. Still, there is much work to be performed, both on the fundamental level in order to follow and understand specific reactions and mechanisms, and in the more complex atmosphere of full-scale boiler where economic and environmental aspects call for increased alloy performance.

1) Variations in test environment conditions

The present corrosion studies were mainly performed in ambient air, HCl- and H2O-laden environments at 600 °C for up to 168 h. In future, an extensive corrosion testing matrix should be included in order to establish clear relations between process parameters, microstructure, properties and corrosion performance. By this, the corrosion performance of a coating used in different types of boiler or deposited on different parts in a boiler could be addresses. Laboratory corrosion tests at varied environments (reducing and sulfidizing depending on application), different temperatures, and longer time could be highly beneficial. Long-term exposures in full-scale boiler under various conditions is recommended in order to evaluate the reliability of such coatings.

Since the KCl deposit has proven very detrimental from a corrosion point of view, laboratory studies investigating alloy performance in the presence of molten and solid chlorides are also highly interesting. The work should focus especially on corrosion attack in gas atmospheres similar to the full-scale boiler, since the reactions taking place on superheater tubes may unintentionally be accelerated or retarded in the laboratory if certain components are excluded from the test atmosphere.

2) Effect of post treatment on corrosion behavior

Post-spray treatments are recommended to be investigated for different purposes. For instance, mechanical treatment like shot peening can be used for splat-size control in order to unify the diffusion of protective scale-forming elements towards the surface. Heat treatment can be implemented in order to pre-oxidize

-Al2O3 on the surface.

66

3) Advanced characterization techniques such as X-ray photoelectronspectroscopy (XPS) and facilities such as Broad Ion Beam (BIB) milling could beimplemented.

The advanced characterization techniques will help to better understand the formation of different complex corrosion products during corrosion processes and the interaction between chlorine and the various alloying elements and how this affects the scale growth. In the thermal spray coatings, the influence of passivation elements such as Cr can be studied by XPS as this technique is a highly surface sensitive research tool in corrosion science. It provides unambiguous chemistry and detailed mechanism of the corrosion process that takes place on the surface. HCl-induced corrosion could be analyzed in more detail with some advanced techniques such as high temperature mass spectroscopy to analyze the different volatile species and quantify their impact on the corrosion attack. The board ion beam (BIB) cross sectioning is also an advanced technique to produce accurate cross sections with limited artifacts and distortion through the oxidation product. The results obtained from the cross sectional images of the oxidized coatings could be more valuable than manual cutting/polishing.

4) Oxide dispersion-strengthened (ODS) coatings

While only SiO2 was used in the present work as ODS, addition of other dispersoids such as Al2O3, Y2O3, CeO2, and Cr2O3, etc. alloyed to the feedstock powder could be highly beneficial for high temperature corrosion protection.

67

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Ni-Based Coatings for High Temperature Corrosion ProtectionCritical load-bearing components in biomass-/waste-fired boilers, e.g., water-wall and superheater tubes are severely deteriorated due to Cl-induced high temperature corrosion. Deposition of a dense and adherent Ni-based coating containing protective scale-form-ing elements such as Al or/and Cr using high velocity air-fuel (HVAF) thermal spray tech-nique is a promising approach to extend the component’s lifetime, hence increase the thermal/electrical efficiency of the boilers.

While the HVAF-sprayed coatings exhibit unique microstructural characteristics, there are a few technical challenges in producing extremely high-temperature corrosion resis-tant coatings including the emergence of pores and splat boundaries. Complex interac-tion of in-flight particles with supersonic flame in the HVAF process, and formation of a complex deposit structure comprising of pores, as well as splat boundaries encumber understanding the inter-relation of process, microstructure, properties, and corrosion per-formance. An additional challenge is development of an in-depth understanding on high temperature corrosion mechanisms of coatings due to their intricate microstructures. This dissertation aims to establish a correlation between the feedstock composition, mi-crostructure, and high temperature corrosion performance of coatings.

Esmaeil SadeghimereshtEsmaeil received his Bachelor’s degree in Material Science & Engineering from Amirkabir University of Technology - Tehran Polytechnic, Iran in 2008. He did his Master’s program in Corrosion Science & Engineering at Tarbiat Modares University, Iran in 2010. His research interests stand broadly in the area of corrosion, coating and additive manufacturing.

ISBN 978-91-87531-70-5




NI-BASED COATIN

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Ni-Based Coatings for High Temperature Corrosion Protection

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