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Bioaccessibility of Stainless Steels

– Importance of Bulk and Surface Features

Gunilla Herting

Doctoral Thesis

Division of Corrosion Science

School of Chemical Science and Engineering

Royal Institute of Technology

Stockholm, Sweden, 2008

This doctoral thesis will, with the permission of Kungliga Tekniska Högskolan, Stockholm, be presented and

defended at a public dissertation on Tuesday June 3, 2008, at 10.00 in room F3, Lindstedtsvägen 26,

Kungliga Tekniska Högskolan, Stockholm, Sweden.

Faculty Opponent: Professor Stuart Lyon, Ph. D. School of Materials,

The University of Manchester, UK.

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Contact information KTH Chemical Science and Engineering Division of Corrosion Science Royal Institute of Technology Drottning Kristinas väg 51 SE-100 44 Stockholm Sweden Copyright © Gunilla Herting All rights reserved Paper I © Journal of Electrochemical Society Paper II © Elsevier Ltd. Paper II © Elsevier Ltd. Paper IV © Elsevier Ltd. Abstract of this thesis is available in electronic version at: http://media.lib.kth.se TRITA-CHE Report 2008:39 ISSN 1654-1081 ISBN 978-91-7178-977-8 www.kth.se

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To my parents and absent friends

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Abstract With increasing environmental awareness, the desire to protect human beings and the

environment from adverse effects induced by dispersed metals has become an issue of great

concern and interest. New policies, such as REACH (Registration, Evaluation, Authorisation

and Restriction of Chemicals) within the European Community, have been implemented to

reduce hazards posed by the use of chemicals on producers and downstream users. The

generation of exposure assessment data and relevant test procedures able to simulate realistic

scenarios are essential in such legislative actions.

This doctoral study was initiated to fill knowledge gaps related to the metal release process of

stainless steels. A wide range of stainless steel grades, fourteen in total, were investigated.

They cover a very broad range of applications, and the focus in the thesis was to simulate a

few selected exposure scenarios: precipitation, the human body and food intake. Comparisons

were made between metal release from stainless steel alloys and the pure metals that

constitute each stainless steel in order to explore the differences between alloys and pure

metals, and to provide quantitative data on metal release rates of different alloy constituents.

Because of similar surface properties between stainless steel and pure chromium, this metal

exhibits similar release rates, whereas iron and nickel exhibit significantly lower release rates

as alloy components than as pure metals. Detailed studies were also performed to elucidate

possible relations between metal release and steel surface properties. Key parameters turned

out to be chromium enrichment of the self-passivating surface film, surface roughness, the

electrochemically active surface area and the microstructure of the steel substrate. The degree

of metal release increased with decreasing chromium content in the surface oxide, increasing

surface roughness, and increasing presence of inhomogeneities in the bulk matrix.

More detailed studies were initiated to possibly correlate the nucleation of metastable pits and

the extent of metal release. Evidence was given that metastable pits exist even when the

stainless steel is passive, and may cause extremely short-lived bursts of released metal before

the surface film repassivates again.

Keywords: Stainless steel, iron, chromium, nickel, corrosion, metal release, artificial rain, synthetic

body fluids, acetic acid, surface oxide, surface finish

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What has the teabag in common with stainless steel?

Corrosion is a process during which a material, usually a metal such as iron or copper, slowly is released whereby part of the dissolved metal can form corrosion products on the surface. Rust forms on iron or steel, and patina on the copper surface. This process can either be beneficial for the metal (if the corrosion product protects from further corrosion), or harmful (if local pits penetrate the surface and the material properties change). Stainless steel is an example of a so called passivating alloy, on which an extremely thin and unvisible oxide film is formed which strongly hinders the corrosion process.

A consequence of corrosion may be a slow dissolution of the corrosion product or the oxide film, during which the metal dissolves and is transferred from the corroded surface into the surroundings. Metal release is a concept usually not familiar to people outside the scientific community. It happens during exposure to aqueous environments such as rain water, drinking water, sea water or body fluid.

To imagine the concept of metal release from stainless steel, think of a tea bag in hot water. The change in colour you observe when tea goes from the teabag into hot water corresponds to the metal released from the steel in contact with the water solution. The protective and very thin oxide film on the stainless steel can be compared with the tea bag which prevents the major portion of tealeaves from dispersing into the hot water.

The content of this thesis illustrates what happens when a stainless steel surface is put in contact with various types of fluids simulating different environments in which it can be exposed. Stainless steels are frequently used in outdoor applications such as buildings or bridges exposed to rain events, cocking pans and cutlery, and as implants in the human body. Due to its wide use in the society, it is important to know how much metal can be released from stainless steels into different environments to ensure a safe use from an environmental and health point of view.

Stainless steel was given the name “stainless” when it was found impossible to stain its surface with vinegar during tests in the early 20th century. From then on stainless steel became the most commonly used material for everyday cutlery. The use of stainless steel for food preparation brings questions about the potential risk of metals being released from the cocking pan and become integrated in the food we are preparing and eating. Results in this thesis illustrate that metals such as chromium, nickel and iron will be released from a stainless steel cocking pan or from cutlery, however in very low amounts. With repeated use the amount of metal will be even less, as long as the pan is not seriously damaged and starts to corrode.

The visual appearance is many times the determining factor for the surface finish of a given application of stainless steel. Do we want the surface to be shiny, dull, or have a rough look to it? A rough surface will be more vulnerable to corrosion than a smooth surface, and more metal will be released as the real surface area on a rough surface is larger compared with a smooth surface. There is, however, more to the surface finish than just visual considerations. The stainless steel structure and composition also determine how efficient the protective oxide film, the teabag, prevents the metal from being released into the surrounding environment.

There is definitively more to stainless steel than what meets the eye, and this doctoral thesis has tried to elucidate a few of the unresolved scientific issues.

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Vad har tepåsen och rostfritt stål gemensamt?

Korrosion är en process när ett material, vanligtvis en metall som järn eller koppar, i kontakt med omgivningen långsamt löses upp varvid en del av den lösta metallen kan bilda korrosionsprodukter på ytan. Rost uppstår när järn rostar och patina när koppar korroderar. Korrosionsprocessen kan antingen ha en gynnsam effekt på metallen (om korrosionsprodukten skyddar för fortsatt korrosion) eller vara skadlig (om korrosionseffekten förstör ytan och ändrar materialets egenskaper). Rostfritt stål är ett exempel på en s.k. passiverbar legering, där en extremt tunn och för ögat helt osynlig skyddande oxidfilm spontant bildas på ytan, som starkt hämmar den fortsatta korrosionen.

En följd av korrosionen är att den bildade korrosionsprodukten/oxidfilmen delvis kan lösas upp, varvid metall frigörs och förs bort från den korroderade ytan ut till den omgivande miljön. Processen är ett för allmänheten rätt okänt begrepp som kan ske vid kontakt med en mängd olika vattenlösningar, exempelvis regnvatten, dricksvatten, havsvatten eller kroppsvätska.

Frigörelsen av metall från ett rostfritt stål kan liknas vid en tepåse som drar i varmt vatten. Den allt kraftigare färgförändring som sker kring tepåsen motsvarar frigörelsen av metall från stålet i kontakt med vattenlösningen. En viktig skillnad är att den frigjorda metallen inte syns för blotta ögat. Den skyddande och mycket tunna oxidfilmen på rostfritt stål motsvarar själva påsen som hindrar tebladen från att spridas fritt i det varma vattnet.

Innehållet i den här avhandlingen beskriver vad som händer när en rostfri stålyta utsätts för vätskor som kan efterlikna olika typer av miljöer som det rostfria stålet kan utsättas för. Rostfritt stål används i många utomhusapplikationer såsom byggnader och broar, i kastruller och bestick, eller som implantat i människokroppen. Eftersom användningsområdet är så brett är det viktigt att undersöka hur mycket metall som frigörs i olika miljöer för att förvissa sig om att det är riskfritt att använda det rostfria stålet, både miljö- och hälsomässigt.

Rostfritt stål (stainless steel på engelska) fick sitt namn under tidigt 1900-tal när det visade sig omöjligt hitta fläckar på ytan (stain the surface) när stålet testades i vinäger. Sedan dess har rostfritt stål blivit det mest använda materialet för vardagsbestick. Men användningen av rostfritt stål för tillagning av mat leder till frågan om eventuella risker med att metall från stålet kan frigöras ut i maten. Resultat presenterade i denna avhandling visar att metaller såsom krom, nickel och järn visserligen frigörs från exempelvis en matkastrull eller bestick, men i mycket låga och riskfria halter. Mängden metall som frigörs minskar dessutom med upprepad användning så länge kastrullen inte är förstörd och börjar korrodera.

Utseendet är ofta den avgörande faktorn som bestämmer varför vi väljer en viss ytfinish för en viss applikation. Önskas en blank, matt eller grov yta? En grov yta består av mer yta än en blank, och resulterar därför också i mer frigörelse av metall än den grova ytan. Det finns dock fler faktorer än ytfinish som behöver beaktas. Även stålets struktur och sammansättning påverkar hur effektivt den skyddande oxidfilmen, tepåsen, hämmar metall från att gå ut i den omgivande miljön.

Bakom den rostfria stålytans blanka finish gömmer sig en hel värld av viktiga men ouppklarade forskningsuppgifter. Den här doktorsavhandlingen har försökt belysa några.

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List of publications

I “A comparison of release rates of Cr, Ni and Fe from stainless steel alloys and the

pure metals exposed to simulated rain events” G. Herting, I. Odnevall Wallinder and C. Leygraf Journal of Electrochemical Society, 152, (2005), B23-29.

II “Metal release from various grades of stainless steel exposed to synthetic body fluids”

G. Herting, I. Odnevall Wallinder and C. Leygraf Corrosion Science 49, (2007), 103-111.

III “Factors that influence the release of metals from stainless steels exposed to

physiological media” G. Herting, I. Odnevall Wallinder and C. Leygraf Corrosion Science 48, (2006), 2120-2132.

IV “Corrosion-induced release of chromium and iron from ferritic stainless steel grade

AISI 430 in simulated food contact” G. Herting, I. Odnevall Wallinder and C. Leygraf Journal of Food Engineering, 87, (2008), 291-300.

V “Metal release from AISI 316L stainless steel and pure Fe, Cr and Ni into a synthetic

biological medium – a comparison” G. Herting, I. Odnevall Wallinder and C. Leygraf Submitted to Journal of Environmental Monitoring, 2008.

VI “Corrosion-induced release of the main alloying constituents of manganese-chromium

stainless steels in different media” G. Herting, I. Odnevall Wallinder and C. Leygraf Submitted to Journal of Environmental Monitoring, 2008.

VII “Multianalytical investigation of stainless steel grade AISI 420 in simulated food

contact” G. Herting, D. Lindström, I. Odnevall Wallinder and C. Leygraf Submitted to Journal of Food Engineering, 2008.

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Author contribution in each paper

Paper I The work was planned together with the supervisor and the author performed all rain simulations and took part in the analysis of data from the metal concentration analysis and the surface analysis together with the supervisor, Inger Odnevall Wallinder. The author wrote a short draft of the paper and took part in the last finishing together with the co-authors.

Paper II

Method development and planning of the work was performed together with Klara Midander. Analysis of data from meal concentration analysis was performed by the author and analysis of surface information was performed together with the supervisor. The paper was written in close cooperation with the co-authors.

Paper III

The planning of the work was performed by the author and all experimental procedures except the metal concentration analysis and the XPS analysis was also performed by the author. The evaluation of the XPS data was discussed with the supervisor and the supervisor and the author wrote the paper together.

Paper IV

All planning and execution of the experimental work was performed or administered by the author, EIS experiments was performed by David Lindström. Analysis with the atomic absorption spectroscopy was performed in cooperation with Klara Midander. Analysis of XPS data was performed with support from the supervisor. Data was mainly analysed by the author with some help from the supervisor concerning the XPS data. The paper was written by the author in cooperation with the co-authors.

Paper V The work in this paper was based on the work in papers I and II and the author planned and executed all the experimental work except the surface roughness measurement that was performed by Swerea KIMAB. The XPS-data was mainly evaluated by the author with some support from the supervisor as was the writing of the paper.

Paper VI

All experimental work, except running the XPS analysis, and the main part of writing the paper was performed by the author.

Paper VII

All work concerning metal release was performed by the author. The microscopy work was all performed by co-author David Lindström. Analysis of metal release data was performed by the author and the microscopy evaluation was performed in cooperation with David Lindström and the supervisor. The paper was mainly written by the author in close cooperation with the co-authors.

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The following peer-reviewed papers, reports proceedings, popular science and presentations have been completed but are not included in this thesis.

“The interaction between concrete pavement and corrosion-induced copper runoff from buildings” B. Bahar, G. Herting, I. Odnevall Wallinder, K. Hakkila, C. Leygraf, M. Virta. Environmental Monitoring and Assessment, 140, (2008), 175-189.

“Corrosion-induced release of Cu and Zn into rainwater from brass, bronze and their pure metals. A two year field study” G. Herting, S, Goidanich, I. Odnevall Wallinder, C. Leygraf Environmental Monitoring and Assessment, DOI 10.1007/s10661-007-0008-5, (2007).

“Corrosion induced metal release from copper-based alloys compared to their pure elements” S. Goidanich, I. Odnevall Wallinder, G. Herting, C. Leygraf Corrosion Engineering, Science and Technology, DOI 10.1179/174327808X286383, (2008).

Reports

“Release of chromium, nickel and iron from stainless steel exposed under atmospheric conditions and the environmental interaction of these metals”, I. Odnevall Wallinder, S. Bertling, G. Herting, C. Leygraf, Heijerick, D. Berggren and P. Koundakjian, report 72 pages, www.eurofer.org (2004).

Popular science

“Release of chromium, nickel and iron from the metals and 304 and 316 stainless steel induced by atmospheric corrosion – a combined field and laboratory investigation” I. Odnevall Wallinder, S. Bertling, G. Herting and C. Leygraf. ACOM (A corrosion management and applications engineering magazine from Outokumpu Stainless), 2, (2004).

“Environmental fate of corrosion-induced metal release from stainless steel” I. Odnevall Wallinder, S. Bertling, G. Herting and C. Leygraf Nordic Steel & Mining Review (2005).

”Nygammal naonteknik - passivfilmer på rostfria stål” C. Olsson, G. Herting och I. Odnevall Wallinder Bergsmannen, 6, (2005), 16-17.

”Passive films on stainless steel – recent nano-range research” C. Olsson, G. Herting och I. Odnevall Wallinder ACOM, 2, (2006), 15.

”Corrosion-induced metal release from stainless steel and its alloy constituents inte different media” I. Odnevall Wallinder, G. Herting, K. Midander, C. Leygraf Stainless Steel Industry, June, 9 (2006).

“Frigörelse av zink från varmförzinkat stål i utomhusapplikationer” I. Odnevall Wallinder, D. Lindström, G. Herting, C. Leygraf Bygg & Teknik, 4 (2008).

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Conference proceedings

“Release rates of chromium, nickel and iron from pure samples of the metals and 304 and 316 stainless steel induced by atmospheric corrosion – a combined field and laboratory study”, I. Odnevall Wallinder, S. Bertling, G. Herting and C. Leygraf. UN/ECE, Convention on long-range transboundary air pollution, workshop on release of heavy metals due to corrosion of metals, Munich, Germany, May 12-14, (2003).

“Release of Cr, Ni and Fe from stainless steel alloys and the pure metals”, G. Herting, I. Odnevall Wallinder and C. Leygraf, Proc. 13th Scandinavian Corrosion Congress, Reykjavik, Iceland, April (2004).

"Corrosion-induced metal release from stainless steel and its alloy constituents in different media" I. Odnevall Wallinder, G. Herting, K. Midander, J. Pan, C. Leygraf. Procedings at Baosteel BAC, Shanghai, China, 25-26 May (2006).

“Corrosion-induced metal release from copper-based alloys compared to their pure elements” S. Goidanich, I. Odnevall Wallinder, G. Herting, C. Leygraf 14th Nordic Corrosion Congress, NKM 14, Copenhagen, Denmark, May, 14-15, (2007)

Conference presentations

"Release of Cr, Ni and Fe from stainless steel alloys and the pure metals" Oral presentation at the 204th Electrochemical Society meeting, ECS 204, Orlando, Florida, USA, October 12-16, (2003).

"Release of Cr, Ni and Fe from stainless steel alloys and the pure metals" Oral presentation at the 13th Nordic corrosion Congress, NKM13, Reykjavik, Iceland, April 18-20, (2004).

"Release of main metal constituents from alloys and the pure metals" Poster, Towards the city surface of tomorrow, Vienna, Austria, June 8-9, (2006).

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Abbreviations

AAS-GF Atomic Absorption Spectroscopy - Graphite Furnace

AAS-F Atomic Absorption Spectroscopy - Flame

AFM Atomic Force Microscopy

AISI American Iron and Steel Institute

ALF Artificial Lysosomal Fluid

EN European standard

EIS Electrochemical Impedance Spectroscopy

ICP-MS Inductively Coupled Plasma – Mass Spectrometry

ICP-AES Inductively Coupled Plasma – Atomic Emission Spectrometry

PBS Phosphate buffered saline, biological fluid

SEM/EDS Scanning Electron Microscopy/Energy Dispersive Spectroscopy

SKPFM Scanning Kelvin Probe Force Microscopy

XPS X-ray Photoelectron Spectroscopy

Definitions

Ra Surface roughness

Corrosion rate The rate at which a metal oxidises per time unit and surface area

Metal release rate The rate at which a metal is dissolved from a surface oxide or corrosion

product per time unit and surface area

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CONTENTS

CHAPTER 1 INTRODUCTION .............................................................................. 17

1.1 Background ............................................................................................................. 17

1.2 A short history of iron and stainless steel ............................................................ 18

1.3 What is stainless steel ............................................................................................. 19

1.4 Atmospheric interaction with stainless steel (Papers I, VI) ............................... 20

1.5 Synthetic Body fluids (Papers II, III, V, VI) ........................................................ 21

1.6 Food related metal release (Papers IV, VII) ........................................................ 22

1.7 Need for correct metal release data ...................................................................... 23

CHAPTER 2 DESCRIPTION OF EXPERIMENTAL PROCEDURES ........................... 25

2.1 Materials ................................................................................................................. 25

2.1.1 Austenitic stainless steels ................................................................................... 26

2.1.2 Ferritic stainless steel ......................................................................................... 26

2.1.4 Martensitic stainless steel .................................................................................. 27

2.2 Artificial media and experimental setup .............................................................. 27

2.2.1 Artificial rain (Paper I) ...................................................................................... 27

2.2.2 Synthetic body fluids (Papers II, III, V, VI)...................................................... 28

2.2.3 Acetic acid (Papers IV, VII)............................................................................... 29

2.3 Chemical and surface analysis .............................................................................. 30

2.4 Metal analysis ......................................................................................................... 30 2.4.1 Atomic absorption spectroscopy, AAS ............................................................... 30

2.4.2 Inductively coupled plasma – mass spectrometry, ICP-MS and

inductively coupled plasma-atomic emission spectroscopy, ICP-AES ............. 31

2.5 Surface analysis ...................................................................................................... 31

2.5.1 X-ray photoelectron spectroscopy, XPS ............................................................ 31

2.5.2 Electrochemical impedance spectroscopy, EIS ................................................. 31

2.5.3 Confocal microscopy .......................................................................................... 32

2.5.4 Profilometry ........................................................................................................ 33

2.5.5 Atomic force microscopy, AFM ......................................................................... 33

2.5.6 Scanning Kelvin Probe Force Microscopy, SKPFM ........................................ 34

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2.5.7 Scanning Electron Microscopy, SEM, and

Energy Dispersive Spectroscopy, EDS .............................................................. 34

2.6 Electrochemical noise measurements ................................................................... 34

CHAPTER 3 IMPORTANT ASPECTS OF METAL RELEASE ................................... 35

3.1 The influence of bulk alloy content on the metal release process ...................... 37

3.1.1 There is a strong correlation between the bulk alloy content

and the total metal release process (Papers I, II, V, VI) ................................... 38

3.1.2 The degree of metal release from stainless steels is not proportional

to either the bulk alloy content or to the behaviour of their pure metal

constituents. (Papers I, V) .................................................................................. 40

3.2 The influence of surface oxide properties on the metal release process ............ 43

3.2.1 Exposure to any media results in an enrichment of the chromium content in

the outermost surface oxide (Papers I, II, III, IV, V, VI and VII)................... 44

3.2.2 The metal release process is highly time-dependent

(Papers I, II, IV, V, VI, VII) .............................................................................. 45

3.2.3 The surface oxide temporarily breaks down locally causing alloy

constituents to be released ................................................................................. 47

3.3 The influence of surface finish and microstructure on the metal release process ............................................................................................. 50

3.3.1 The surface finish and other surface parameters have a considerable

influence on the metal release process (Papers III, IV and VII) ..................... 51

3.3.2 A highly inhomogenous microstructure favours high metal release rates ...... 55

3.3.3 Changes in experimental conditions highly influence metal release............... 57

CHAPTER 4 SUMMARY ...................................................................................... 59

CHAPTER 5 FUTURE WORK ............................................................................... 60

CHAPTER 6 ACKNOWLEDGEMENT ................................................................... 61

CHAPTER 7 REFERENCES .................................................................................. 62

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Chapter 1 Introduction

1.1 Background

Stainless steel has become one of the most used groups of materials in the world due to its

beneficial mechanical and corrosion resistance properties. The widespread use has made it

important to increase the knowledge about the metal release process and the amount of metal

released into different media. Accurate and relevant data on metal release of individual alloy

constituents is vital for accurate assessment of potential adverse effects on human health and

the environment. Investigations on the interaction between stainless steel and the surrounding

environment have been investigated for a long time in terms of corrosion resistance in a wide

range of media [Arnold, 2004, Asami and Hashimoto, 1996, Beddoes, 1999, Heidersbach,

1987, Hudson et. al., 2002, Kearns and et. al., 1988, Outokumpu Corrosion Handbook, 2004,

Uggowitzer, 1996, Wallinder et. al. 2003]. Metal release investigations however are not as

commonly found even though the number of investigations has lately increased, Figures 1, 2,

3. The corrosion resistance and the degree of metal release should not be mixed since the

concepts reflect different properties governed by different mechanisms.

In June 2007 a new chemicals policy, REACH, Registration Evaluation, Authorisation and

Restriction of Chemicals entered into force in the European Community. According to

REACH, all components of an alloy that are deliberately added during production and exceed

0.1 % of the content may have to be registered. Products, such as stainless steel, have to be

assessed in terms of metal release, and prove not to pose a adverse health effects on human

health or on the environment.

All research activities presented in this thesis aim to improve the knowledge on the

mechanisms behind the metal release process by using a multidisciplinary approach.

Quantitative bioaccessibility data is generated combined with surface analytical studies of

changes in surface composition and passivating properties on stainless steels used in different

applications. Key aspects that are discussed and investigated within this thesis are metal

release rates and their time-dependence from different stainless steels exposed to different

media, the difference in metal release between stainless steel alloys and the pure metals, and

the importance of surface finish and experimental parameters.

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This thesis is the sixth in a series of doctoral and licentiate theses performed at the division of

Corrosion Science, KTH, aiming to reflect and elucidate aspects of metal release from metals

and metal alloys in different media [He, 2002, Karlén, 2001, Bertling, 2005, Sandberg, 2006,

Midander, 2006].

1.2 A short history of iron and stainless steel

The ancient Egyptians are believed to be among the first to use iron 5000 to 6000 years ago.

The earliest artefacts have been dated to around 3500 BC, [Encyclopædia Britannica Online,

2008]. In the beginning, iron was considered very valuable since it was only found in

meteorites and hence difficult to find. However, the use of iron has not been found to be of

any extent during this period. During the second millennium BC, iron is believed to be used

regularly in Asia Minor and an iron industry was most likely established during the latter half

of that millennia. The knowledge of iron working was transmitted throughout Europe during

the first millennia BC. During the Iron Age, iron was increasingly used for weapons as it was

found to have superior properties compared with copper and bronze.

The quality of iron was during the early days strongly depending upon the quality of iron ore

used, and the methods used for purifying the ore were in their cradle. During the middle ages

and the Renaissance, the processes for iron production were refined and low carbon steel was

soon produced at increasing volumes. However, the introduction of stainless steel did not

occur until the early 20th century. Stainless steel was invented by Harry Brearely, Sheffield,

England, [ISSF, What is stainless steel] as he was searching for a steel with improved

corrosion resistance. He started with steels containing some chromium as these were known

to have a higher melting temperature than ordinary steels. During his experiments he found

that chromium containing steels resisted chemical attack during etching and also resisted

corrosion in contact with food and recognised the potential for this revolutionary new material

in the cutlery industry. Brearely named this new material “rustless steel” and Earnest Stuart,

cutlery manager of the local cutler’s R. F. Mosely, was the first to call the material “stainless”

as tests with vinegar did not stain the surface.

The use of stainless steel today is widely spread and the applications range from medical

implants to food related items to construction materials used in buildings. The variety of

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applications creates a higher demand for suitable materials that are not only corrosion

resistant but also environmentally friendly.

1.3 What is stainless steel

Stainless steels are called “stainless” because they are resistant to corrosion (i.e. they do not

rust to a large extent) in conditions under which iron or non-stainless steels would rust.

Stainless steels are defined as iron-based alloys containing at least,10.5 wt% chromium [ISSF,

2005] and a maximum carbon content of 1.2 wt%. At a concentration of 10.5 wt% chromium,

chromium in the alloys reacts with oxygen and moisture in the environment to form a very

thin chromium-rich oxide layer (“passive film”) on the surface of the steel. This passive oxide

film is so thin 1-3 nm [Olsson, 2003] that it does not affect the natural brightness of the steel,

but effectively separates the material from the surrounding medium. The oxide is continuous,

fairly non-porous and insoluble under ambient conditions. If locally destroyed, e.g. by

scratching, it is immediately self-healing under the influence of oxygen in air or water.

Chromium is the essential element for the formation and stabilisation of the passive film.

Other alloying elements may influence the effectiveness of chromium in forming, or

maintaining the film (for example, nickel promotes re-passivation, especially in reducing

environments. Molybdenum stabilises the passive film in the presence of chlorides).

Increasing the chromium content, from the minimum of 10.5 wt% necessary for “stainless”

stainless steel, to 17 to 20 wt%, greatly increases the stability of the passive film.

Stainless steels are sub-divided into a number of categories based on the metallurgical phases

present in their micro-structure e.g. austenitic, ferritic, martensitic and austenitic-ferritic

(duplex). Within each of these categories different “grades” of stainless steel exist. Austenitic

grades are the most numerous and the most widely used. They are used primarily for

applications where corrosion resistance is of high importance, e.g. exterior architectural trim,

street furniture, food processing and handling equipment, hospital equipment, dairy and

beverage equipment and pharmaceutical equipment. Ferritic grades are used in applications

with lower demands on corrosion resistance as, e.g. vehicle mufflers and catalytic converters,

containers, drums for dryers, interior architectural trim. Duplex stainless steels combine the

beneficial properties of the austenitic and the ferritic microstructure, and show generally high

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corrosion resistance and high strength. Martensitic steels have a high carbon content allowing

for hardening, making them suitable for cutting tools such as cutlery knives and scissor

blades.

There are several systems available for designating individual stainless steels based on their

elemental composition (usually given as wt% ranges e.g. for AISI grade 304 the composition

is <0.8wt% C, 17.5-20wt% Cr, 8-11wt% Ni, <2wt% Mn, <1wt% Si, <0.045wt% P, <0.03wt%

S (balance Fe)). Commonly used designations include the American Iron and Steel Institute

(AISI) system and the European Standard (EN), adopted for use in the European Union.

1.4 Atmospheric interaction with stainless steel (Papers I, VI)

Interaction between stainless steel (e.g. buildings or bridges) and the surrounding

environment determines corrosion rates and release rates. Metal release data from a variety of

metals and alloys in different environments has been obtained during the past decade, Figure

1. However, corrosion resistance and corrosion rate measurements are more commonly found,

Figure 1. It is crucial to realise the difference between corrosion rate and metal release rate.

The corrosion rate is a strictly electrochemical procedure and reveals how quickly the metal

oxidises and forms corrosion products/oxides on the metal surface. Corrosion occurs at the

interface between the metal surface and the corrosion product. Metal release occurs at the

interface between the corrosion product and the surrounding corrosive media; consequently,

the release rate is the speed at which metal ions are dissolved from the corrosion

product/surface oxide and released into the surrounding media.

A heavily polluted environment, such as an industrial or a marine location, generally induces

higher corrosion and release rates as the presence of for example chlorides increase the

corrosion rate. A high enough alloying content of chromium, and an addition of molybdenum

will improve the corrosion resistance and thus the life span of the appliance [Johnson and

Pavlic, 1982]. Increased alloying also generally reduces the metal release rate.

_________________________________Chapter 1_________________________________

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~ 21 ~

Baroux, 1997

Edwards, 2002

Miyake, 1995

Odnevall Wallinder, 2002, 2004a, 2004b

Persson, 2001

Elfström-Broo, 2001

Waltersson, 2001

Beddoes, 1999

Hilbert, 2003

Sivakumar, 1993

Baker, 1988, 1990

Johnson, 1982

Heidersbach, 1987 NiDI, 2002

Leygraf, 2000

Kearns, 1988

Wallinder, 2003

Hudson, 2002

Asami, 1996

Karlsson, 1975

Tochihara, 1996

Figure 1 Compilation of reported literature on corrosion rates and metal release in water

solutions, given by the first authors. Photo by David Shankbone

A large number of investigations regarding the corrosion resistance of stainless steels in

various atmospheric environments, rural, urban, industrial and marine, for different time

spans are performed and reported in the literature, Figure 1.

1.5 Synthetic body fluids (Papers II, III, V, VI)

Stainless steels may come into contact with a wide range of body fluids such as sweat (e.g.

when wearing a wristwatch with a stainless steel case), saliva (e.g. from orthodontic wires),

blood/plasma/lymphatic fluid (from body piercing applications), a range of other body fluids

from stainless steel surgical implants (e.g. synovial fluid), various respiratory tract fluids

(from occupational exposure to stainless steel particles), gastric juice (following swallowing

of particles cleared from the respiratory tract). Reliable information on metal release rates is

invaluable when assessing the potential for risk of adverse effects arising from exposure to the

alloying metals in stainless steel. No simple relationship between corrosion rate and metal

release rates exist, nor between the bulk composition of the alloy and the relative release rates

of main alloying metals [ISSF, 2005. Paper I].

_________________________________Chapter 1_________________________________

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~ 22 ~

Reported metal release rates available in the literature are primarily based on austenitic

stainless steels immersed in various synthetic body fluids such as artificial sweat and artificial

saliva, Figure 2. In recent studies, metal release rates of individual alloy constituents have

been reported for stainless steel grades in phosphate buffered saline, artificial lysosomal fluid

(ALF) and Gamble's solution [He, 2002a, 2002b]. However, simulations of fundamental body

functions using artificial internal body fluids (including Gamble's solution and ALF) have

been performed in a number of reported studies to evaluate the behaviour of materials other

than stainless steel such as man-made fibres, various cobalt-containing materials, indium

phosphide, and uranium tetra fluoride, Figure 2.

Agarwal, 1997Ansoborolo, 1990

Bailif, 1994Bauer, 1994Christensen, 1994

Fisher, 1984 Gjerdet, 1991

Haudrechy, 1993, 1994, 1997

He, 2002a, b

Hultquist, 1984Hwang, 2001 Ito, 2001

Jia, 1999Johansson, 1980

Johnson 1994

Kabe,1996

Kanerva, 1994

Katz, 1975

Lidén, 1998

Mattson, 1994

Menné, 1987

Meinert, 1998

Moura, 1993

Nestle, 2002Okazaki, 2005

Pönkä, 1998 Randin, 1988

Rondelli, 2005Samitz, 1975

Sivakumar, 1993

Staffolani, 1999Sundararajan, 1999Théolan, 1994

Tomás, 1994Wever, 1998

Stopford, 2003

Midander, 2007

Hostynek, 2002

Figure 2 Compilation of scientific literature (listed after the first author) assessing corrosion

rate and metal release in various fluids simulating exposure in the human body.

1.6 Food related fluids (Papers IV, VII)

As stainless steel is highly corrosion resistant and possesses properties that recommend its use

in a wide variety of applications including food processing, in cutlery and cooking

paraphernalia, high alloyed stainless steel grades as AISI 304 and 316 with 8-10 wt % nickel

are the most commonly used grades in these circumstances [Council of Europe, 2001]. As the

presence of nickel is becoming progressively more debated from a health perspective and as

the market price of nickel increases, an interest for reducing or removing nickel in stainless

steel has received growing attention. Evaluating the performance of stainless steel in food

_________________________________Chapter 1_________________________________

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~ 23 ~

related applications is difficult as standardised methods are few and contains a large number

of degrees of freedom. Frequent studies are reported in the literature concerning cleanability

of stainless steels in the food industry and the importance of surface roughness, etc [El

Shawesh et. al. 2001, Faller et. al, 2005, Hilbert, 2003]. Metal concentrations in actual

foodstuff after preparation in stainless steel vessels have been measured, compiled in Jellesen,

2006, see Figure 3 for further references. Generated results have limited value as there are

several uncertainties involved when performing studies with complicated systems and

undefined parameters such as different foodstuffs. More distinct boundaries in the test

methods are essential to produce reproducible data for wider use.

Accominotti, 1998

Flint, 1997

Jellesen, 2006

Kuligowski, 1992

Offenbacher, 1983

Hausch, 1996

Hilbert, 2003

Arnold, 2004

Faller, 2005Kumar, 1994

Kawamura, 1997

Figure 3 Compilation of references investigating the interaction between metals and food stuffs

in various fluids (listed after the first author).

1.7 Need for correct metal release data

For correct use in different applications, the use of stainless steel and their alloying metals

will always call for accurate knowledge and evaluation of their properties, both from a

mechanical and a chemical perspective. A stainless steel grade that is successfully used in for

example rural environments may fail before the planned life time is reached when used in

marine environments. Metal release rates that are acceptable in one application can be outside

the limit for another. To select the most suitable material for a given application it is therefore

essential to have relevant and accurate data. Corrosion rates have for a long time been

available [Outokumpu corrosion handbook, 2004] giving producers and downstream users

valuable information on materials suitable for use in different environments. However, as

_________________________________Chapter 1_________________________________

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~ 24 ~

stressed in a previous paragraph corrosion rates are not sufficient to assess any environmental

interaction, which in addition requires information on metal release rates.

Recommendations and regulations on threshold values for the metal content in for example

drinking water or food have been in use for long times for protection of the environment and

human health [WHO, 2004]. However, few publications concerning metal release rates are

available, although the number is growing [Figures 1, 2, 3]. Few standardised test methods

exist concerning metal release into media and those that are used have large degrees of

freedom for the user to exploit [D. M. 21-03-1773, OECD Transformation and dissolution

Protocol, Food Standards Agency]. These regulations and protocols are seldom developed for

metal alloys, rather expanded from e.g. chemical compounds. Proper test methods generally

accepted for metallic materials in biological applications are even more difficult to find.

The interest for metal release has intensified with the implementation of REACH. The new

chemicals policy in the EU requires the industry to ensure that their products do not adversely

affect the environment or human health [Reach in Brief, 2007]. To follow these new

regulations, steel producers, among others, must produce bioaccessibility data for their

products to ensure no harmful effects will follow their use.

_________________________________Chapter 2_________________________________

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~ 25 ~

Chapter 2 Description of experimental procedures The work described in this thesis was all performed under laboratory conditions in order to i)

simulate outdoor rain events on stainless steel constructions, ii) the use of stainless steel in for

example as implants in the human body, and iii) in food related applications such as cutlery

and cooking pots and pans. A multidisciplinary approach was used to link the metal release

process to surface properties of massive sheet of stainless steel of different grades. Total

metal release concentrations were measured by flame atomic absorption spectroscopy (AAS-

F), graphite furnace atomic absorption spectroscopy (AAS-GF) and inductively coupled

plasma mass spectrometry (ICP-MS). Changes in composition of the outermost surface oxide

were recorded by means of x-ray photoelectron spectroscopy (XPS). The electrochemically

active surface area was evaluated using electrochemical impedance spectroscopy (EIS).

Estimates of the surface roughness were performed on selected surfaces using profilometry

and contact angle measurements. In addition, analysis using atomic force microscopy, AFM

combined with scanning Kelvin probe force microscopy, SKPFM, and scanning electron

microscopy energy dispersive spectrometry, SEM-EDS were performed on selected polished

surfaces to observe relative differences in surface nobility, and to perform microstructural

studies.

2.1 Materials

The metal release process was investigated for fourteen grades of stainless steel of varying

microstructure upon exposure to different media. For comparison, the main pure metal alloy

constituents, iron, chromium and nickel, were exposed to identical conditions. Table 1

summarises the different materials investigated in this thesis.

_________________________________Chapter 2_________________________________

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~ 26 ~

Table 1 Summary of materials used within the scope of the different papers included in this

thesis.

Material Microstructure Solution Surface finish Paper Grade 1Ni Austenitic acetic acid 0.46±0.053 µm VI Grade 1.5Ni Duplex acetic acid 0.35±0.028 µm VI Grade 4Ni Austenitic acetic acid 0.38±0.050 µm VI Grade 5Ni Duplex acetic acid 2.4±0.88 µm VI Grade AISI 2205 Duplex ALF, Gamble 2B, abraded III Grade AISI 201 Austenitic ALF, Gamble Shotblasted, abraded III

Grade AISI 304 Austenitic Art. rain ALF, Gamble 2B, 2D, BA, electroploished, electropolished+passivated III

Grade AISI 310 Austenitic ALF, Gamble BA, abraded III Grade AISI 316 Austenitic Art. rain 2B, abraded I Grade AISI 316L

Austenitic ALF, Gamble 2B, abraded III, V

Grade AISI 409 Ferritic ALF, Gamble 2B, abraded III Grade AISI 420 knife Martensitic acetic acid Polished VII

Grade AISI 420 scissor Martensitic acetic acid Polished VII

Grade AISI 430 Ferritic ALF, Gamble, acetic acid BA, abraded, polished III, IV Pure Fe Art. rain, ALF As-received, abraded I, V Pure Cr Art. rain, ALF As-received, abraded I, V Pure Ni Art. rain, ALF As-received, abraded I, V Pure Fe Art. rain, ALF As-received, abraded I, V

2.1.1 Austenitic stainless steels

Austenitic stainless steel grades are the most common grades and are frequently used in areas

including outdoor constructions, vehicles, food related applications, jewellery and to some

extent as implants. Austenitic grades usually have a chromium content of 16-25 wt % and 8-

10 wt % nickel [ISSF, 2005]. Austenitic grades investigated within this thesis are AISI 201,

AISI 304, AISI 310, AISI 316, AISI 316L, 1Ni and 4Ni.

2.1.2 Ferritic stainless steel

The ferritic grades, such as AISI 409, AISI 430, do not contain any deliberately added nickel

but generally 13-30 wt % chromium [ISSF, 2005]. These grades are frequently used where the

demands on corrosion resistance is lower compared with austenitic and duplex grades. The

appeal for using non-nickel stainless steels is increasing due to high nickel prices and the

possibility of nickel release into the surrounding media from nickel containing steels.

_________________________________Chapter 2_________________________________

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2.1.3 Duplex stainless steels

Duplex stainless steels, or austenitic-ferritic grades, AISI 2205, 1.5Ni and 5Ni, are composed

of a mixture of the austenitic and ferritic phases in the microstructure, usually close to a 50 -

50 relation [ISSF, 2005]. The mix of structures results in improved yield and tensile strength,

and high corrosion resistance, compared to ferritic and austenitic grades individually. The

chromium content is typically between 20-25 wt %. Depending on application, other common

alloying elements are molybdenum and nickel although the nickel content usually is between

1.5 to 7 wt %.

2.1.4 Martensitic stainless steel

Due to beneficial mechanical properties, martensitic stainless steels, like AISI 420, are

commonly used in cutlery and knife applications. To obtain a suitable tempering of the steel

for cutting applications, the carbon content has to be higher compared to other grades of

stainless steel, 0.1-1.2 wt %. A fast cooling process during manufacturing is necessary to

avoid precipitation of chromium carbides that will reduce the corrosion resistance. The

chromium content is usually within the range of 11-13 wt % resulting in lower corrosion

resistance compared with other grades of stainless steels [ISSF, 2005].

All materials are described in detail in Papers I-VII.

2.2 Artificial media and experimental setup

2.2.1 Artificial rain (Paper I)

Outdoor scenarios were simulated in a rain chamber able to mimic rain events of different

intensities. Experiments were performed on metal samples, inclined 45° from the horizontal,

by using artificial rain with an intensity of 4 mm/h and a pH of 4.4. Runoff water samples

were collected after 30 min, 1, 2, 3, 4, 5, 6, 7 and 8 hours, to gain information on the time-

dependence of the metal release process, and to provide quantitative data on metal release

rates for the different alloys and pure metals investigated. The artificial rain was prepared

using ultra pure water and analytical grade chemicals. The experimental set-up is

schematically illustrated in Figure 4. Experimental parameters are further described in Paper

I.

_________________________________Chapter 2_________________________________

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~ 28 ~

artificial rain

metal release

AAS‐F/GF

ICP‐MS

stainless steel grades:1Ni, 4Ni, AISI 304, AISI 316, pure metals: Fe, Cr, Ni

XPS

as‐receivedabraded

1st 8 h rain event4 mm/h, pH 4.4

dry period

2nd 8 h rain event4 mm/h, pH 4.4

Figure 4 Experimental setup for artificial rain exposure of stainless steel grades 304 and 316

and the pure metals iron, chromium and nickel.

2.2.2 Synthetic body fluids (Papers II, III, V, VI)

Two different synthetic body fluids were used to simulate deep lung conditions, Gamble’s

solution, and an inflammatory response, ALF (artificial lysosomal fluid). The kinetics of the

metal release process for the stainless steel grades investigated in these fluids were

determined after immersion time periods varying between 1 and 168 hours at constant

temperatures (37 ± 1 °C) and dark conditions. The experimental set-up is schematically

illustrated in Figure 5. Both fluids and the experimental procedure are thoroughly described in

Papers II, III, V and VI.

_________________________________Chapter 2_________________________________

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~ 29 ~

ALF Gamble’s

as‐receivedabraded

metal release

AAS‐GFICP‐MS

immersion periods of 1 ‐ 168 h 

stainless steel grades:AISI 2205, AISI 201, AISI 304, AISI 310, AISI 316L, AISI 409, AISI 430, 1Ni, 4Ni 

pure metals: Fe, Cr, Ni

XPS EISSurface roughness 

Profilometry Figure 5 Experimental setup for immersion in synthetic body fluids of stainless steels and the

pure metals.

2.2.3 Acetic acid (Papers IV, VII)

Simulating contact with food was achieved using acetic acid as food simulant. Depending on

the stainless steel application, different recommendations exist as to which temperature and

time to use in such simulations. The experimental setup, Figure 6, was based on the Italian

law text D.M. 21-03-1973, Art. 37. More thorough descriptions of the procedures are given in

Papers IV and VII.

EISsurface 

roughness 

as‐received,well polished,badly polished

metal release

AAS‐GFICP‐MS

XPS

immersion periods of 1 ‐ 10 days

AFM

SEM

SKPFM

SEM

stainless steel grades

AISI 430AISI 420

as‐received,glossy, line,abraded

Figure 6 Experimental setup for immersion in acetic acid of stainless steels for simulated food

contact.

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~ 30 ~

2.3 Chemical and surface analysis

A multi-analytical investigation was undertaken to measure released metal concentrations

(metal release) and to characterise the stainless steel surfaces (surface analysis) investigated

prior to, and after immersion into, or exposure to, different media. A summary of techniques

used in each paper is given in Table 2.

Table 2 Summary of experimental techniques used within this thesis.

Technique Paper I Paper II Paper III Paper IV Paper V Paper VI Paper VII AAS-F X X X AAS-GF X X ICP-MS X X X XPS X X X X X X X EIS X X X X X Confocal microscopy X X

Profilometry X SEM-EDS X AFM X SKPFM X

2.4 Metal analysis

2.4.1 Atomic absorption spectroscopy, AAS

Atomic absorption spectroscopy is a very useful tool for analysis of total concentration of

metals in fluids. One element is analysed at a time giving the method low detection limits and

a high accuracy. Depending on concentration, there are two possible choices for AAS, flame,

AAS-F and graphite furnace, AAS-GF. Solutions with metal concentrations above 500 ppb

are preferably analysed with the flame technique, whereas lower concentrations yield an

improved result when analysed with the graphite furnace accessory. Metal release into

Gamble’s solution (Paper III), ALF (Papers V and VI) and acetic acid (Papers IV and VII)

were analysed with this method.

All AAS-F and AAS-GF analysis were performed using a Perkin Elmer AA800 analyst

instrument at the division of Corrosion Science, KTH.

_________________________________Chapter 2_________________________________

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2.4.2 Inductively coupled plasma – mass spectrometry, ICP-MS and inductively

coupled plasma-atomic emission spectroscopy, ICP-AES

ICP-MS is another practical and useful method for the determination of total metal

concentration in liquid solutions. It differs from AAS as it uses a significantly higher

atomisation temperature, approximately 8000 °C compared with 2000 °C for AAS. One

advantage is that several elements can be analysed at one time. However, metal

concentrations in some fluids such as Gamble’s solution cannot be successfully analysed with

ICP-MS due to interference of the salts in the solution with the metal peaks in the mass

spectrum. Metal release samples of artificial rain (Paper I) and ALF (Papers II and III) were

analysed with ICP-MS, Perkin-Elan 6100.

ICP-AES is preferably used when high metal concentrations are expected. Selected metal

release samples of acetic acid (Paper VII) was analysed using ICP-AES - Spectro CirosCCD,

Smart Analyzer Vision, SPECTRO A. I., D-47533 Kleve, Germany.

All ICP measurements were performed by the Environmental Research Laboratory, Swedish

University of Agricultural Sciences, Umeå, Sweden.

2.5 Surface analysis

2.5.1 X-ray photoelectron spectroscopy, XPS

The composition of the outermost surface layer of metal surfaces can be determined using

XPS. It is a very sensitive technique and only reaches a depth of 5-10 nm into the sample

surface. The sample is irradiated with x-rays from a monochromatic AlKα X-ray source

exciting photoelectrons from specific electron shells each with its own specific kinetic energy

[Ernstsson, 1999]. The surface composition is then determined by the detected kinetic energy.

It is thereby possible to determine the chemical composition and also to determine the relative

amount of oxidised and metallic species in the surface film. More details on this method are

given in Papers I - VII. All XPS analysis was performed using a Kratos AXIS HS instrument.

2.5.2 Electrochemical impedance spectroscopy, EIS

Relative estimations of the electrochemically active surface area can be made using EIS since

this area is proportional to the capacitive response of the system. It is important to note that

_________________________________Chapter 2_________________________________

____________________________________________________________________

~ 32 ~

the measurements performed within this thesis were only performed for comparative purposes

between stainless steels of different surface finish, and not to obtain an absolute measure of

the surface area. Some assumptions were therefore necessary to make. All samples were

assumed to have a native oxide film of similar thickness and the same dielectric constant. This

method is further described in Papers II – VII. All EIS measurements were performed using

an electrochemical interface, Solatron 1287 and a frequency response analyser Solotron 1250.

2.5.3 Confocal microscopy

The surface roughness, Ra, of selected stainless steels surfaces investigated were determined

using a Olympus LEXT confocal microscope OLS 3000 with 408 nm LD laser. The lateral

resolution of the method is 0.12 µm. Ra is a quantitative measure of the mean surface

roughness determined from variations (peaks and valleys) observed on a 256 x 256 µm large

scanned surface area. Surface roughness variations for an abraded stainless steel surface are

illustrated in Figure 7. Surface roughness measurements using this technique are provided in

Paper IV.

All surface roughness measurements were performed by Swerea Kimab AB, Stockholm.

_________________________________Chapter 2_________________________________

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~ 33 ~

30 µm Figure 7 Surface topography of an arbitrary chosen surface, abraded AISI 430, as measured by

confocal microscopy (top) and the corresponding light optical image (bottom) of the

investigated surface.

2.5.4 Profilometry

Another approach to measure the surface area is by using profilometry. A View 5010 Zygo

Ltd (USA) instrument with a lateral resolution of 1.1 µm was utilised for this purpose. This

resolution proved not to be sufficient to provide a proper estimation of the surface area on the

stainless steels investigated in this study (Paper II).

The profilometry investigations were performed at the Institute of Surface Chemistry.

2.5.5 Atomic force microscopy, AFM

The surface topography of a smooth (diamond polished) sample can be investigated with

AFM. An ultra sharp tip mounted on a cantilever spring is swept over the surface of the

sample and a laser beam focused on the cantilever will reflect the light and the motions of the

tip will be registered by a detector. More information on this method is given in Paper VII.

All AFM investigations were performed with a Veeco NanoScope IV MultiMode AFM in the

Nano-Fab-Lab at the Royal Institute of Technology, further information in Paper VII.

_________________________________Chapter 2_________________________________

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~ 34 ~

2.5.6 Scanning Kelvin Probe Force Microscopy, SKPFM

Differences in relative nobility between phases on a polished surface of a material can be

detected by SKPFM. The Volta potential is the difference in potential between the AFM tip

and the sample surface. This technique was tested and applied using the Nanoscope IV

instrument at the Nano-Fab-Lab, KTH in the SKPFM mode. Further information is given in

Paper VII.

2.5.7 Scanning Electron Microscopy, SEM, and Energy Dispersive Spectroscopy,

EDS

Secondary electrons from the surface of the sample are excited by an electron beam from a

tungsten filament. These electrons are detected, converted into a voltage and transformed into

a two dimensional image. By applying the SEM-EDS technique, elemental compositional

bulk analysis of the sample can be performed. The electron beam causes x-rays to be emitted

from the sample at µm-depth. It is a useful tool for elemental analysis of the bulk content or

of large inclusions; however, small inclusions < 1 µm cannot be analysed as the surrounding

bulk material will contribute with information not related to the area intended for analysis. A

Gemini Leo 1530 with an EDS application, model INCA-energy from LINK, was used for the

investigations. The method was applied for stainless steel surfaces in Paper VII. All

measurements were performed at Swerea Kimab AB, Stockholm

2.6 Electrochemical noise measurements

Electrochemical noise was measured from a single electrode with an applied potential of 0.4

V and from an identical twin electrode at the open circuit potential at 39 °C in ALF and

modified ALF for the two electrode types respectively. The area of the electrodes was 2 x 10-5

cm2 and an Ag/AgCl counter/reference electrode was used. The electrochemical cell was set

up in double Faraday cages to minimise the background noise and the potential was supplied

using a bank of dry batteries, regulated by a voltage divider. A current amplifier, also housed

in a Faraday cage, was connected immediately next to the cages containing the

electrochemical cell and operated at 1010 V A-1.

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 35 ~

Chapter 3 Important aspects of metal release The metal release process depends on several different parameters that will influence the rate

of release from both alloys and pure metals. Within the framework of the research projects in

this thesis, several issues have been raised concerning metal release from stainless steel.

• Can the bulk alloy content (degree of alloying) affect the metal release process, if so,

how?

• Is there a difference in metal release between the stainless steel alloys and the

corresponding pure metals?

• How does the surface oxide influence the degree of metal release?

• How does the surface oxide change during exposure to different environments?

• What effect will changes in the surface area to solution volume ratio have on the metal

release process?

• Is the metal release process time-dependent independent of media and way of

exposure, thin film exposure or bulk immersion?

• Can the degree of metal release be explained by the formation of metastable pits?

The following paragraphs will address aspects of these issues and endeavour to clarify them

in three main paragraphs.

The influence of bulk alloy content on the metal release process and comparisons between

different stainless steel grades and their corresponding pure metals will be exemplified with

stainless steel grades of different microstructure (Papers II, VI, V) immersed in the synthetic

body fluid ALF.

The influence of surface oxide properties on the metal release process will be discussed and

primarily illustrated using generated data for AISI 316L during exposure to artificial rain,

describing the build up of the surface oxide and how the maturing of the oxide film affects the

metal release process (Papers I, VI). In addition, to illustrate the non-continuous metal release

process, possibly related to the formation of local metastable pits, generated data for 316L

immersed in ALF will be presented. The scientific argument for not illustrating the results

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 36 ~

with artificial rainwater, as above, is related to too small currents involved in the process (pA)

to be measured using the noise technique (unpublished data).

The influence of surface finish on the metal release process, demonstrated with differences in

surface roughness and electrochemically active surface areas, is illustrated with results

generated for stainless steel grades AISI 430 immersed in food related media (Paper IV). The

influence of inhomogeneities present in the bulk matrix and the subsequent effect on the metal

release process is in addition illustrated by results for the stainless steel grade AISI 420 in

acetic acid (Paper VII). Changes in experimental parameters are in addition studied in paper

IV.

All features described in each paragraph are representative examples illustrating the general

behaviour of all stainless steel grades tested, independent of exposure parameters. Main

conclusions from each paragraph are compiled below.

Important aspects of metal release

Influence of bulk alloy content on the

metal release process

Influence of surface oxide properties on the metal release process

Influence of surface finish and

microstructure on the metal release process

Papers I, II, V, VI Papers I, II, III, IV, V, VI, VII

Papers II, III, IV, V, VI

• There is a strong correlation between the bulk alloy content and the metal release process

• The degree of metal release from stainless steels is not proportional to either the bulk alloy content or to the behaviour of their pure metal constituents

• Exposure to any media will result in an enrichment of the chromium content in the outermost surface oxide

• The metal release process is highly time dependent

•The surface film temporarily breaks down locally, causing alloy constituents to be released

• The surface finish and other surface parameters have a considerable influence on the release process

• A highly inhomogeneous microstructure favours high metal release rates

• Changes in experimental conditions highly influence the metal release process

Figure 8 Outline of the thesis and the corresponding papers to each paragraph

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 37 ~

3.1 The influence of bulk alloy content on the metal release process

Stainless steel, for example grade 316L, has been used as a surgical material for short time

fixation of broken bones in the human body and in a variety of surgical tools [Rimnac et. al.

1991]. As the interaction between the surrounding media and the surface characteristics of the

stainless steel alloy determines the amount of metal release, it is important to evaluate how

different alloying constituents influence the metal release behaviour. Nine stainless steel

grades with various alloying content, were immersed in ALF for comparative purposes, and to

elucidate how changes in the degree of alloying would influence the metal release process. A

compilation of the range of the main alloy constituents and the different microstructures of the

stainless steel investigated are illustrated in Figure 9.

Austenitic

Cr 16.0-24.2Ni 1.0-19.1

Mo 0.02-2.2 Mn 0-9.7

MartensiticCr 12.6-13.6Ni 0.1-0.2

Mo 0.01-0.03 Mn 0.3-0.5

DuplexCr 21.4-22.7 Ni 1.5-5.6

Mo 0.3-3.1 Mn 1.5-5.1

FerriticCr 11.4-16.0Ni 0.1-0.2Mo 0-0.06Mn 0.3-0.5

201304

310

316 316L

1Ni

420 knife steel

420 scissor steel

4304092205

1.5Ni

5Ni

wt %

Figure 9 Compilation of the stainless steel grades investigated of different microstructure and

alloying content to assess their effect on the metal release process.

In the following paragraphs, the influence of bulk alloy content on the metal release process

and comparisons between different stainless steel grades and their corresponding pure metals

will be exemplified with stainless steel grades of different microstructure (Papers II, VI, V)

immersed in the synthetic body fluid ALF.

_________________________________Chapter 3_________________________________

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~ 38 ~

3.1.1 There is a strong correlation between the bulk alloy content and the total

metal release process (Papers I, II, V, VI)

As generally known, the corrosion resistance of the stainless steel strongly depends on the

degree of chromium alloying. A steel earns the title “stainless” when the chromium content

exceeds 10.5 wt % [ISSF, 2005], however, such a low chromium content will not provide

sufficient protection in most environments, which at least requires 13 wt % for proper

corrosion protection. A low content of chromium will slow down the rate of repassivation of

the surface oxide and make the stainless steel more susceptible to corrosion attacks, hence

increasing the metal release rate. Another problem associated with a low chromium content is

the possible formation of chromium carbides. Chromium has a strong affinity for carbon and

can, at favourable conditions, easily form chromium carbides [Bardal, 2004] and thereby

deplete the neighbouring area from chromium. This is common for incorrectly heat treated

stainless steels with a carbon content higher than 0.03 wt %. For steels with a high chromium

content, such a depletion will have a less prominent influence as there is more chromium

available for reparation of the chromium film.

The grade with lowest chromium content, AISI 409 with only 11.4 wt % chromium, reveals

significantly higher release rates compared to other grades investigated of higher chromium

bulk content. The lowest metal release rates are observed for the highest alloyed grade, AISI

310 with 24.2 wt % chromium also with highest degree of total alloying content. However,

most of the grades with intermediate chromium content, 16-18 wt % are clustered at release

rates between 1 and 3 µg cm-2 week-1.

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 39 ~

0

1

2

3

4

5

10 12 14 16 18 20 22 24 26tota

l met

al re

leas

e / µ

g cm

-2w

eek-1

Cr content in bulk wt %

409

310

2205

304

201316L

430 4Ni

1Ni

ALFduplexausteniticferritic

Figure 10 Relation between measured average total metal release rates and the chromium bulk

content of nine stainless steel grades after immersion in ALF for 168 hours.

The results presented above illustrate that the metal release process for stainless steels is

complex, however correlated to the degree of alloying, but not simply explained by the

chromium bulk content. The total alloy content and other parameters including surface

properties and experimental parameters are of outmost importance. The replacement of some

alloying elements with the addition of others will influence the metal release process as will

be discussed in the following paragraph. To endure specific environments a wide variety of

alloying constituents are used to improve the performance and durability of stainless steels.

These changes simultaneously influence the metal release process. The most frequently used

alloying element after chromium is nickel. The addition of nickel stabilises the austenitic

structure and facilitates repassivation [ISSF, 2005]. Nickel is, however, expensive and also

debated from a health perspective. The nickel content can, for some applications, be reduced

by replacing it with other elements that will provide the stainless steel with similar properties.

Molybdenum is added to improve repassivation properties to grades such as AISI 316L,

designed for an improved protection against localised corrosion in chloride rich environments.

The addition of manganese, replacing some of the nickel, will maintain the austenitic

structure and reduce the cost, but the removal of nickel will also reduce the maximum

chromium content of the alloy possible to maintain the microstructure? [ISSF , 2005]. By

reducing the chromium content, the risk for corrosion is enhanced, and thereby most likely

also the degree of metal release. Depending on alloying content, stainless steels are produced

with different microstructures, austenitic, ferritic, duplex (austenitic-ferritic) or martensitic, as

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 40 ~

main bulk matrix. As the requested microstructure is obtained by using different alloying

elements, it can be difficult to establish if properties such as metal release depend primarily

on alloy content or if differences in microstructure will influence the metal release behaviour

and if so, to what degree.

Generated results illustrate that the austenitic grades in general have lower metal release rates

than the ferritic grades, Figure 10, with exception for grade 1Ni. The nickel content of grade

1Ni is very low and has almost completely been replaced with manganese. The higher metal

release rates may indicate that manganese is not as preferable as nickel for aiding

repassivation, an observation calling for further investigations. Despite a higher chromium

content, Figure 9, the duplex (austenitic-ferritic) grade, AISI 2205, shows similar metal

release rates compared with the six austenitic grades. Grade AISI 310, with the highest degree

of alloying, displays the lowest rate of metal release, Figure 10. These results clearly illustrate

that a high degree of alloying often is beneficial from a metal release perspective, in contrary

to a common belief that possible adverse health effects induced by corrosion increases with

increasing bulk content of alloy constituents.

3.1.2 The degree of metal release from stainless steels is not proportional to either

the bulk alloy content or to the behaviour of their pure metal constituents.

(Papers I, V)

An alloy consists of two or more metals where the intrinsic properties of the individual metals

have been changed during production. A stainless steel contains several different metals

where iron is the major constituent. Chromium is the main alloying component of every grade

of stainless steel whereas nickel is primarily present in austenitic and duplex stainless steels

and only at very moderate levels in ferritic and martensitic grades. Upon air exposure,

stainless steel forms a few nano-meter thin passive surface oxide rich in chromium and iron

[Olsson, 2003]. The passivating properties of this passive film are substantially improved

compared with the oxide formed on the pure alloy constituents, typically having poorly

adherent and highly defective surface oxides. The importance of the passivating properties of

the passive oxide on the metal release process is illustrated in Figure 11 for stainless steel

grade 316L immersed in ALF for one week.

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 41 ~

A relative comparison of the bulk alloy content and the proportions of released alloy

constituents clearly demonstrate a non-proportional relationship. The relative bulk (left)

content is significantly higher compared to the released fraction (middle). The degree to

which nickel is released is somewhat lower compared with the nickel content of the bulk. In

addition, the released fraction of alloy constituents compared to the surface oxide composition

differ even more (right).Observed differences between the alloy content and the degree of

metal release unambiguously prove that it is not possible to predict the degree of metal release

by the bulk composition alone.

To illustrate that stainless steel alloys have totally different properties compared to their pure

metal components also from a metal release perspective, a set of identical experiments were

performed in ALF for one week for the stainless steel alloy AISI 316L and its pure metal

components iron, chromium and nickel, Figure 12. The approach enables differences and

similarities between the materials to be illuminated, and clearly demonstrates that the pure

metals cannot be used to estimate the degree of metal release rates from stainless steel alloys.

Fe

Ni

Cr

bulk composition metal release

Fe

NiCr

Fe

Ni

Cr

surface oxide composition Figure 11 Relative proportions of iron, chromium and nickel in the bulk of stainless steel AISI

316L (left), released into ALF after one week of immersion (middle), and in the

outermost surface oxide measured by using XPS (right).

Iron and nickel are released at rates approximately 50 and 100 times higher, respectively,

from the pure metals compared with the alloy. Chromium is released at similar rates for the

stainless steel and the pure metal, except for the first day during which the release rate was

significantly higher from the stainless steel. This difference is probably related to a more

effective surface oxide on pure chromium acting as a barrier for metal release compared with

the gradually built-up of a surface oxide on stainless steel (Papers I and V).

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 42 ~

As a result of significant knowledge gaps related to metal release data on individual alloy

constituents from alloys, e.g. stainless steels, estimates on metal release from alloys have, by

environmental consultants among others, been made by multiplying the nominal bulk alloy

composition with existing metal release rates from pure metals. The consequence is illustrated

for stainless steel in Figure 13 and is based on data from its pure metals generated in the

comparative study presented above. Calculated release rates of iron and nickel are highly

erroneous, and significantly higher compared with actual measured release rates, whereas

calculated values of chromium release are about one order of magnitude lower than actual

release rates.

0

100

200

300

400

0 20 40 60 80 100 120 140 160 180

rele

ase

rate

/ µg

Fe

cm-2

wee

k-1

exposure time / h

Fe/50

AISI 3160

2

4

6

8

10

0 20 40 60 80 100 120 140 160 180rele

ase

rate

/ µg

Cr

cm-2

wee

k-1

exposure time / h

Cr

AISI 316

0

10

20

30

40

50

0 20 40 60 80 100 120 140 160 180rele

ase

rate

/ µg

Ni c

m-2

wee

k-1

Ni/100

AISI 316

exposure time / h Figure 12 Accumulated metal release rates, µg cm-1 week-1, from abraded stainless steel grade

AISI 316L and the pure metals iron, chromium and nickel immersed in ALF for 168

hours.

The results clearly illustrate the error in such an estimate and the importance of an improved

understanding of the metal release process for metal alloys.

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 43 ~

0

0.5

1

1.5

2

Fe Cr Nimet

al r

elea

se r

ate

/ µg

cm-2

wee

k-1

3199 211

0.0040.03

ALF

Measured

Calculated

Figure 13 Comparison between measured and estimated metal release rates of the main alloy

constituents iron, chromium and nickel from stainless steel based on the erroneous

approach to multiply the bulk content of each alloy constituent with metal release data

for the pure metal exposed to identical conditions.

3.2 The influence of surface oxide properties on the metal release process

Stainless steel is used in a multitude of outdoor constructions including the Atomium in

Brussels, Belgium, the Petronas Twin Towers in Kuala Lumpur, Malaysia and the Chrysler

building in New York, USA, all clad with stainless steel, Figure 14. The Chrysler building

was constructed during the late 1920s and has not required any special treatment for corrosion

protection or cleaning until 1995. The secret to the great performance of this building is the

protective surface oxide on the austenitic stainless steel cladding.

In the following paragraphs, the influence of surface oxide properties on the metal release

process will be discussed using generated data for AISI 316 during exposure to artificial rain,

describing the build up of the surface oxide and how the maturing of the oxide film affects the

metal release process (Papers I, VI). In addition, to illustrate the non-continuous metal release

process, possibly related to the formation of local metastable pits, generated data for 316L

immersed in ALF will be presented. The scientific argument for not illustrating the results

with artificial rainwater, as above, is related to too small currents involved in the process (pA)

to be measured using the noise technique (unpublished data).

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 44 ~

Figure 14 The Petronas Twin Towers in Kuala Lumpur, Malaysia, [Wikipedia], the Chrysler

Building in New York, USA, photo by David Shankbone, and Atomium in Brussels,

Belgium. [Wikipedia].

3.2.1 Exposure to any media results in an enrichment of the chromium content in

the outermost surface oxide (Papers I, II, III, IV, V, VI and VII)

The outermost surface oxide on a stainless steel is the last line of defence against corrosion

and prevention of metal release, which is strongly dependent on its relative chromium

content. Changes in this relative content of the outermost surface oxide of all stainless steels

exposed to, or in, the different media investigated were determined by means of XPS. The

results are compiled in Figure 15 for two duplex (5Ni, 1.5Ni) and four austenitic grades (AISI

304, AISI 316, 1Ni, 4Ni) all exposed to identical artificial rain events. All grades, except AISI

316, demonstrated an increasing relative content of chromium in the surface oxide upon

exposure to an eight-hour simulated rain event. Prolonged exposure to artificial rain results in

a gradual improvement of the relative chromium content and consequently a more protective

surface oxide with time. This is confirmed by decreasing metal release rates with time,

described further in paragraph 3.2.2. The initial surface oxide composition is higher in iron

compared with chromium. Since iron is predominantly released during the initial rain

volumes impinging the surfaces, the outermost surface becomes depleted in iron and enriched

in chromium.

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 45 ~

0

0.5

1

1.5

2

15 16 17 18 19 20 21 22 23tota

l met

al r

elea

se /

µg c

m-2

wee

k-1

Cr content in bulk wt %

1Ni

5Ni1.5Ni

304

4Ni

316

duplexaustenitic

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1.5Niduplex

304austenitic

prior to exposure after exposure

Cr/

(Cr+

Fe)

Figure 15 Total metal release rates related to the chromium bulk content for austenitic and

duplex stainless steels and corresponding enrichment of the relative chromium content

in the outermost surface oxide illustrated for AISI 304 (austenitic) and 1.5 Ni (duplex)

stainless steel prior to, and after 8 hours of exposure to artificial rain.

Similar to the findings in paragraph 3.1.1, the austenitic grades reveal reduced total metal

release rates with increasing chromium bulk content. However, the presence of other alloy

constituents such as molybdenum and nickel are essential ingredients to improve the

repassivation properties of each grade.

3.2.2 The metal release process is highly time-dependent (Papers I, II, IV, V, VI,

VII)

Previous investigations have shown the metal release process from different metals and

alloys, for example copper, zinc, brass, bronze, exposed to different media to be highly time-

dependent [Bertling et. al., 2006, He et. al. 2000, He et. al. 2002c, Herting et. al. 2008, Brune

and Hultquist, 1985, Karlén et. al. 2002, Zhang et. al. 2002]. Similar observations have been

made for the release process of the main alloy constituents of stainless steels, independent on

media and surface properties. All fourteen stainless steel grades investigated within this thesis

displayed initially high release rates of different magnitude followed by a consecutive

decrease to lower and relatively constant release rates. This time-dependent behaviour is

illustrated in Figure 16 for iron, chromium and nickel released from stainless steel grade AISI

316 during an eight hour continuous rain event. The time-dependent release process is not

unique for thin film exposures during simulated rain events, but also relevant during

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 46 ~

immersion in all fluids investigated within this thesis, i.e. ALF, Gamble’s solution and acetic

acid.

Decreasing metal release rates with time are attributed to the formation and stabilisation of the

passive chromium rich surface oxide on stainless steels, gradually becoming enriched in

chromium, independent of stainless steel grade and exposure/immersion procedure.

Freshly abraded surfaces of stainless steel grade 316 were prepared to obtain more uniform

surfaces to increase the reproducibility of the experiments and to enable studies of the gradual

formation of a chromium-enriched surface oxide of improved passivating properties with

time. In contrast to the air-aged as-received surface showing decreasing release rates of

chromium with time, the abraded surface of the same grade revealed an initially increasing

release rate, reaching a maximum after a few hours and subsequent decreasing rates, Figure

17.

0

5

10

15

20

0 1 2 3 4 5 6 7 8exposure time / h

met

al r

elea

se r

ate

/ µg

cm-2

wee

k-1

Fe

Ni

Cr

Figure 16 Time dependent release rates of chromium, nickel and iron from as-received grade

AISI 316 stainless steel exposed to an eight hour continuous rain event.

Differences in the metal release process between newly formed surface oxide on the abraded

surface and the air-aged, more stable surface oxide on the as-received surface provide

important knowledge on how the surface oxide is formed. Surface analytical studies with XPS

of the as-received surface revealed an initially high relative chromium content of the surface

oxide, a content which only changed moderately with time. The relative chromium content of

the surface oxide on the freshly prepared abraded surface, was significantly lower, however

continuously increasing with time, improving its passivating properties, thereby reducing the

metal release rate. Repeated and prolonged exposures result in the gradual improvement of

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 47 ~

the passivating properties and the chromium content of the surface oxide, with decreasing

release rates as a consequence.

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5 6 7 8exposure time / h

rele

ase

rate

/ µg

Cr

cm-2

wee

k-1

as-received

abraded

0

0.1

0.2

0.3

0.4

0.5

0 0.5 2 5 8

Cr/

(Cr+

Fe)

abraded surfaces

exposure time / h Figure 17 Time-dependent release rates of chromium from as-received and abraded surfaces of

stainless steel grade 316 exposed to a continuous eight-hour rain events (left) and

corresponding changes in the relative chromium content of the outermost surface

oxide on abraded surfaces

3.2.3 The surface oxide temporarily breaks down locally causing alloy constituents

to be released

This oxide of stainless steel is believed to consist of two layers, an inner layer rich in

chromium and iron oxides, and an outermost surface layer mainly composed of chromium

hydroxide [Clayton and Olefjord, 2002]. The presence of the passive film is responsible for

the beneficial corrosion resistance of stainless steels and acts as a fairly efficient barrier

preventing metal release, depending on environment. Pure chromium forms a similar surface

oxide at ambient conditions, whereas pure iron and nickel typically forms thicker surface

oxides being highly defective and loosely attached to the bulk matrix [Wegrelius, 1995]. The

passivating properties of the latter oxides are substantially lower compared to the oxide

formed on stainless steel and on chromium metal. The surface oxides present on stainless

steels and on the pure metals are schematically illustrated in Figure 18. The layer adjacent to

the passive surface oxide, the alloy surface layer, is believed to be enriched in iron and nickel,

and of other alloying constituents such as molybdenum [Olefjord and Clayton, 1991, Femenia

2004]. As nickel is rarely observed in the passive film, but often detected in the metal release

solution, it has to pass through the surface oxide before being released. This indicates that the

passive film contains defects, possibly varying in time and space, facilitating passage.

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 48 ~

Stainless steel matrix

Alloy surface layerSurface oxide

Pure metal matrix

Surface oxide

Figure 18 Schematic pictures of the surface oxide present on stainless steel (left) and on pure

metals (right)

In addition, formation of metastable pits on the surface, temporarily rupturing the passive

film, allowing a short burst of released alloy constituents before it is re-passivated, is believed

to be a major process for the metal release phenomena [Burstein and Marshall, 1984, Burstein

et. al. 1993, Burstein and Vines, 2001, Gonzáles-García et. al. 1991, Pistorius and Burstein,

1992, 1994].

Pitting of stainless steel is known to occur especially in chloride containing environments.

Nucleation of corrosion pits does not, however, necessarily mean that the pits propagate into

fully developed propagating pits. Depending on the ambient environment, metastable pits can

repassivate and become dormant. Several studies have proven the presence of metastable pits

[Burstein and Marshall, 1984, Burstein et. al. 1993, Burstein and Vines, 2001, Pistorius and

Burstein, 1992, 1994] by registering current transients, electrochemical noise, during

immersion of metals and metal alloys in various media. However, the connection between

nucleation of pits and metal release has not yet been established. In an attempt to elucidate

this connection, the formation of metastable pits recorded on polished surfaces of stainless

steel AISI 316L immersed in ALF, using the electrochemical noise technique. This method is

extremely sensitive able to measure small current transients that occur when a pit is nucleated.

The possibility to detect these events depends on the possibility to obtain an as low

background noise as possible at the same time as the corrosivity of the electrolyte is

aggressive enough to generate transients of measurable amplitudes. For this purpose artificial

rain was not suitable as immersion media as it has a very low conductivity, 40 µS cm-1, and is

fairly non-aggressive with a chloride content of 10 µM. Current transients occurring during

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 49 ~

exposure in this media were too few and often too small to be separated from the background

noise.

To enable measurable metastabile pit formation, studies of transients formed on stainless steel

316L was performed in ALF at 39 °C with an increased chloride content (0.16 M) and a

higher conductivity, 15 mS-1 than defined in Papers II, III, V and VI (0.06 M), but similar to

other commonly used synthetic body fluids, such as Hank’s and Ringer’s solution [Burstein

and Liu, 2007, Assis et. al. 2005].

Experiments performed using a freshly polished microelectrode of AISI 316L at open circuit

potential display an initial increase in the number of transients (number of metastable pits)

recorded, reaching a maximum after a certain time period followed by a decreasing number of

events per time period. Observed results reveal a similar scenario as observed and measured

for the release of chromium and iron from freshly abraded surfaces of AISI 316 exposed to

artificial rain, see Figure 17 (left).

0

50

100

150

200

250

300

450 900 1350 1800 2250 2700 3150 3600 4050 4500 4950 5400

ITME_14OCPT=39±1 °C143 Hz2500 P

0.16 M Cl-

Ag/AgCl

num

ber

of tr

ansi

ents

time / s Figure 19 Number of current transients per time interval, equivalent to numbers of metastable

pits, during immersion of a 316L microelectrode in modified ALF at 39 °C at open

circuit potential.

Metal release studies performed within the context of this thesis have illustrated the metal

release process to be highly irregular with time (Papers V, VI and VII). Immersion of abraded

AISI 316L in ALF revealed most metal to be released during the first few hours of

immersion, after which the metal release process was lower but highly irregular in time and

magnitude, Figure 20 (top). These observations are in agreement with observed variations in

time of the number of transients registered (number of metastable pits) during successive time

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 50 ~

periods, which strongly implies that the release process may be governed, at least partly, by

this kind of process.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0

0.002

0.004

0.006

0.008

0.01

0-1 h 1-4 h 4-8 h 8-24 h 24-48 h 48-168 hmom

enta

ry r

elea

se r

ate

mgc

m-2

wee

k-1

FeCr

Fe Cr

exposure time / h

050

100

150

200

250

300

350

400

450 900 1350 1800 2250 2700 3150 3600 4050 4500 4950 5400 5850 6300 6750 7200

num

ber

of tr

ansi

ents

time period / s Figure 20 Momentary metal release rates calculated from accumulated release rates of iron and

chromium during immersion of abraded AISI 316L in ALF at 37 °C (top) and number

of transients registered per time period (number of metastable pits initiations) during

immersion of an AISI 316L microelectrode in modified ALF at 39°C at an applied

potential of 0.4 V (bottom).

3.3 The influence of surface finish and microstructure on the metal release

process

The visual and aesthetic appearance of a surface is a common way to select a specific surface

finish for a given application. For a cooking pot for example, surfaces in contact with the food

stuff should be smooth and easily cleaned, whereas outside surfaces of the pot can receive a

decorative surface finish, Figure 21. There is, however, a lot more to a surface than simply its

appearance.

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 51 ~

In the following paragraphs, the influence of surface finish on the metal release process,

demonstrated with differences in surface roughness and electrochemically active surface

areas, is illustrated with results generated for stainless steel grades AISI 430 immersed in food

related media (Paper IV).

Figure 21 Stainless steel cooking pot with its interior and exterior of different surface finish,

each fulfilling its purpose, smooth and easily cleanable, and decorative, respectively.

The influence of inhomogeneities present in the bulk matrix, and the subsequent effect on the

metal release process is in addition illustrated by results for the stainless steel grade AISI 420

in acetic acid (Paper VII).

3.3.1 The surface finish and other surface parameters have a considerable

influence on the metal release process (Papers III, IV and VII)

The influence of surface finish on the metal release process has been thoroughly investigated

for stainless steels used in different applications. The studies include commercially available

finishes, modified commercial finishes and company specific surface finishes generated for

specific applications.

The surface area of a given surface varies depending on its surface finish, and is hence

different from the geometric surface area of the sample. The concept of surface area is not

straightforward since it depends not only on its surface finish, but also on other parameters

including the presence of surface inhomogeneities, and areas available for e.g. metal release.

A selection of different methods was selected to elucidate its complexity and if possible,

relate the results to the degree of metal release. The results are illustrated for stainless steel

grade 430 of different company specific surface finishes, glossy and line, used for the interior

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 52 ~

and exterior, respectively of cocking pans and pots. Metal release data reflect 10 days of

immersion into 3 vol-% acetic acid at 40°C. An abraded surface of the same material was

used as a reference (Ra: 0.23±0.007 µm).

Surface roughness, Ra, measurements were performed on all surfaces to establish and quantify

differences in surface topography (confer Table 1 in experimental). In addition, the relative

electrochemically active surface area, assumed to primarily take part in the metal release

process, was estimated by applying EIS measurements. This approach is justified since this

surface area is directly proportional to the capacitive response of the interface between the

electrolyte and the metal surface, but requires the assumption that all surfaces possess the

same native surface oxides of the same dielectric constant and film thickness [Norlin, 2005].

Generated results were only used for comparative purposes without any attempts made to

quantify the electrochemically active surface area. The results are illustrated in Figure 22 with

all results normalised to the abraded surface finish, acting as a reference surface.

The Glossy surface revealed significantly lower electrochemically active surface area and a

lower surface roughness (Ra: 0.08±0.006 µm) compared with the abraded surface, and

consequently significantly lower metal release rates. However, even though the measured

surface roughness and the electrochemically active surface area was significantly higher for

the Line surface (Ra: 0.37±0.04 µm) compared with the abraded surface (Ra: 0.23±0.007 µm),

rather similar total metal release rates were observed.

Generated results illustrate that observed total metal release rates are influenced both by the

surface roughness and by the electrochemically active surface area, both showing large

discrepancies compared with the geometric area (quotient of 1)

Compared to smooth surfaces, a rough surface finish provides more potential nucleation sites

for localised corrosion as the total surface area increases. Similar conclusions were confirmed

by testing grade AISI 304 of different surface finishes and surface roughness in artificial rain

(unpublished data).

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 53 ~

EIS measurements of the relative electrochemically active surface area were also performed

to investigate whether this surface area would change during ageing of a freshly abraded

stainless steel surface (AISI 430), stored at ambient room temperature in a desiccator for 48

hours and one year, respectively. The results are illustrated in Figure 23 normalised to the

response of the sample aged for 1 year.

0

0.5

1

1.5

2

Glossy Line

surface roughness total metal release

surf

ace

finis

h X

/ su

rfac

e fin

ish

abra

ded

Surface finish X

electrochemically active area

metal release

Figure 22 A relative comparison between the surface roughness (Ra), the electrochemically

active surface area, and total metal release rates for stainless steel AISI 430 of

different surface finish immersed in acetic acid for ten days. Each parameter is

normalised to the response of an abraded surface of the same material exposed to

identical conditions.

The electrochemically active surface area is significantly higher (1.5 times) for a freshly

polished surface compared to the aged surface. No large differences were however observed

between the sample aged for 48 hours and one year respectively. This indicates that the

characteristics of the surface oxide formed within a few days do not change significantly upon

prolonged air exposure from an active surface area perspective. The results are in agreement

with literature findings showing the passive film on stainless steel to achieve its best

passivating properties if allowed to form and stabilise at room temperatures [Hultquist et. al,

1986] Previous investigations have also illustrated improved passivating properties upon

ageing with lower metal release rates as an obvious consequence (Papers I-VII) [Leygraf et.

al, 1979, Olefjord et. al. 1985].

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 54 ~

The surface finish of stainless steel grade AISI 420 was deliberately modified to investigate if

the way of polishing could influence the barrier properties of the surface oxide and thus also

the degree of metal release.

0

0.5

1

1.5

2

freshly abraded aged 48 hours

surf

ace

finis

h X

/ su

rfac

e fin

ish

aged

1 y

ear

Figure 23 A relative comparison between the electrochemically active surface area of freshly

abraded surfaces of stainless steel grade AISI 430 air-aged compared to identical

samples aged in a desiccator for 48 hours. Both samples are normalised to the

electrochemically active surface area of samples aged for one year at identical

conditions.

The results clearly illustrate that polishing by creating high surface temperatures (obtained by

applying a high pressure and lack of polishing agents between the polishing wheel and the

surface) is beneficial from a metal release perspective. This procedure seems to improve the

passivating properties of the surface oxide compared to a polishing procedure using excess of

polishing agents and frequently changed contact points between the surface and the polishing

wheel. The results are illustrated in Figure 24 showing total metal release rates after 10 days

of immersion in acetic acid at 40°C.

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 55 ~

02468

10121416

high pressure low pressure

tota

l met

al r

elea

se r

ate

/ µg

cm

-2 w

eek-1

Figure 24 Differences in total metal release rates for stainless steel grade 420 of two surface

finishes immersed in acetic acid at 40° for ten days. The surfaces are deliberately

created by applying different ways of polishing with excess or insufficient polishing

agents and opposed applied pressures to generate differences in surface temperatures

during the polishing procedure.

3.3.2 A highly inhomogenous microstructure favours high metal release rates

Grades AISI 420 knife steel and AISI 420 scissor steel displayed surprising features during

immersion in acetic acid. As low alloyed martensitic stainless steels, total alloy content of

14.4 wt % (13.6 wt % Cr) and 13.4 wt % (12.55 wt % Cr) for the knife- and scissor steel

respectively, their metal release rates were expected to exceed those of the ferritic stainless

steel grade AISI 430 (total alloy content 16.8 wt %, 16.0 wt % Cr)) tested at the same

experimental parameters. However, some of the samples, especially for the scissor steel,

displayed significant changes during the testing procedure, the acetic acid changed colour

from perfectly clear to a deep red. Analysis proved these acetic acid samples to contain

extremely high released concentrations of chromium and iron and the corresponding surfaces

of the stainless steel were covered with a blackish non-adherent layer of corrosion products,

Figure 25. The outermost surface was found, by analysis with XPS, to be enriched in iron and

copper and depleted in chromium. A closer study of the martensitic microstructure revealed

bands of ferrite and small chromium carbide particles in the knife steel. The scissor steel

contained amounts of inclusions and tempered martensite. The inclusions were rich in

manganese and sulphur and silicon and oxide and were distributed unevenly through the bulk

_________________________________Chapter 3_________________________________

____________________________________________________________________

~ 56 ~

matrix. As some samples displayed release rates that were considered normal, i.e. no

discolouration of the acetic acid and release rates in the vicinity of those measured from grade

AISI 430, it is believed that the samples that showed extreme metal release rates revealed a

microstructure with a very high number of inclusions and other inhomogeneities.

Unfortunately it was not possible to study each individual sample before and after immersion

in acetic acid to document characteristics and changes as the surface would have been

compromised due to sample preparation and no longer have the predetermined surface

finishes. Inclusions and defects are well known to decrease the corrosion resistance [Baroux,

2002, Stewart and Williams, 1992, Strehblow, H. H., 2002, Wranglén, 1969, 1974] and

disrupt the protective surface oxide and are therefore believed to reduce the passivating

properties of the surface oxide, hence increasing the metal release rates.

0

10

20

30

2500

3000

Cr

Fe

Cr

FeS

Cu

black

shiny

high pressure low pressure

met

al re

leas

e ra

te /µ

g cm

-2w

eek-1 Surface composition

Surface composition

Figure 25 Schematic figure of observed total metal release rates from shiny and blackish

surfaces of stainless steel grade AISI 420 of different modified surface finishes

immersed in acetic acid at 40 °C for ten days.

_________________________________Chapter 3_________________________________

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~ 57 ~

3.3.3 Changes in experimental conditions highly influence metal release

Temperature, pH and mechanical influence are some parameters well known to influence the

corrosion resistance of metals in our surroundings [Baroux, 2002]. Recent studies have also

been performed to evaluate their influence on the metal release process from various metals

and alloys in different media, such as atmospheric exposure of brass, copper and zinc in

urban, rural and marine environments [Paper IV, Odnevall Wallinder et. al. 2002, He et. al.

2000].

The influence of temperature and surface area to solution volume ratio on the metal release

process has been given special attention within this thesis, in particular when related to food

contact applications.

Immersion of samples into any kind of aqueous media will bring a certain surface area of the

sample to come in contact with a defined volume of the media. Depending on whether the

media is agitated or stagnant the transport of fresh media will be restricted and the freshness

will decrease as metals are released from the sample. In the Italian law text, D.M. 21-03-1973,

ratios of 0.5 to 2 are recommended for simulation of stainless steel in food contact giving

rather large degrees of freedom for a specific investigation. Detailed studies of differences in

metal release rates using two extreme ratios, 0.5 and 2 cm-1, and two intermediate ratios, 0.9

and 1.3 cm-1 disclosed a strong dependence on the ratio, Figure 26. As anticipated, high ratios,

large surface area and small solution volumes, resulted in higher released metal

concentrations than low ratios. Released concentrations at the ratio 0.5 cm-1 was more than

three times less compared with 2 cm-1.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 0.5 1 1.5 2 2.5

Cr

rele

ase

/ mg

L-1

(surface area)/(solution volume) / cm-1

Figure 26 Released concentrations of chromium from grade AISI 430 immersed in acetic acid at

different surface area to solution volume ratios.

_________________________________Chapter 3_________________________________

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~ 58 ~

Changes in temperature will affect diffusion rates, chemical reactivity etc. of surfaces of

metals and alloys and can potentially cause them to perform unexpectedly if not kept

constant. The influence of temperature on pitting potential, and consequently corrosion

resistance of stainless steels, has been thoroughly studied [Outokumpu Corrosion Handbook,

2005] providing important information about the performance of these alloys in an assortment

of media. Few records exist on metal release rates from stainless steels at different

temperatures in various media, but that does not reduce the importance of correct and up to

date information. Stainless steel grade AISI 430 was immersed in 3 vol % acetic acid at 20,

40 and 60 °C for ten days (Paper IV). The surface area to solution volume ratio was kept at 1

cm-1 for each sample to minimise the risk of error due to the large influence of that parameter

on metal release. As expected, the metal release rates increased with increasing temperature

following an almost linear relationship with highest release rates at 60 °C consecutively

decreasing for 40 and 20 °C.

_________________________________Chapter 4_________________________________

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~ 59 ~

Chapter 4 Summary The results produced within the framework of this thesis aim at improving the knowledge on

the mechanisms behind the metal release process from stainless steels by using a

multidisciplinary approach. Quantitative bioaccessibility data is generated in combination

with surface analytical studies of changes in surface composition and passivating properties

on stainless steels. Key aspects discussed and investigated within this thesis are metal release

rates and their time-dependence from different stainless steels exposed to various media, the

difference in metal release rates between stainless steel alloys and the pure metals, and the

importance of surface finish and experimental parameters.

Main conclusions are schematically illustrated in Figure 27. Detailed results are provided in

Papers I-VII.

Figure 27 Main conclusions from research projects within this thesis, acquired by a multi-

analytical approach.

_________________________________Chapter 5_________________________________

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~ 60 ~

Chapter 5 Future work Even though several aspects of the metal release process have been investigated within this

thesis, questions remain that the author would like to address.

For a further understanding of the corrosion and metal release processes of different stainless

steel investigations of all grades exposed to identical experimental parameters would be

preferable for further understanding of the influence of the degree of alloying and of

microstructural differences.

The composition of the oxide film has been investigated by means of XPS. The way the

measurements were performed within the framework of this thesis, no investigations of

changes in composition as a function of depth and the adjacent alloy surface layer were

performed. Further work on the compositional variations in the oxide film and additional

analysis using TEM would improve further understanding of the correlation between the

oxide film and the metal release process.

The role of manganese was investigated in Paper VI, however, to acquire a better

understanding of how manganese influences the metal release process further investigations

are required using other stainless steel grades alloyed with manganese, using a thorough

surface analytical approach.

The potential of using electrochemical noise combined with measurements of metal release

rates for an improved understanding of how the metal release process occur needs to be more

thoroughly investigated. Measuring metal release at time intervals coordinated with

electrochemical noise measurements at identical exposure conditions would be a proper

continuation of the investigations briefly described in this thesis.

_________________________________Chapter 6_________________________________

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~ 61 ~

Chapter 6 Acknowledgement The making of this thesis had not been possible without the help and support from several people that I wish to extend my gratitude to.

• My main supervisor Associate Professor Inger Odnevall Wallinder, I could not have asked for a better one, your guidance and your encouragement have been very valuable during this time.

• I am very grateful to Professor Christofer Leygraf for allowing me to join the corrosion division and for providing such an inspiring workplace, I could not have had a better place to spend these past five years.

• Klara, the best roommate possible, through good times and bad times, you have always been great, putting up with my never ending chatter and sometimes short memory.

• My past roommates, Sofia and Anna for showing me what roommates are for and for guiding me.

• My co-authors Sara, David and Babak for performing exciting research projects together with me.

• All my brilliant colleagues, past and present, at the Division of Corrosion Science, no one mentioned no one forgotten, you have given me such good laughs and great times.

• Professor Tim Burstein, of the Corrosion and Electrochemistry group, Cambridge University, for giving me the opportunity to study electrochemical noise and spend time with his group learning about other ways of studying corrosion.

• Leif, Connor, Eric, Monica, Ben and Gang, Corrosion and Electrochemistry group, Cambridge University, thanks for making my time in Cambridge such a brilliant experience.

• Pat Koundakjian and Tony Newson, Eurofer, and Staffan Malm, ISSF, for all your help.

• Jan Andersson and Lennart Ringsjö at IKEA of Sweden AB for all your help and providing interesting projects.

• Perkin Elmer for technical support and being there Friday afternoons when help was needed.

• Eurofer, the International Stainless Steel Forum (ISSF), IKEA Sweden AB and Outokumpu are greatly acknowledged for their financial support.

• Jernkontoret for providing funding for my stay in Cambridge and financing my thesis work.

• Bengt Andersson and Birgitta Olsson, SLU, Umeå, James Oliver and Pontus Rydgren at Swerea KIMAB, Mikael Sundin, YKI, for helping me analyse my samples.

• My parents, Anita and Carl-Johan, for all your support and encouragement during my studies It would not have been possible without you.

• All my friends at Kalvsvik/Lövhagen for providing the perfect place for recreation. • Finally, my fellow Goldfish, thank you for supporting me during the short but happy

memories that we share.

_________________________________Chapter 7_________________________________

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~ 62 ~

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