Zinc(II) Silicate based pigments for Corrosion Inhibition of ...

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Zinc(II) Silicate based pigments for Corrosion Inhibition of

Cold Rolled Steel

A thesis submitted to the Division of Research and Advanced Studies at the

University of Cincinnati

in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE

In the Materials Science and Engineering Graduate Program,

College of Engineering and Applied Science

by

Prajyula Pemmaraju

November 2016

Bachelors of Science, Mechanical Engineering,

Vasavi College of Engineering, Hyderabad, India 2012

Thesis Advisor and Committee Chair: Dr. Relva C. Buchanan

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ABSTRACT

In solution form, zinc silicate and phosphates are widely used as corrosion inhibitors for prevention

of corrosion, especially in cooling water systems and pipelines. Zinc silicate in powder/pigment

form (derived from reaction of ethyl silicate and zinc tailings) has been extensively used in organic

based primer coatings, for long-term corrosion protection of large steel structures in harsh

environments. However, not much work has been done on characterizing these pigments on the

basis of their structure and composition, which specially relate to their corrosion inhibiting action.

This research investigates, therefore, the corrosion inhibition behavior of a novel Zinc silicate –

pigment system, prepared using different synthesis techniques, with focus on the characteristics of

the pigment along with the corrosion mechanism on cold rolled steel substrates. The aim was to

develop a non-toxic, environment friendly, and low cost alternative zinc silicate processed

pigment.

The unique feature of this work is the development of the zinc (meta) silicate pigments using a

combination of Zinc sulfate and Sodium silicate powders with solution based co-precipitation

technique. By controlling the stoichiometry temperature, time, and stirring regime, followed by

controlled drying and heat treatment. The particle size and structural characteristics of the powders,

were thereby determined, with respect corrosion inhibition efficiency, as determined from

electrochemical analysis and immersion testing. Further the structural study of these pigments

were carried out by standard characterization techniques such as XRD, FTIR, SEM and EDAX to

analyze the crystallite size, structural bond vibrations and morphological changes which brings out

the inhibition effect. Optical measurements of the immersed samples show the corrosion inhibition

of these pigments. This study explains the structural dependence of the inhibitors on the

electrochemical behavior of the zinc silicate powders produced and also gives an optimum way of

producing zinc silicate powders which were proved to be the best suited phase for anti-corrosive

behavior.

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ACKNOWLEDGEMENT

I would like to take this opportunity to express my sincere respect and gratitude to my advisor Dr.

Relva C Buchanan for providing continuous guidance throughout my research work. I would like

to thank Dr. Rodney Roseman and Dr. John Yin for taking time to review my work and being a

part of my thesis committee.

I would like to thank my research mate Shivani and Yongkun who taught all the processes and

guide me throughout. Siddhartha and Dhaval for their continuous encouragement and help with

analysis. Amulya for asking me questions and having thoughtful discussions. Nitin, Dushyant and

Aman for always providing a helping hand when needed.

I would also like to thank my parents for always being loving and supportive. My sister and my

boyfriend who were a strong support both emotionally, financially and without whom I wouldn’t

have gotten so far. I would like to thank all my cousins and friends here, who are my extended

family in US.

I would take this opportunity to mention about Sudheera, my best friend and the spark of

inspiration in my life, to make a change and dream big.

Also I would like to thank the Almighty for what I am today.

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Table of Contents

ABSTRACT……………………………………………………………………………………………………………………………………………… 2

ACKNOWLEDGEMENT…………………………………………………………………………………………………………………………….3

TABLE OF CONTENTS………………………………………………………………………………………………………………………………4

LIST OF FIGURES …………………………………………………………………………………………………………………………………… 6

LIST OF TABLES……………………………………………………………………………………………………………………………………… 7

LIST OF ABBREVIATIONS…………………………………………………………………………………………………………………………7

1. INTRODUCTION ................................................................................................................................ 9

1.1. Significance of Corrosion ................................................................................................... 9

1.2. History of Corrosion ........................................................................................................... 9

1.3. Electrochemical behavior of Aqueous Corrosion ............................................................. 10

1.4. Kinetics of Equilibrium at Electrode: ............................................................................... 12

1.5. Activation Polarization: .................................................................................................... 12

1.6. Polarization Resistance ..................................................................................................... 19

1.7. Corrosion Measurements: ................................................................................................. 21

1.8. Concentration Polarization: .............................................................................................. 22

1.9. Corrosion of Cold Rolled Steel ......................................................................................... 23

1.10. Corrosion control in steels: ............................................................................................... 25

1.11. Corrosion Inhibitors .......................................................................................................... 26

1.12. Water based corrosion inhibitors ...................................................................................... 27

1.13. Zinc and Silicates as Corrosion Inhibitors review: ........................................................... 28

1.13.1 Mechanism of Zinc Silicate primer: ............................................................................................ 28

1.13.2 Pigment preparation techniques:.................................................................................................. 31

2. RESEARCH PLAN............................................................................................................................ 34

2.1. Objective: .......................................................................................................................... 34

2.2. Approach: .......................................................................................................................... 34

3. EXPERIMENTAL ............................................................................................................................. 36

3.1. MATERIALS:................................................................................................................... 36

3.1.1. Precursor materials: ...................................................................................................................... 36

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3.1.2. Substrate ....................................................................................................................................... 36

3.1.3. Corrosive Environment ................................................................................................................. 36

3.2. Pigment Processing: .......................................................................................................... 36

3.2.1 Co- Blending: ................................................................................................................................ 36

3.2.2 Co- Precipitation Technique: ......................................................................................................... 37

3.3. Substrate Preparation: ....................................................................................................... 38

3.4. Electrochemical Characterization: .................................................................................... 38

3.4.1 Evaluation of corrosion inhibition of zinc metasilicate ................................................................. 38

3.4.2. Echem Analyst .............................................................................................................................. 39

3.5. Structural Characterization: .............................................................................................. 39

3.5.1: Thermogravimetric Analysis: ....................................................................................................... 39

3.5.2: X- Ray Diffraction: ....................................................................................................................... 40

3.5.3 Fourier Transform Infrared Spectroscopy (FTIR) ......................................................................... 41

3.5.4. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy ............................. 42

3.5.5. Immersion and Optical Microscopy: ............................................................................................ 42

4. RESULTS .......................................................................................................................................... 43

4.1. TGA: ................................................................................................................................. 43

4.2. ELECTROCHEMICAL ANALYSIS: .............................................................................. 44

4.2.1. Electrochemical Results: .............................................................................................................. 45

4.3. FTIR ANALYSIS ............................................................................................................. 55

4.4. XRD ANALYSIS ............................................................................................................. 59

4.5. SEM: ................................................................................................................................. 66

4.6. Immersion Testing: ........................................................................................................... 73

5. DISCUSSION: ................................................................................................................................... 75

6. CONCLUSION: ................................................................................................................................. 77

7. SUGGESTIONS FOR FUTURE WORK……………………………………………………………………………………………..77

8. REFERENCES………………………………………………………………………………......... 78

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LIST OF FIGURES

Figure.1.1 Schematic for equilibrium state of metal

Figure 1.2 Schematic Representation: Free energy curve of the electrode for activation over potential. (---)

polarized state, (---) equilibrium state

Figure 1.3 Mixed Potential and Corrosion for zinc in acid solution

Figure 1.4 Polarization curve with Tafel Extrapolation

Figure 1.5 Illustration for polarization resistance measurement

Figure 1.6 Corrosion of Metal M under diffusion control

Figure 1.7 Mechanism of zinc rich coatings for corrosion inhibition

Figure 1.8 Mechanism of Corrosion inhibition of zinc silicate coatings

Figure 1.9 Schematic of the top-down and bottom-up approach with the techniques used

Figure 2.1 Inhibitor Co-Blending Processing Procedure

Figure 4.1. Thermogravimetric analysis of co-precipitated Zinc silicate pigment

Figure 4.2 Potentiodynamic Curve of 1% NaCl + Coblended ZnO:SiO2 at 80°C

Figure 4.3 Potentiodynamic curve of 1% NaCl + Coblended pigment at 450°C

Figure 4.4 Potentiodynamic curve of 1%NaCl + Coprecipitated pigment at 80°C

Figure 4.5 Potentiodynamic curve of 1% NaCl + coprecipitated powder at 450°C

Figure 4.6 Potentiodynamic curve of 1%NaCl + comparison of Coblended and Coprecipitated powders

Figure 4.7 Potentiodynamic curve of 1%NaCl + coprecipitated pigment with stirring at 80°C





Figure 4.12 Potentiodynamic curve of 1%NaCl + comparison of all the temperatures of copreciitated powders

Figure 4.13 Potentiodynamic curve of 1%NaCl + coprecipitated pigment with 1:3 ratio of precursors


Figure 4.15 Potentiodynamic curve of 1%NaCl + comparison of different ratios

Figure 4.16 FTIR analysis of the processed ZS powders with change in processing techniques

Figure 4.17 FTIR for ZS inhibitors at different heat treatment temperatures

Figure 4.18 JCPDF of zinc silicate (ZnSiO3)

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Figure 4.19 XRD of coprecipitated pigment with Stirring at 80°C

Figure 4.20 XRD of coprecipitated pigment without Stirring at 80°C







Figure 4. 27 Cummulative XRD of all the powders at different temperatures

Figure 4.28 Coprecipitated Zinc silicate at 500X maginifcation for powders sintered at 80°C

Figure 4.29 Coprecipitated Zinc silicate at 12000X magnification for powders sintered at 80°C









Figure 4.38 EDS of the powder at 80°C where the composition of the pigment is measured

Figure 4.39 Graph showing the elements present in the pigment

Figure 4.40 Optical microscopy samples in 1% Nacl solution at 100 μm a) with the inhibitor b) without

inhibitor

Figure 4.41 Optical microscopy samples in 1% Nacl solution at 50 μm a) with the inhibitor b) without

inhibitor

Figure 4.42. Optical microscopy samples in 1% Nacl solution at 20 μm a) with the inhibitor b) without

inhibitor

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LIST OF TABLES

Table 1 Icorr values for CRS from Potentiodynamic scan with different temperatures

Table 2 Polarization resistance for CRS with different temperatures

Table 3 Corrosion Rate for CRS from Potentiodynamic Scan for different temperatures

Table 4. Icorr values for all the pigments at different temperatures.

Table 5. Icorr values for the pigments prepared out of precursors with different stoichiometry

Table 6. FTIR peaks for bonds present in the zinc silicate pigment

Table 7 Crystallite sizes of the zinc silicate pigment at different sintering temperatures

Table 8. The percentage composition of the elements in the pigment

LIST OF ABBREVIATIONS

CRS Cold Rolled Steel

UNHT Un-Heat Treated

HT Heat Treated

ZS Zinc Silicate

PTD Potentiodynamic

ZnSO4 Zinc sulfate

Na2SiO3 Sodium metasilicate

SEM Scanning Electron Microscopy

XRD X-ray Diffraction

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1. INTRODUCTION

1.1. Significance of Corrosion

Corrosion is the destructive attack of a material by reaction with its environment. The serious

consequences of the corrosion process have become a problem of worldwide significance. In

addition to our everyday encounters with this form of degradation, corrosion causes plant

shutdowns, waste of valuable resources, loss or contamination of product, reduction in efficiency,

costly maintenance, and expensive overdesign. It can also jeopardize safety and inhibit

technological progress [1].

1.2. History of Corrosion

The word “corrode” is derived from the Latin word rodere, meaning “to gnaw.” The term “corrosion”

entered the French language in 1314 to designate the act of gnawing, or wearing away because of chemical

effect. The term did not formally enter the English lexicon until the beginning of the 19th century, at which

point it was first adopted into medical vocabulary.

Corrosion is not an easy process to define. It is either a slow, progressive or rapid deterioration of

a metal's properties, such as appearance, surface aspects, or mechanical properties, under the effect

of the surrounding environment (seawater, water, various solutions and organic environments).

Previously, “oxidation” was a more commonly used term than corrosion. The former was a more

accurate term as corrosion usually consists of an electrochemical reaction during which metal is

oxidized, transforming into the state in which it existed in the mineral. Pure metal, with the

exception of noble metals, is thermodynamically unstable at normal conditions and tries to return

to its oxide state. [3]

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Corrosion phenomena have baffled humans since the discovery of metals. Pliny the Younger was

already complaining about the rusting of Roman soldiers' weapons. Scientists began investigating

corrosion in the early 19th century with Nicholson and Carlyle's discovery of the electrolyte

decomposition of water due to the electric current supplied by a galvanic battery.

Humphrey Davy established a relationship between the production of electricity and the oxidation

of zinc, in which one of the two metals was copper and the other acted as the generator of

electricity. In 1830, the Genevan chemist Auguste de la Rive developed the basis of the

electrochemical theory of corrosion. At the beginning of the 20th century, this theory was taken

up by Whitney and then completed by Hoar and Evans at the end of the 1920s [2][3][8].

1.3. Electrochemical behavior of Aqueous Corrosion

All metallic corrosion processes involve transfer of electronic charges and a conductive media for

its transfer mostly an aqueous phase called electrolyte, illustrated in Figure 1. If we take the

example of basic corrosion reaction:

Zn + 2HCl = ZnCl2 + H2 (1)

The ionic form of this reaction can be split into two reactions and the same reaction can be written

as two separate reactions (3) and (5)

Anodic or oxidation reaction, involving dissolution of metal in aqueous solution, with release of

electrons and of the form:

M = M n+ + ne-

(2)

Zn = Zn 2+ + 2e-

(3)

Cathodic or reduction reaction, involving electron consumption with decrease in oxidation state

and of the form:

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M n+ + ne- = M (4)

2H + + 2e- = H2 (5)

If excess electrons are supplied to reaction (3) and (5), the rate of reaction (5) increases and that

of reaction (3) decreases, so all aqueous corrosion reactions are electrochemical in nature.

The Free Energy change ∆G associated is given as:

∆G = -nEF (6)

Where F is Faraday’s constant, n is number of electrons and E is electrochemical potential at

equilibrium. E, therefore, is the sum of the corresponding oxidation and reduction half-cell

electrochemical reactions with their individual half-cell electrode potentials. If the reactants have

unit activity it defines their standard state, which is called standard half- cell electrode potential.

The listing of such electrode potentials for reduction reaction is called the Electromotive Force

Series (emf). But since it is impossible to measure absolute potentials, the half-cell hydrogen

reaction at standard state is selected as reference, and the Standard Half Electrode potential is

assumed to be Zero. For the reaction to take place spontaneous, E must have positive potential so

that the Free energy change in reaction (6) is negative.

When the current flows, or enters, from electrode to electrolyte, that electrode is called the anode:

conversely when the current leaves the electrolyte to enter electrode then this electrode is called

the cathode. Reactions at anode therefore are called anodic reactions and those reactions at cathode

are called cathodic reactions. At the anode, metal ions go into solution and electrons are generated.

In consequence an anodic current is generated, with direction flow from the anodic electrode to

the electrolyte solution. At the cathode, electrons are consumed and the cathodic current

accordingly flows from the electrolyte solution to the cathode. [1]

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1.4. Kinetics of Equilibrium at Electrode:

At equilibrium the net current in the system is zero or:

i a = -ic = io (7)

Where, io is exchange current density, depending on the kinetics of the reaction, it gives us

understanding of how charge transfer takes place at equilibrium. And if the equilibrium system is

polarized in positive or negative direction a net current will flow depending on which direction it

is polarized.

Polarization is the potential change from equilibrium electrode potential, because of

electrochemical reactions occurring on the electrode surface, preferential electron transfer takes

place, current is generated depending on kinetics of reactants. In cathodic polarization the electrode

surface has potential negative to equilibrium potential, electrons are consumed by positive

electrode ions and the electrode has a surface negative potential to referenced to the equilibrium

potential. The opposite happens in case of anodic polarization where the electrons are released

from the electrode surface at a faster rate and cause a positive potential change from equilibrium

potential. Polarization is of two types:

1.5. Activation Polarization:

The reactions on the electrode surface happens in steps, if we consider the cathodic half-cell

reaction for Hydrogen reduction, it occurs in steps

Step 1: Formation of adsorbed Hydrogen

H+ + e- = H adsorbed (8)

Step 2: Formation of Hydrogen molecule

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H adsorbed + H adsorbed = H2 (9)

Step 3: Nucleation of hydrogen bubble

If any of the steps in the above reaction controls the rate of charge (electron) flow that step would

be rate controlling, and the overall reaction will be under activation polarization control.

Figure1.1 Schematic for equilibrium state of metal [16]

The equilibrium state of the system is as shown in Figure 1.2. As mentioned above at equilibrium

the net current in the system is zero or: ia= -ic=i0, but when the equilibrium is disturbed, for example

in case of cathodic polarization - ic >ia

and net cathodic current will flow and if anodic polarization ia > -ic and net anodic current will

flow, with low over-potential, the driving force for each reaction being low, the concentration of

ionic species on the surface and bulk can be assumed to be same , the rate of current can be given

as : the rate of oxidization(rr) and reduction reaction (rf) which is dependent on activation energy

of each reaction given by the Arrhenius equation (applicable to many reactions over reasonable

temperature range) :

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Reduction: r𝑓 = K𝑓 𝑒𝑥𝑝(−

∆𝐺𝑓 ∗

𝑅𝑇 ) (10)

Oxidation: r𝑟 = K𝑟 𝑒𝑥𝑝(−

∆𝐺𝑟 ∗

𝑅𝑇 ) (11)

Where ∆G is energy barrier for specific reaction, and Kf and Kr are the rate constants , as seen

above from Equation 7 at equilibrium rf = rr. But, as an over potential is applied the activation

energy of the reduction reaction in cathodic polarization is reduced and activation energy for

anodic polarization is increased. Schematic representation by Free Energy curve is shown in

Figure 1.2. The reduction reaction’s Free energy is decreased by αnFηc and the activation energy

for oxidation reaction is increased by (1- α)nFηc. Where , α and (1- α)is the specific fractional

contribution for the reduction and oxidation reaction respectively from the over potential For such

electrochemical system the movement of charged species generate current , so the rate rr, rf can be

replaced by i , where :

Figure 1.2 Schematic Representation: Free energy curve of the electrode for activation over potential.

(---) polarized state, (---) equilibrium state [17]

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Reduction: i𝑓 = K𝑓 𝑒𝑥𝑝(−

∆𝐺𝑓 ∗

𝑅𝑇 ) (12)

Oxidation: i𝑟 = K𝑟 𝑒𝑥𝑝(−

∆𝐺𝑟 ∗

𝑅𝑇 ) (13)

Oxidation: i𝑎 = k𝑟 𝑒𝑥𝑝(−

∆𝐺𝑟∗ + (1 − α)n𝐹η𝑐

𝑅𝑇 )

(16)

Oxidation: i𝑎 = k𝑟 𝑒𝑥𝑝(−

∆𝐺𝑟∗

𝑅𝑇 ) 𝑒𝑥𝑝(−

(1 − α)n𝐹η𝑐

𝑅𝑇 )

(15)

As seen in Equation 7 if = ir = io , the reaction current can be given as:

Oxidation: i𝑎 = i𝑜 𝑒𝑥𝑝(−

(1 − α)𝑛𝐹η𝑐

𝑅𝑇 ) (16)

Reduction: i𝑐 = k𝑓 𝑒𝑥𝑝(−

(∆𝐺𝑓∗ − αn𝐹η𝑐)

𝑅𝑇 ) (17)

Reduction: i𝑐 = k𝑓 𝑒𝑥𝑝(−

(∆𝐺𝑓∗

𝑅𝑇 )e𝑥𝑝(

αn𝐹η𝑐)

𝑅𝑇 )

(18)

Reduction: i𝑐 = i𝑜 e𝑥𝑝(

αn𝐹η𝑐

𝑅𝑇 ) (19)

The net current for cathodic over potential in the system is given then as

i= ic –ia

i = i𝑜 e𝑥𝑝 (

αn𝐹η𝑐

𝑅𝑇 ) − i𝑜 𝑒𝑥𝑝(−

(1 − α)𝑛𝐹η𝑐

𝑅𝑇 ) (20)

Similarly, for anodic polarization the net current will be

i = i𝑜 e𝑥𝑝 (

αn𝐹η𝑎

𝑅𝑇 ) − i𝑜 𝑒𝑥𝑝(−

(1 − α)n𝐹η𝑎

𝑅𝑇 ) (21)

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The above equation (21) is called Butler Volmer equation , for high over voltage one of the two

anodic or cathodic current is dominant over the other and the Equation 21 can be simplified, if

there is large cathodic polarization, the anodic part of the current will be negligible then we get :

𝑖𝑐= 𝑖𝑜 𝑒𝑥𝑝(𝑎n𝐹ηc

𝑅𝑇) (22)

log (𝑖𝑐

𝑖𝑜) =(

𝑎n𝐹ηc

𝑅𝑇) (23)

η𝑐 = b𝑐log (𝑖𝑐

𝑖𝑜) (24)

η𝑐 = b𝑐log (𝑖𝑐) − b𝑐log (𝑖𝑜) (24)

𝑖𝑎= 𝑖𝑜 𝑒𝑥𝑝(𝑎n𝐹 η𝑎

𝑅𝑇) (25)

log (𝑖𝑎

𝑖𝑜) =(

𝑎n𝐹η𝑎

𝑅𝑇) (26)

η𝑎 = b𝑎log (𝑖𝑎

𝑖𝑜) (27)

And, can be written as a single equation for both anodic and cathodic reaction as:

η = b log (𝑖𝑜) ± b𝑐log (𝑖) (28)

This approximation was developed by a German investigator Tafel in 1905, and is known as

Tafel’s Equation and the Constant β is known as Tafel Slope, estimation of Tafel Slope, exchange

and limiting current values are important to describe and understand any electrochemically

corroding system. By knowing the half-cell reactions, which occur simultaneously on the electrode

surface and using the basis of charge conservation for these reactions , underlines the basis of

Mixed Potential Theory, and so for each of the half-cell reaction potentials are polarized to an

intermediate potential value , this potential is known as the Mixed Corrosion Potential or Ecorr,

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cathodic polarization is given by η𝑐 and anodic polarization by η𝑎 from their equilibrium half-cell

electrode potentials respectively, until they reach the intermediate potential value to coexist , at

this potential the current densities of anodic and cathodic reactions are equal ,which gives the

value of icorr , corrosion current. This is represented in Figure 1.3 for Zinc in acid. Mixed Potential

Theory can be used for determination of corrosion rate from Tafel equation.

Figure 1.3 Mixed Potential and Corrosion for zinc in acid solution [18]

In Polarization curve , measured as Potential versus Current, which is a useful tool for

understanding the kinetics of the corroding system , and indirectly measuring corrosion rates, in

this a potentiostat ic used to polarize the system from steady state corrosion potential , if the

potential is shifted to negative value from Ecorr, it is cathodic over potential , this over potential

is because of applied current to the system which causes potential to change , in cathodic over

potential then, the cathodic reaction rate increases , because of excess electrons and anodic reaction

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rate decreases, and to satisfy charge conservation principle, this increase and decrease should be

compensated by the applied current :

i applied= ic-ia (29)

This is shown by the dotted experimental (.) curve in Figure 1.4. As can be seen from figure 1.4

above at lower over potential, the difference between ia and ic is very low, so there is a curve in

the plot, but at relatively higher over potential, the dotted curve coincides with the dashed curve,

and becomes linear at higher over potential (anodic or cathodic), applied current reflects the

kinetics of only that particular reaction. This linearity is termed as Tafel behavior. Thus the

experimental Polarization curves can be extrapolated from their linear portions, to find the point

of intersection, which gives the value of Ecorr and the corresponding corrosion current. Tafel

behavior is based on the basic assumption that Concentration Polarization is absent.

Figure 1.4 Polarization curve with Tafel Extrapolation [16]

Applied Current Density

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1.6. Polarization Resistance

In the one of the most early work [19][20], Stern and Gary derived an equation which established

a linear relation between slope of linear region of polarization measurements , corrosion rate and

Tafel Slopes , it is important to note here that the in these measurements over potential is very

close to corrosion potential , avoiding any surface damage to the electrode surface . To avoid

confusion with Linear Tafel region, it is important to note here is that the linear Tafel region is not

seen until an applied current of approximately 10 times the magnitude of icorr is reached. The

background and illustration of this condition is given in Figure 6 below:

The applied current, derived from the potentiostatic measurements, as given in Equation (29), as

(iapplied = ic-ia). The value of individual currents ia and ic can be obtained by rearranging Equation

(27) and (24), hence Equation 29 can be written as:

i applied= icorr (10(-η /βc) -10( η /βa) ) (30)

The exchange current is analogous to corrosion current icorr in the corroding system.

Approximating it with power series expansion, with the assumption that (η /β) value is very small,

that is β greater than η [19][20][22].

10 η/βa = 1 + 2.3 η/βa

10 -η/βc = 1 - 2.3 η/βc

We obtained, therefore:

η

i𝑎𝑝𝑝𝑙𝑖𝑒𝑑= 2.3𝑜 i𝑐𝑜𝑟𝑟 (

β𝑎 + β𝑐

β𝑎β𝑐 ) (31)

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icorr = βa β𝑐

2.3(βa + β𝑐) *(

∆𝑖

(∆𝐸)) (32)

Where the slope of the curve is (∆𝑖

(∆𝐸)) is polarization Resistance Rp, so polarization resistance is

given as shown from the experimental illustration is given in Figure below and can be written as:

Rp = βa β𝑐

2.3(βa + β𝑐)i𝑐𝑜𝑟𝑟 (33)

From the icorr the Corrosion rate can be measured by use of the Faraday‘s Law and given as

C.R (mpy)= 0.13i𝑐𝑜𝑟𝑟(E.W)

d (34)

Where, mpy= milli-inches per year, Icorr=corrosion current density (µΑ/cm2), E.W=equivalent

weight of the corroding species, (g). d = density of the corroding species, (g/cm3).

Figure 1.5 Illustration for polarization resistance measurement [23]

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1.7. Corrosion Measurements:

The speed of electrochemical measurements makes them a useful tool for corrosion studies with

quick measurements of corrosion rates. In polarization techniques, external voltage is imposed,

and this done by polarizing the electrode, for Polarization Resistance measurements over voltages

of 25mV are usually allowed [22]. And Tafel constant’s absolute value are in the range 0.03 to 0.2

V and are not required to be equal for cathodic and anodic reactions, for speedy measurement of

Polarization resistance analysis the values of Tafel Constants βa =βc =0.1 to 0.12V are acceptable,

in this work the values are assumed to be 0.12 as representative of the average of all corroding

system [19][20][23]. The exact values can be determined from Tafel plots. Advantages of using

this technique is that test can be completed within few minutes, since the over potential is small

no damage is done to the electrode surface and so Polarization Resistance can be considered as the

non–destructive technique of corrosion rate measurement and the same sample can be used for

other further measurement. It can also be used as important technique for corrosion analysis of

structures, for which visual inspection is not possible and weight loss studies cannot be performed.

However using only Polarization resistance test results can be unreliable, since other effects like

Ohmic solution resistance, effect of charge transfer and resistance due to protective film formation

are not considered in Rp calculations .So, along with DC Polarization Resistance, Potentiodynamic

and Immersion studies are used together in this work. For Potentiodynamic curves, over potential

(both anodic and cathodic) in the range of 250 -300 mV can be used. For correct and accurate

measurements by Tafel extrapolation method at least one of the anodic or cathodic branch should

show Tafel behavior, and the extrapolation should be done at least 50 to 100mV away from Ecorr

[16]. Potentiodynamic curve act like identification or figure print of behavior of a metal in a

particular environment, with advantages of providing various information, as corrosion current

22

from Tafel analysis, passivation region, and rapid determination of corrosion currents with Tafel

analysis makes it the most advantageous for studies of corrosion inhibitor efficiency evaluation

studies [15]. The logarithmic scale of current can lead to higher error for measurements in

corrosion currents with error factor in calculation of current up to 5 in manual extrapolations. The

time required for potentiodynamic scans can take 1-2 hours which is more than that required for

Polarization resistance [22]. However, since the over potential applied in this case is high it can

cause damage to the electrode surface, so it is not useful for continuous corrosion rate

measurements. Polarization measurements can be complicated if different phenomenon are

interfering or taking place at the same time, for higher over potential the surface of electrode can

change or damage and concentration polarization can also interfere [22].

1.8. Concentration Polarization:

As the system is further polarized more from Ecorr , subsequent to net current in system in

accordance to Tafel equation , current ceases to increase as the electrode surface gets depleted of

the concentration of dissolved ionic charges carrying charge from solution to the electrode surface

for reaction , and the reaction is diffusion controlled ,dependent on diffusion rate of ions from

solution to the electrode surface and back, for anodic reaction the charged ions are produced at

faster rate at surface and concentration becomes high compared to bulk solution to surface,

conversely in cathodic reaction the concentration is depressed compared to the bulk solution.

Consequently, the change of potential is greater than that of Tafel Equation. It can be seen for

reduction reaction in Figure 1.6. First it is in activation polarization and then in concentration. This

excess potential is Concentration Polarization ηc. which can be calculated by Nernst Equation

taking the difference in potential of electrode at un-polarized E state and the polarized state E`.

This is given as: ηc = E` − E =0.059

𝑍log

[𝑎Mz+]

[𝑎Mz+]p [35]

23

Figure 1.6 Corrosion of Metal M under diffusion control [23]

1.9. Corrosion of Cold Rolled Steel

Steel is one of the most industrially used materials. Steel has had a major influence on our lives,

the cars we drive, the buildings we work in, the houses in which we live and countless other facets

in between. Steel is used in our electricity-power-line towers, natural-gas pipelines, machine tools,

military weapons-the list is endless. Steel has also earned a place in our homes in protecting our

families, making our lives convenient, its benefits are undoubtedly clear. Steel is by far the most

important, multi-functional and most adaptable of materials. The development of mankind would

have been impossible but for steel. The backbone of developed economies was laid on the strength

and inherent uses of steel [27].

Corrosion involves a chemical reaction between the material and environment as a result of which

electric current flows. Two reactions takes place, one an anodic reaction, by oxidation process

produces electron and other electron consuming reaction, called cathodic reaction or reduction

process. The corrosion product formed is the thermodynamic ground state of the materials for that

24

particular corrosion environment. Essentially, the iron in the steel is oxidized to produce rust,

which occupies approximately six times the volume of the original material. The rate at which the

corrosion process progresses depends on a number of factors, but principally the 'micro-climate'

immediately surrounding the steel structure.

The use of corrosion inhibitors for corrosion resistance of steels has received significant attentions,

because of its wide applications and understanding the process .The corrosion of steel which is

considered as an electrochemical process which occurs in stages. Initial attack occurs at anodic

areas on the surface, where ferrous ions go into solution. Electrons are released from the anode

and move through the metallic structure to the adjacent cathodic sites on the surface, where they

combine with oxygen and water to form hydroxyl ions. These react with the ferrous ions from the

anode to produce ferrous hydroxide, which itself is further oxidized in air to produce hydrated

ferric oxide (i.e. red rust.) Since the major component of steel is iron (Fe) , at the surface of a

component its undergoes changes :

Fe → Fe n+ + n electrons

(This allows it to bond to other groups which are negatively charged)

Since the corrosion product of steel is a variant of iron oxide the cathodic reaction involve water

and Oxygen

O2 + 2H2O + 4e- → 4OH – (1)

The negative and positively charged material combine

2Fe + O2 + 2H2O → 2Fe (OH)2 (2)

Iron + Water with Oxygen dissolved in it → Iron Hydroxide

Oxygen dissolves quite readily in water and because there is usually an excess of it, reacts with

the iron hydroxide.

25

4Fe (OH)2 + O2 → 2H2O + 2Fe2O3.H2O (3)

Or

4Fe + 3O2 + 2H2O = 2Fe2O3H2O (4)

(Steel) + (Oxygen) + (Water) = Hydrated ferric oxide (Brown Rust)

Chloride ions also act as catalyst to corrosion when present in sufficient amount at surface, they

are not consumed in the process and permit the corrosion to proceed

Fe +2 + 2Cl- = FeCl2 (5)

FeCl2 + 2OH- = Fe (OH)2 + 2Cl- (6)

Over long periods the loss of metal is reasonably uniform over the surface, and this is usually

described as 'general corrosion'. A schematic representation of how corrosion reaction proceed on

the steel surface is shown in Figure 1. The above reactions helps us understand how corrosion

takes place.

1.10. Corrosion control in steels:

There are four common methods used to control corrosion. They include protective coatings and

linings, cathodic protection, materials selection, and corrosion inhibitors.

A) Coatings and linings are principal tools for defending against corrosion. These substances are

often applied in conjunction with cathodic protection systems to provide the most cost-effective

protection for a structure.

B) Cathodic Protection (CP) is a technology that uses direct electrical current to counteract the

normal external corrosion of a structure that contains metal, such as an underground petroleum

storage tank or natural gas pipeline. On new structures, CP can help prevent corrosion from

starting; on existing structures; CP can help stop existing corrosion from getting worse.

26

C).Materials selection refers to the selection and use of corrosion-resistant materials such as

stainless steels, plastics, and special alloys to enhance the life span of a structure. Some of the most

common materials used in constructing a variety of facilities, such as steel and steel-reinforced

concrete, can be severely affected by corrosion.

D).Corrosion inhibitors are substances that, when added to a particular environment, decrease the

rate of attack of that environment on a material such as metal. They can help extend the life of

equipment, prevent system shutdowns and failures, avoid product contamination, prevent loss of

heat transfer, and preserve an attractive appearance of structures. The prevention of corrosion on

the surfaces of metallic pipes, heat exchangers, and the like which are in contact with industrial

cooling waters, and particularly industrial cooling waters containing utilizing a corrosion

inhibiting amount of a composition .Superior corrosion inhibition can be achieved using the

corrosion inhibitors, particularly in cooling water systems. Costly problems facing drinking water

utilities. A large number of parameters affect pipe corrosion, including water quality and

composition, flow conditions, biological activity, and corrosion inhibitors.

Steel is highly prone to corrosion during storage and transportation air borne salinity, micro-

environments and industrial pollution can affect the corrosion efficiency of these steels. Lately,

demands for plated sheet products having even better corrosion resistance are increasing in view

of the requirements for a very long service life, enhanced performance, etc. in housing and other

buildings [8].

1.11. Corrosion Inhibitors

Corrosion Inhibitors are widely used in the steel industry to control corrosion. There are two main

concepts that needs to be addressed in order to understand the observed phenomena. One is

adsorption and the other is micelles in the solution. These two phenomena differentiate corrosion

27

inhibitors from other chemical entities. Corrosion inhibitors are found to protect steel corrosion in

acid solutions by adsorbing themselves on steel surface. Adsorption is a separation process

involving two phases between which certain components can become differentially distributed.

Adsorption can be described by two main types of interaction:

• Physisorption, involves electrostatic forces between ionic charges or dipoles on the adsorbed

species and the electric charge at the metal/solution interface. The heat of adsorption is low and

therefore this type of adsorption is stable only at relatively low temperatures [37].

• Chemisorption, involves charge sharing or charge transfer from the inhibitor molecules to the

metal surface to form a coordinate type bond. In fact, electron transfer is typically for transition

metals having vacant low-energy electron orbital. Chemisorption is typified by a much stronger

adsorption energy than physical adsorption. Such a bond is therefore more stable at higher

temperatures.

1.12. Water based corrosion inhibitors

Water-based corrosion inhibitors rely on modifying the characteristics of the metal surfaces

themselves to decrease their susceptibility to oxidation and corrosion formation. Water-based

inhibitors are typically sold as a concentrate and diluted with water for use, making their cost less

than most oil/solvent-based products. Water-based corrosion inhibitors generally have very

desirable characteristics. The chemical films formed are thin and typically transparent when dry.

Application methods for these coatings include spray, brush or immersion. They are cleaner to use

and eliminate concerns of solvent fumes. The films rarely have to be removed prior to subsequent

operations but if removal is necessary it is done easily with mild water-based cleaners. Water-

based inhibitors can be effective in preventing corrosion for extended periods of time (weeks to

28

months) under reasonable plant storage and protected shipping conditions. Since water-based

products work by decreasing the susceptibility of the metal to oxidation rather than by completely

excluding water and air, these inhibitors often are not as effective as oil/solvent-based coatings in

shipping and outside storage of parts.

1.13. Zinc and Silicates as Corrosion Inhibitors review:

Various zinc rich and silicate based coatings are widely available for corrosion protection in the

industry.

1.13.1 Mechanism of Zinc Silicate primer:

The figure 1.7 gives an idea of the basic mechanism of how a zinc coating works on a substrate.

When the metallic zinc is coated on a substrate it is completely isolated from the environment.

This basically acts as a barrier between the environment and the substrate preventing any kind of

contact between both of them. So far it acted like any other coating causing a barrier effect. But

once a defect is made in the coating the substrate is exposed to the coating and the zinc coating

acts as a sacrificial protection. This protection depends on certain factors like amount and type of

moisture present, the electrical connectivity of the zinc particles to each other and the substrate,

the purity of zinc and thickness of the coating. After some time the zinc layer will be depleted and

the sacrificial protection will cease.

So to protect the steel substrate we have developed a novel corrosion inhibiting pigment using zinc

and silicate ions. Zinc offers threefold protection since it, seals the underlying metal from contact

with its corrosive environment, provides galvanic protection and repairs minor damages in a

coating forming a barrier to further electrochemical action. It really adds to performance and

29

longevity of the material. Even silicates are known for the Silicate layer acts as a barrier to oxygen

diffusion to the metal surface thereby impeding but not preventing corrosion

.

Figure 1.7 Mechanism of zinc rich coatings for corrosion inhibition

Zinc Silicate coating:

Corrosion of metals, such as steel and zinc, requires the presence of water (H2O), oxygen (O2)

and ions such as chloride ions (Cl¯), all of which exist on the atmosphere. Atmospheric chloride

ions are in greatest abundance anywhere near the coastline. In addition to these, the atmosphere

also carries emissions from human activity, such as carbon dioxide (CO2), carbon monoxide (CO),

sulphur dioxide (SO2), nitrous oxide (NO2) and many other chemicals

To understand how Inorganic Zinc silicates work and why they offer superior corrosion protection,

particularly in coastal environments, you need to understand some environmental corrosion

chemistry. Here’s how it works.

H2O + CO

2 ￫ H2CO

3 (Carbonic Acid)

30

And the figure 1.8 explains the mechanism of zinc silicate in corrosion inhibition.

The carbonic acid then reacts with the zinc metal on the surface of the IOZ matrix:

H2CO

3 (Carbonic Acid) + Zn + O

2 ￫ (Zn)

χ (OH) CO

3 (zinc hydroxy carbonate - insoluble).

The insoluble and inert zinc hydroxy carbonate product then gets locked into the surface pores of

the IOZ, creating an effective barrier to further oxidation. The zinc hydroxy carbonate is

symbolised as Zn+.

Figure 1.8 Mechanism of Corrosion inhibition of zinc silicate coatings

The advantages of zinc silicate compounds are:

• It has outstanding mechanical properties and is capable of resisting impact, scratching and

scuffing.

• These coatings are resistant to heat as high as 750°F.

• Some formulations of zinc-rich paint may be immersed in salt water, while others cannot.

• Using zinc-rich paint as a top coat is harder compared with other types of coatings due to

its porous nature.

• Inorganic zinc-rich paint needs to be applied on a totally clean surface for best outcomes.

31

1.13.2. Inhibitor mechanism:

The role of inhibitor pigments in the process of corrosion prevention of a metal is protecting

the surface of the substrate by releasing cathodic and anodic species like calcium, zinc, silicate

and phosphate ions in to the solution there by forming a protective layer on the surface of the

metal [38]. The pigments with these ions are non-toxic and environment friendly since there

are no chromate ions present in it. As per the literature we already know that calcium silicate,

calcium phosphate and zinc phosphate are efficient corrosion inhibitor pigments used for

steels. So the mechanism of these pigments was explained as [59][61].

The figure above describes the release of inhibitor from the silicon calcium cation exchange

pigment in the solution for corrosion protection. With Calcium ions present, reactions proceed to

formation on cathode areas of a diffusion barrier film of unknown structure containing both

calcium and phosphorus ions. Similarly in the zinc silicate system, the zinc ions form a diffusion

barrier film, forming zinc hydroxide by reacting with the hydroxide ions formed due to the free

electrons, water molecules and excess oxygen. This film appreciably reduces the corrosion rate,

with control of corrosion. Prominent early research was also done in 1985 published by Goldie

[40] who studied the silicon/calcium cation exchange pigments for corrosion protection of steel

in chloride corrosive environment with saturated pigment extract. The corrosion protection

action was studied to be a cation exchange between the H+ and Ca 2+ ions ( from the medium

and pigment respectively).In ion exchange process the pigments immobilize the aggressive ions

32

like Cl- (which corrodes the Metal), while the ions from the pigments migrate to form a

protective layer on the metal. So in relation to the calcium and silica system, for zinc silicate

system, the protective film on the surface is formed from zinc and silica particles.

The undergoing reactions would be:

Fe → Fe n+ + n electrons

O2 + 2H2O + 4e- → 4OH –

With the presence of zinc silicate pigment,

2ZnSiO3 + 4OH – → 2Zn(OH)2 + 2SiO2

There by controlling the reaction of ferrous ions reacting with hydroxide ions and forming

corrosion product. The corrosion inhibitive effect of zinc silicate is studied to be due to its

comprised effect of zinc ions and protective film of SiO2 that blocks the anodic reaction, a

barrier effect occurs, due to zinc silicate, depressing oxygen reduction because of zinc chloride

formation [16].

1.13.3. Pigment preparation techniques:

Recent trends in nanotechnology enabled us manipulate particle properties both in atomic and

molecular scale. The nanometer products which are used for super capacitive applications such as

metal oxides can be processed in two different ways, i.e., Top–Down and Bottom–Up approach.

In the top-down approach we start with the bulk material and break it down in to smaller fragments.

In this method we use large structures, which are controlled externally in the development or

production of nanostructures. This method includes ball milling, attrition and lithography.

In the bottom up approach we basically build the structure up from an atomic scale to nano scale.

Some of the examples of this processing technique include Self-assembly, liquid- solid

transformation. The liquid solid transformation can be sub classified in to co precipitation, sol gel

33

processing, micro emulsion technique and solvothermal methods. In the coprecipitaion technique

we dissolve the precursor salt (silicates, sufates) in water or other solvent mixtures to precipitate

the oxohydroxide form using a base. Whereas, solgel processing techniques involve production of

metal oxides via hydrolysis of precursors. Microimmulsion technique is based on the formation of

micelles in a mixture containing water, surfactant and oil. During the production of nano metal

oxide particles using the solvothermal method, the metal complexes are decomposed by applying

thermal energy in an inert atmosphere under application of pressure. Figure 1.9 is the schematic

of the top-down and bottom-up approach with the techniques that are used. [6]

Figure 1.9 Schematic of the top-down and bottom-up approach with the techniques used [6]

34

2. Research Plan

2.1. Objective:

Silicate itself being an effective corrosion inhibitor for Fe-based substrates has been shown to

provide significantly enhanced corrosion efficiency. Preliminary studies has proved that zinc

phosphate and calcium silicate pigments show superior corrosion inhibition of Cold Rolled steel,

with good ability of each individual ions and their combined efficiency to inhibit corrosion that

increases their efficiency. The objective of this work, therefore, was to combine the zinc ions and

silicate ions in the form of sodium silicate and zinc sulfate, which are known to be good corrosion

inhibitors in order to form a mixed corrosion inhibitor for higher inhibition efficiency and explore

in-depth the mechanisms by which this increased efficiency occurs, in terms of: processing with

co-precipitation and heat treatment procedures during the pigment preparation phase. Structural

study of the zinc silicate pigment formed and in what way it effects the inhibition behavior on the

steels. Effect of external factors like temperature, stoichiometry of the precursors, concentration

of the pigment on the corrosion inhibition efficiency of the pigment.

Though there have been patents and papers published on zinc silicate as a good corrosion inhibiting

primer there hasn’t been any electrochemical analysis and characterization analysis provided. Also

the zinc silicate pigment is not commercially available in the market so an optimum method to

make zinc silicate is deduced using experiments. Therefore to explain all these aspects this

particular topic has been chosen.

2.2. Approach:

To realize these objectives, the pigment samples are prepared by coprecipitation and heat

treatment techniques, followed by electrochemical characterization using Linear Polarization

35

resistance and Potentiodynamic techniques for the selection of the efficient inhibiting pigment.

Supported by the Immersion and Optical, SEM micrograph are studied. A major part for pigment

characterization includes XRD and FTIR analysis for crystallite size and bond vibrational energy

changes, to study the cell distortions in the system. A manifestation of this is related to what makes

the pigments more active, with increased release of inhibiting ions in the corrosive environment

and the enhanced inhibitor efficiency. This work bridges the processing of pigment to their

inhibition behavior, facilitating their optimal utility.

36

3. EXPERIMENTAL

This chapter gives the information about the materials, various techniques used in preparation and

characterization of the pigment.

3.1. MATERIALS:

3.1.1. Precursor materials:

Reagent grade Zinc Sulfate and Sodium Metasilicate were used for processing of the pigments.

3.1.2. Substrate

Un-pretreated cold rolled steel sheets (ACT code APR10157;3 × 6 × 032;unpolished) were used

as the substrate material.

3.1.3. Corrosive Environment

The corrosive environment for the electrochemical and immersion testing is 1% sodium chloride

solution.

3.2. Pigment Processing:

The pigment preparation involved co-blending and co-precipitation of zinc sulfate and sodium

metasilicates in different stoichiometric ratios. The goal is to prepare the zinc silicate pigment with

maximum corrosion inhibition capacity by using co-blending, co-precipitation and heat treatment

techniques which leads to enhanced corrosion control for steel. This is achieved by the

collaboration of individual ions (Zn, Si).

3.2.1 Co- Blending:

Ball milling allows for the control of the particle size, structural and morphology changes with

enhanced leaching behavior. Along with the optimum heat treatment this will not allow unwanted

37

expansion of moisture. In this work 60/40 was used as the dispersing agent, with the mixing for

10 hours so there is particle size reduction and uniform mixing is achieved, after milling

the solution was washed and centrifuged in vacuum for 10 hours. Together, with the optimum heat

treatment will not allow unwanted expansion of moisture.

Figure 2.1 Inhibitor Co-Blending Processing Procedure

3.2.2 Co- Precipitation Technique:

Zn2SiO4 powders were produced by co-precipitation of ZnSO4 and Na2SiO3 as precursor solutions.

ZnSO4 and Na2SiO3 precursor solutions were made by dissolving in 95:5 solution (95% De-ionized

water; 5% Iso propyl alcohol) separately. The solution is allowed to stir for few hours for equal

distribution of ions. After the solution was stirred the powders were filtered using filter paper. The

powders were then washed with distilled water and were filtered out again. This process was

repeated several times to obtain purer powders. These powders were further dried in a vacuum

oven at around 60oC overnight. These dried powders were then separated in to batches and were

heat treated in reducing atmospheres at different temperatures fixed as per TGA analysis data.

Batch preparation of co-blending

Ball milling of the batch (10 hours)

Centrifuging (5 rpm)

Vacuum Drying of the Centrifuged

powder (10 hours)

Heat Treatment of the powder at desired temperature (450oC )

38

ZnSO4 + Na2SiO3 → ZnSiO3 ↓ + Na2SO4

3.3. Substrate Preparation:

Cut Panels of cold rolled steel were purchased from ACT Test Panel Technologies, the below

procedure was used to clean and prepare the samples for testing in corrosive environment for

corrosion testing.

3.4. Electrochemical Characterization:

3.4.1 Evaluation of corrosion inhibition of zinc metasilicate

The techniques used to evaluate the corrosion inhibition of zinc metasilicate for cold rolled steel

are DC polarization test and immersion test.

All the electrochemical analysis in the current work is performed in a three electrode setup using

Gamry Potentiostat 300G. The software used in the potentiostat is called the Gamry Framework.

All the analysis done is recorded directly in the computer by this software. A three-electrode cell

system including a working electrode, an auxiliary electrode and a reference electrode was used

for the electrochemical measurements. The auxiliary electrode was platinum and the working

The steel panels were cut into

coupons of 2x4 cm2

Cleaned with 7% alkaline solution

Ultrasonic cleaning with acetone

and methanol

Air dried

39

electrode was made of CRS with exposed area of 1cm2. The corrosive solution with the pigment

was stirred for twelve hour and then left to settle after which the saturated solution was taken for

the electrochemical tests. The working electrode was immersed in the test solution at open circuit

potential for 2 hours before polarization measurement until steady state is achieved. The

potentiodynamic polarization curves were carried out by polarizing in the range between -500 and

500 mV with respect to OCP at a scan rate of 1 mV s-1. Electrochemical measurements i.e.,

polarization resistance and potentiodynamic curves were carried out at the open circuit potential

using Gamry Potentiostat at room temperature in deaerated solution. Icorr is measured which is

directly related to corrosion rate.

3.4.2. Echem Analyst:

Echem analyst is the analysis software used to analyze all the data obtained from Gamry. Charge

discharge potentials, charge stored and specific capacitance can be estimated using this analysis

technique.

3.5. Structural Characterization:

3.5.1: Thermogravimetric Analysis:

TGA measures weight changes in the material as a function of temperature or time in a controlled

environment. It is typically used to understand the compositional and stability of the material with

temperature. The weight loss or gain, with temperature give us understanding of various reaction

temperatures in which reactions like decomposition, dehydration and oxidation be taking place.

For the analysis 10 milligram of powder was placed in a platinum pan, which is placed in an

electrically heated furnace in compressed air and nitrogen atmosphere with a thermocouple for

40

appropriate measurements with heating rate of 100C/min up to 10000C. TA, TGA Q50 was used

to perform the TGA analysis [53][54].

3.5.2: X- Ray Diffraction:

X- Ray diffraction technique is used to do the qualitative and quantitative analysis of the material.

It is also used to understand the crystalline state and size, structure, the amount of phases present

and the lattice parameters of the material. X- Ray diffraction is operated on the basis of Bragg’s

law. A diffraction pattern of the monochromatic beam which hits the sample at different angles is

used to understand materials in this technique.

The diffracted beam will be composed of mutually reinforcing scattered beams. The detectors

detect these x-rays and the signals are processed to get a pattern of intensity versus 2 theta. Pattern

matching (PDF or Powder Diffraction Files database) are used for matching and peak indexing

according to the standard file. More symmetry the easier it is to identify the pattern.

Using the peak of maximum intensity the crystallite size can be calculate for each pigment using

the Scherrer Equation published in 1918

cos2

L

KB

Peak width (B) is inversely proportional to crystallite size (L), the size were calculated from the

Full Width Half Maxima of the high peak intensity. The constant of proportionality, K (the

Scherrer constant) was taken as 0.89.λ l for Cu- Kα used is 1.54 A.

X-Ray analysis was done in X'Pert Pro MPD , using diffractomer at 40 KV and 25mA, Cu Kα =

1.5418 Å. Scans were run from 20 -70o 2theta at a speed 0.4 seconds per step and a step of 0.02o

.The base line or control chosen for XRD was JCPDS of Zinc silicate. The structure of which was

41

identified by fitting it to the standard from PDF (Powder diffraction file). X-ray diffraction was

done for the all the heat treated pigments to see if there are peak shifts from that of the baseline

pigments, Scherrer formula was used to measure the crystallite size. Attempts are made to

understand these changes and relate them to corrosion inhibition behavior [58].

3.5.3 Fourier Transform Infrared Spectroscopy (FTIR)

In FTIR analysis IR radiation (2.5 m to 25 m) is passed through the sample, with some radiation

absorbed by the sample and some passed through or transmitted. This wavelength includes

frequencies corresponding to vibrations of mostly all the organic molecules and some inorganic

compounds, so they can be studied. The resulting spectrum of absorption versus wavelength is a

typical characteristic representation of molecular structure. Like a figure print no two molecular

structure have similar spectrum.

In FTIR the infrared source emits radiation of all wavelengths and selected range is sent to the

interferometer from which reconstructed interference beam is passed through sample and focused

on the detectors. The signal from the detector is an interferogram, which undergoes Fourier

transform through a computer to produce infrared spectra usually presented as intensity versus

wave number.

With the IR figure print of the molecular structure, the absorption peaks represent corresponding

frequencies of vibration between bonds of atoms the material is composed of, making it a valuable

resource for qualitative analysis. The intensity of the peak is indication for the amount of material.

So, FTIR can help us understand the consistency of the powder processed (zinc metasilicate)

[56][57].

42

3.5.4. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy

(EDS):

SEM is an electron microscopy technique used to study the morphology and surface properties of

the materials. It is a highly sophisticated technique which has a very high resolution and

magnification. Microstructures of heat treated zinc metasilicate pigments were obtained using

SEM. Energy Dispersive Spectroscopy is used to determine the elemental composition of the

material. It helps in obtaining the distribution of elements at a certain region on the sample area.

3.5.5. Immersion and Optical Microscopy:

Immersion tests measure the progress of corrosion damage obtained from the immersion length

within a corrosive environment, as well as other factors that can accelerate the corrosion process.

To carry out the immersion test firstly saturated solutions of 1% Nacl with and without the zinc

silicate inhibitor pigment should be made. Prior to be test the CRS substrates are polished with

100-1000 grade emery paper. The specimens should were washed with distilled water and

degreased with acetone and dried with a cold air blaster. After polishing and cleaning these

substrates, are then immersed in the solutions with inhibitor and without inhibitor for 3 days. Post

immersion test these samples are tested under an optical microscope to see the corrosion product

on the samples, whether the inhibitor in the solution prevents the sample from getting corroded.

43

4. RESULTS

This chapter gives the information about the results of corrosion testing and the pigment

characterization, relating both the aspects and justifying the corrosion inhibition with respect to

structural analysis.

4.1. TGA:

Figure 5.1 shows the thermogravimetric analysis of the processed zinc silicate pigment using co-

precipitation technique (It is a plot between weight loss in mg vs temperature in degrees Celsius).

There is weight loss of about 10% within the temperature range 100°C- 250°C which can be

attributed to the processing routes(coprecipiation), also the formation of willemite (Zn2SiO4)

occurs only after 250C.[Takesue, 2009 #6]. It is observed that there is no weight loss after 300°C

until 900°C since the curve stabilizes until this temperature and there is a phase change after this

temperature. The calcining temperature was chosen to be 450°C, as at a higher temperature the

0 200 400 600 800 1000

80

90

100

Ma

sslo

ss(%

)

Temperature(°C)

TGA ZnSiO3

Figure 4.1. Thermogravimetric analysis of co-precipitated Zinc silicate pigment

44

activity of the pigments starts decreasing. The heating rate for the calcinations was designed to be

10°C/min, which seemed to be the optimum rate according to the TGA results.

4.2. ELECTROCHEMICAL ANALYSIS:

The electrochemical tests were conducted on samples with variations in the powder processing

techniques, temperatures and stoichiometry. All the data is presented and compared to find the

best sample which gives the maximum corrosion inhibition efficiency.

Rate of corrosion and corrosion efficiency are measured with respect to corrosion current denoted

as icorr. For corrosion current (icorr) measurement, potentiodynamic curves are used. From the

corrosion measurement technique discussed in Chapter 1. Tafel extrapolation from

potentiodynamic curve method is used to get the values of icorr. According to corrosion rate

Equation (34) of Chapter 1, more the corrosion current more is the rate of corrosion, resulting in

more damage of the metal. The Potentiodynamic curve analysis is used to study the effectiveness

of pigments prepared using coblending and coprecipitation techniques at different heat treated

temperatures. The individual potentiodynamic curve for each of the samples is shown and the

behavior is explained below.

The Corrosion rate of the inhibitor pigments was calculated from the icorr values obtained from

tafel plots using the equation below

C.R (mpy) = 0.13(icorr)*E.W / d

Where:

C.R = corrosion rate

E.W = Equivalent weight of the metal

45

D = density of the metal

Inhibitor Efficiency (%) = 100*(CRuninhibited - CRinhibited)/CRuninhibited

Where:

CRuninhibited = corrosion rate of the uninhibited system

CRinhibited = corrosion rate of the inhibited system

4.2.1. Electrochemical Results:

The coblended powders of zinc oxide and silica powder in 1:1 molar ratio heat treated at 80°C

were measured for electrochemical tests. From the figure 5.2 it is clearly observed that the icorr

value is above 10μA which proves that pigment corrosion efficiency is low since the icorr value is

linearly related to the corrosion rate. Higher the corrosion current higher is the rate of corrosion.

It is also observed that there is no passivation region in the anodic curve as the electrode potential

is increasing.

Figure 4.2 Potentiodynamic Curve of 1% NaCl + Coblended ZnO:SiO2 at 80°C

icorr= 10µA

E corr=-600mV

46

Further the powders were heat treated at higher temperatures to check for the reactivity, but still

no change is observed in the corrosion inhibition behavior of the sample which shows that the

coblended powders were not good inhibitors.

Figure 4.3 Potentiodynamic curve of 1% NaCl + Coblended pigment at 450°C

Figure 4.4 Potentiodynamic curve of 1%NaCl + Coprecipitated pigment at 80°C

47

Figure 4.5 Potentiodynamic curve of 1% NaCl + coprecipitated powder at 450°C

Figure 4.6 Potentiodynamic curve of 1%NaCl + comparison of Coblended and Coprecipitated

powders

48

The process of coprecipitation was improved fiurther more by adding another step into the

process i.e., stirring the slurry for another day after the precipitate is formed. This helps in the

interaction of the particles, which would help to make the solution homogenous. Also more

interaction of the particles leads to nucleation and grain growth, there by leading to formation of

more prominent grains which can be attrubuted to the activity of the powders. In the figure 5.7 it

is clearly seen that stirring has strongly affected the activity of the powder by decrease in the

corrosion current, also there is a strong passivation region in the anodic region as the potential is

increasing. And further more figure 5.9 shows that at 450°C the powders has reached an

optimum level of activity since the corrosion current for this powders has the minimum value.


49



50



51

Figure 4.12 Potentiodynamic curve of 1%NaCl + comparison of all the temperatures of copreciitated

powders


52




Figure 4.15 Potentiodynamic curve of 1%NaCl + comparison of different ratios

53


Table 2 Polarization resistance for CRS with different temperatures

0.2

2

0.78

0.48 0.5

0.2

0.5

1.1

0

0.5

1

1.5

2

2.5

SrCrO4 NaCl ZS 80°C ZS 300°C ZS 350°C ZS 450°C ZS 550°C ZS 800°C

Ico

rr(µ

A)

Corrosion Current from Tafel Plots

65

0.697

29.26

50.151.95

57.51

48.4

41.22

0

10

20

30

40

50

60

70

SrCrO4 NaCl ZS 80° ZS 300°C ZS 350°C ZS 450°C ZS 500°C ZS 800°C

Rp

(K

oh

ms)

Polarization Resistance from Linear Polarization Resistance Curve

54

Table 3 Corrosion Rate for CRS from Potentiodynamic Scan for different temperatures

Table 4. Icorr values for all the pigments at different temperatures.

ZnSiO3 pigment at different temperatures icorr (µA)

80°C (8th) 0.8-0.9

80°C(6 decant) 0.6-0.7

80°C 0.7-0.8

80°C 0.5-0.6

300°C 0.6

400°C 0.4

400°C 0.6

450°C 0.1-0.2

450°C 0.3

450°C 0.3

500° 0.5-0.6

600° 1.0-2.0

800°C 2.0-3.0

From the above results it can be seen that with increasing temperature up to 450°C and the

stirring time of the coprecipitated solution there is an increase in the corrosion inhibition. The

processing technique can lead to various surface morphology changes like giving more time and

0.09

1.8

0.3510.216 0.225

0.090.225

0.495

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

SrCrO4 NaCl ZS 80° ZS 300°C ZS 350°C ZS 450°C ZS 500°C ZS 800°C

Co

rro

sio

n R

ate(

mp

y)

Sample

Corrosion rate in mpy

55

Table 5. Icorr values for the pigments prepared out of precursors with different stoichiometry

ZnSiO3 with ZnSO4: Na2SiO3 ratio icorr (µA)

1:1 at 450°C 0.2-0.3

1:3 at 450° 0.6-0.7

3:1 at 450°C 0.7-0.8

window for nucleation and grain growth, as is discussed in Chapter1. Increased corrosion

inhibition is due to increased activity of the ions as the temperature is increasing which can be

attributed to the reactivity of the powder and also particle size, to understand further how this

increased availability of inhibiting ions is present, various characterization are performed on the

inhibitors to develop our understanding further. Also the corrosion inhibition of the powders

processed using different molar ratios showed high corrosion current when compared to the

equal molar ratios which proves that the reaction of the precursor materials is in equal molar

ratios.

4.3. FTIR ANALYSIS

In the FTIR spectrum, absorption of the infrared spectrum is plotted versus wavenumber. The

bending, stretching, twisting symmetric and asymmetric vibrations of bonds, or such vibrational

bond deformation in the molecule can also lead to changes in the absorption spectrum. The

structural deformations in terms of changes observed in these vibrations, are studied through

FTIR .It takes more energy for stretching and asymmetric vibration of the bond so these

vibrations are seen at high wavenumber, and conversely bending and symmetric vibrations need

less energy, so are seen at lower wavenumbers [49]

For FTIR, the processed zinc silicate heat treated inhibitor pigments at different temperatures with

variation in the processing are analyzed with the help of standard FTIR peaks for silicates, zinc

56

oxygen and silicon bond peaks, by the concept of elimination. The silicate bonds are seen as a very

complex group of absorption bands in wide range of wavenumbers, with higher wavenumber 700-

1300 cm-1 for stretching vibrations and with lower wavenumber associated with bending

vibration’s absorption bands broadly seen in the range of 400-550 cm-1 and 600-700 cm-1.

The bending vibrations of Si-O-Si, O-Si-O bonds are seen at 458cm-1, asymmetrical vibrations of

the SiO4 tetrahedral chain. The absorption band at 819 cm-1 is due to the stretching of the O-Si-Ο

bond. In 800–1300 cm-1 wavenumber range for the stretching vibrations of the Si–O–Si. At higher

wavenumber value, as mentioned above the asymmetric stretching of Si–O–Si bond and SiO4

tetrahedral occur at higher wavenumber broadly in 800-1300 cm-1 and 900-1100 cm-1 respectively.

The absorption peak at 1080 cm-1 is due to the symmetric stretching vibration of the Si-Ο-Si bonds

[50][51].

Based on the above wavenumber values for specific vibrations, or range of vibration energy

associated, the processed inhibitor can be compared with the baseline to assign the specific peak,

the peak in the 550-570 cm-1 and 640-644 cm-1 can be attributed to Si-O-Si bending vibrations,

Heat treated silicate peaks in the range 890 – 970 cm-1 can be attributed to the stretching vibrations

of Si-O-Si The peak in the range of 1006 – 1030 cm-1 may be indicative of symmetric stretching

of Si-O-Si bonds. The peak in the range 1495- 1508 cm-1 can be attributed to Zn-O-Si bonds. The

peak in 1640 -1670 cm-1 is due to bending vibration of molecular H2O, the vibrations in the range

f 3000-4000 cm-1 is due to hydroxyl in water, which is due to stretching vibrations of O-H group

in H2O, wide range is due to formation of Hydrogen bond in wide range of strengths.

The O-H bonds of water molecules which occur around 1640-1670 cm-1 are not observed in the

pigments since all the powders are vacuum dried and sintered at 80°C to remove any residual

moisture of water from them.

57

Table 6. FTIR peaks for bonds present in the zinc silicate pigment [48]

500 1000 1500 2000 2500 3000 3500 4000 4500

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

ZS processes with stirring at 450°C

Ab

so

rba

nce

Wavenumber(cm-1)

without stirring 450°

with stirring 450°C

ZS processes without stirring at 450°C

Figure 4.16 FTIR analysis of the processed ZS powders with change in processing techniques

Group wavenumber(cm-1)

Si-O-Si 460

O-Si-O 800

Si-O-Si 1080

H2O 1640-1670

Zn-O-Si 2400

Zn-O, Zn-O-Si 2800

O-H 3400

58

1000 2000 3000 4000

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

Ab

so

rba

nce

Wavenumber(cm-1)

ZS at 600°HT

ZS at 450°HT

ZS at 300°HT

ZS at 80°C HT

Figure 4.17 FTIR for ZS inhibitors at different heat treatment temperatures

And for ZnSO4, the absorption peaks were found to be 1190, 1150, 1085, 1075, 1005, 695, 605, 451 [48].

Reports assigned some peaks to ZnO such as 2865-2971 cm-1 and 3419 cm-1 and 3409 cm-1 but the peaks are not

really characteristic to ZnO, and a registration of FTIR spectra between 400-500 cm-1 could have proved the

presence of ZnO. The 2450 cm-1 to 2560 cm-1 can be attributed to the zinc, silicon and oxygen peaks which can

be specific to zinc metasilicate bonds.

59

4.4. XRD ANALYSIS

Figure 4.18 JCPDF of zinc silicate (ZnSiO3)

The baseline xrd for ZnSiO3 were available as PDF (Powder Diffraction File) with the number

PDF- 01-078-5350. The figure 4.23 shows the xrd pattern for the zinc silicate with main peaks

between 30°- 45° 2theta. The powdered pigments at different sintered temperatures were measured

for xrd patterns. The crystallite sizes are measured for the different peaks.

60

20 30 40 50 60 70

0

20

40

60

80

100

120

140

160

80°C HT

inte

nsity

2theta

(2,1,2)

(3,2,2)


15 20 25 30 35 40 45 50 55 60

20

40

60

80

100

120

140

160

inte

nsity

2theta

ZnSiO3 No stirring 80°C

Figure 4.20 XRD of coprecipitated pigment without Stirring at 80°C

61

10 20 30 40 50 60 70

0

50

100

150

200

250in

tens

ity

2theta

300HT


10 20 30 40 50 60 70

0

20

40

60

80

100

120

140

160

180

200

220

inte

nsity

2theta

400HT


62

10 20 30 40 50 60

0

50

100

150

200

inte

nsity

2theta

450HT


10 20 30 40 50 60

0

50

100

150

200

250

300

350

400

inte

nsity

2theta

550HT


63

10 20 30 40 50 60

0

50

100

150

200

250

300in

tens

ity

2theta

600HT


10 20 30 40 50 60 70

0

100

200

300

400

500

inte

nsity

2theta

800HT

(4,0,0)

(3,1,1)

(4,2,0)1,3,1)

(3,0,2)

(3,2,2)

(2,3,2)


64

The crystallite size can be calculated from Debye – Scherer equation, which is,

𝑑 =𝑛 𝜆

𝛽 𝑐𝑜𝑠𝜃

where,

d – crystallite size

n – shape factor (constant = 0.9)

λ – Wavelength of the X - ray source

β – Full width and half maximum of the 100% intensity peak in radians

θ – Angle between the incident ray and the scattering planes.

Table 7 Crystallite sizes of the zinc silicate pigment at different sintering temperatures

Zinc silicate at temperatures

Peak 1(at 38)(nm)

peak 2 (at 44)(nm)

80° 32.16 21.23

300° 35.71 34.89

400° 42.29 34.16

450° 45.29 37.23

550° 44.4 33.82

600° 35.11 42.13

800° 16.53 10.33

The crystallite sizes are given in the table 5.

65

20 25 30 35 40 45 50 55 60 65

0

200

400

600

800

1000

1200

1400

Inte

nsity (

a.u

)

2theta (degrees)

800

600

550

450

300

80

XRD ZnSiO3

Figure 4. 27 Cummulative XRD of all the powders at different temperatures

The above figures show that XRD analysis performed on all the prepared powders with different

temperatures. From the literature we have observed that the characteristic peaks for ZnSiO3

compound is between 30-45° 2theta. As the temperature increases the crystallinity of the powder

was increasing. The particle size of the pigments was increasing in relation to the crystallinity as

the temperature was increasing. The powders at 450°C can be has the optimum particle size and

chemical bonding to give the maximum corrosion inhibition efficiency.

66

4.5. SEM:

SEM images of the zinc silicate inhibitors with different calcination temperatures and

magnifications are presented to observe the surface morphology of the inhibitors.



67



68



69



70



71

As the temperature was increasing the particle size of the powders was increasing. From the SEM

it is observed that as the temperature is increasing there is an increase in the grain size of the

pigments. And that can be attributed to the corrosion activity of these inhibitors.

The powders prepared at 80°C shows active phases and flocks of the particles which can be

attributed that there is a zinc silicate phase forming in the pigment. As the temperature is increasing

slowly the flock size starts decreasing and the growth of nuclei is clearly visible. At the higher

magnification that is 12000X as there is increase in the temperature the size of the crystals become

larger and the coagulation of the flocks is more transparent. And it can be claimed that figure 4

450°C the powders have the optimum level of flocks of grains and crystal size, since as the crystal

size is increasing the surface area decreases. As the surface area decreases the amount of area

present for releasing of the inhibitor species would be low there by reducing the corrosion

inhibition efficiency.

EDS:

EDS gives the information of the elemental composition of the powders. In relation to the SEM

testing the compositional analysis of the pigment can be done. The eds results shown in the figure

that the powder consists of Zinc, silicon and oxygen atoms. So it is evident that the powders

obtained are zinc metasilicate. There is slight amount of gold and platinum visible in the graph

which can be attributed to the sputtering of the pigments to make the powders conductive so as to

increase the electronic interaction to acquire information of the particles.

72

Figure 4.38 EDS of the powder at 80°C where the composition of the pigment is measured

O K 37.95 45.20 384.65 9.59

AlK 0.12 0.09 4.45 67.57

SiK 13.76 9.34 612.47 5.42

AuM 3.97 0.38 64.96 9.91

PdL 0.97 0.17 19.32 35.03

ZnK 18.34 5.35 139.07 5.59

Table 6. The percentage composition of the elements in the pigment

Figure 4.39 Graph showing the elements present in the pigment

73

4.6. Immersion Testing:

The immersion samples show that the inhibitor pigments protected the substrate from getting

corroded by forming a passive layer when comparison to the immersed samples in corrosive

environment without inhibitor.

Figure 4.40 Optical microscopy samples in 1% Nacl solution at 100 μm a) with the inhibitor b)

without inhibitor

Figure 4.41 Optical microscopy samples in 1% Nacl solution at 50 μm a) with the inhibitor b)

without inhibitor

74

Figure 4.42. Optical microscopy samples in 1% Nacl solution at 20 μm a) with the inhibitor b)

without inhibitor

From the figures 4.40, 4.41 and 4.42 it is clearly seen that the zinc silicate pigment acts as a

protection layer on the substrate from corroding.

Since zinc silicate is a mixed inhibitor the mechanism of protection could be attributed to both

anodic and cathodic reaction. There would be a zinc silicate layer forming on the substrate, slowing

down the whole cathodic reaction to prevent the corrosion of substrate. Free zinc ions in the

pigment present in the solution acts as cathodic reaction inhibiting species react with the corrosion

accelerating ions( chlorides in this case) and form zinc chloride. And the silicate ions act as anodic

reaction inhibiting species there by forming a passive layer on the substrate from reacting with the

corrosive environment.

75

5. Discussion:

The electrochemical data , and characterization results of pigments , now it can be clearly related

to that fact that for increased corrosion inhibition processing along with temperature plays a

significant role, from the electrochemical data it is clearly established that zinc metasilicate is an

effective corrosion inhibitor comparable to strontium chromate powders. The powders prepared

with precursor materials zinc sulfate and sodium metasilicate using coprecipitation technique

showed best performance and this performance is enhanced by increasing the stirring time which

yields to added time for nucleation and grain growth in the precipitated solution. So in the

coprecipitation technique the zinc metasilicate which is precipitated becomes more reactive as

there is an increase in the temperature. Also it is important to mention the key step in the

coprecipitation process, the stirring time of the precipitate had a major role in the activity of the

pigment. With increased stirring there is more scope for homogeneity in the system along with

particle interaction, this leads to increase in grain size due to nucleation and grain growth. This

increase in the crystallite size and grain size is seen in the SEM images of the inhibtors. The

increase in the inhibition is noticed until it reaches a temperature of 450°C. At this temperature

there is the optimum crystallite size of the pigment along with the reactivity. These processed

pigments were electrochemically tested to understand their corrosion inhibition behavior.

Along with electrochemical study we have the characterization to prove that the pigments

formed were zinc metasilicate. XRD and FTIR provide deeper understanding regarding the

structural characteristics of the pigment. JCPDF files of zinc metasilicate XRD were taken as

baseline to compare if the pigment had the zinc metasilicate peaks, it is also observed that theer

is increase in the crystallite size which is calculated from then Full width half maximum of the

76

peaks using scherrer’s formula. From the literatures we have known that zinc silicate is an

effective corrosion inhibition primer, so the peaks of zinc metasilicate can be attributed to the

corrosion activity of the pigment. Stirring of the precipitate made the powders more amorphous

leading to more surface area available for leaching and corrosion inhibition. FTIR analysis shows

that there are Si-O-Si peaks and also peaks which can be attributed as Zn-O-Si peaks. Also

distortion are introduced in the silicate system in the form of bond vibration energy. There are

changes in the whole system, and these changes are more specific to Si-O-Si bending and Zn-O-

Si stretching vibrations obtained from the FTIR data. These powder characteristic techniques

show that the basic zinc metasilicate system is confirmed and the phase formed with zinc

metasilicate at lower temperatures also has good activity in relation to corrosion.

The surface morphology changes of the substrate studied through the SEM, optical and EDX

show the presence of inhibitor. This inhibition is provided by the Zn and Silicates.

The inhibition mechanism of these pigments can be related to the zinc silicate primer coatings

which is studied in the literature. Zinc and silicate ions along with the zinc silicate compound

form a protective layer or passivation layer on the substrate to reduce the cathodic and anodic

reaction rates since these are mixed inhibitors. The processing of zinc silicate pigments has been

studied in the literatures there was no technique for making zinc metasilicate pigment with low

temperatures. This thesis provides a unique method of making this pigments along with relating

them to the corrosion inhibition activity, since they have corrosion inhibition efficiency

comparable to chromate inhibitors and can be easily made.

77

6. Conclusions:

1) Zinc metasilicate pigments were successfully developed using co-precipitation and

solution blending techniques from commercially available chemicals. The pigments developed

were characterized using XRD, TGA, SEM and DC-polarization techniques. The principles of

anion-cation exchange behavior were proven by DC-polarization testing, in which the metal

substrates showed corrosion resistance characteristics comparable to that of the chromate solution

2) Zinc metasilicate show the best result for CRS, with further increase in efficiency by heat

treatment process. Electrochemical analysis show corrosion inhibition of the processed pigments

for CRS steel. The corrosion values dropped from 10μA to 0.1μA which is substantial difference.

3) XRD, FTIR, SEM, EDX and Optical characterization provide in-depth understanding of

the pigments, which show that inhibition is due to the presence of zinc silicate bonds and inhibiting

species. These studies showed the structural relationship to the corrosion inhibition of the inhibitor

species.

This works successfully correlates the characterization of the pigments to its processing, providing

us with the understanding of processing of pigment for their optimal utility and corrosion

protection. Overall, this research has demonstrated that alternative corrosion inhibitive pigments

which are non-toxic in nature can be developed. These pigments have the potential to replace

chromate based pigments and can be developed from inexpensive zinc-silicate based compounds.

78

7. Suggestions for Future Work:

1. ICP-MS studies can be performed on the pigment to study the behavior of

inhibitor species over a period of time in a solution. This gives an initial idea

about how the anions and cations are behaving in the solution: a higher

concentration indicates that there is a higher rate of release of these ions which

could then take part in the corrosion inhibition process, whereas a lower

concentration means the ions would be less effective.

2. Different Heat treatment settings can be studies in the coprecipitation technique

of the zinc and silicate compounds with varying time for stirring.

79

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