Corrosion protection of low carbon steel by cation

APPROVED:

Seifollah Nasrazadani, Major Professor Nourredine Boubekri, Committee Member Phillip R. Foster, Committee Member Enrique Barbieri, Chair of the Department of Engineering Technology Costas Tsatoulis, Dean of the College of

Engineering Mark Wardell, Dean of the Toulouse Graduate

School

CORROSION PROTECTION OF LOW CARBON STEEL BY CATION

SUBSTITUTED MAGNETITE (Fe3O4)

Ameya Phadnis, B.E

Thesis Prepared for the Degree of

MASTER OF SCIENCE

UNIVERSITY O F NORTH TEXAS

May 2013

Phadnis, Ameya. Corrosion protection of low carbon steel by cation substituted magnetite

(Fe3O4). Masters of Science (Engineering Systems - Mechanical Systems), May 2013, 60 pp., 8

tables, 44 illustrations, references, 36 titles.

Surfaces of low carbon steel sheet were modified by exposure to highly caustic aqueous

solutions containing either chromium or aluminum cations. Corrosion resistances of such

surfaces were compared with that of steel surfaces exposed to plain caustic aqueous solution. In

all cases a highly uniform, black coating having a spinel structure similar to magnetite (Fe3O4)

was obtained. The coated steel surfaces were characterized using X-ray photoelectron

spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive spectroscopy

(EDS), and Fourier transform infrared spectrophotometry (FTIR). Polarization resistances (Rp)

of modified steel surfaces were measured and compared with that of bare steel surfaces. Results

indicate that chromium (Fe2+ Fe3+x Cr3+

1-x) or aluminum (Fe2+ Fe3+x Al3+

1-x) substituted spinel

phases formed on steel surfaces showed higher Rp values compared to only magnetite (Fe2+

2Fe3+O4) phase formed in the absence of either chromium or aluminum cations. Average Rp

values for steel surfaces with chromium containing spinel phase were much higher (21.8 kΩ) as

compared to 1.7 kΩ for bare steel surfaces. Steel surfaces with aluminum containing spinel phase

and steels with plain magnetite coated samples showed average Rp values of 3.3 kΩ and 2.5 kΩ

respectively. XPS and EDS analysis confirmed presence of cations of chromium and aluminum

in Fe3O4 in cation substituted samples. FTIR results showed all coating phases were of spinel

form with major absorption bands centered at either 570 cm-1 or 600 cm-1 assigned to Fe3O4 and

γ-Fe2O3 respectively.

Copyright 2013

by

Ameya Phadnis

ii

ACKNOWLEDGEMENTS

I would like to thank Dr. Seifollah Nasrazadani, Major professor for his encouragement,

suggestions, invaluable guidance and support throughout this Research. His morale support and

continuous guidance has enabled me to complete my work.

I appreciate the help that Dr. Nourredine Boubekri, committee member for helping me

with the suggestions for data Analysis and his recommendation for improvement of thesis.

I would like to thank Dr. Phillip Foster for his assistance as a committee member and

providing useful suggestions for enhancing the Thesis work.

I would like to mention special thanks to Dr. Manuel Quevedo-Lopez and Mr. Victor

Martinez-Landeros for their help with XPS work.

I am, as ever, especially indebted to my parents Mrs. and Mr. Suresh Shamrao Phadnis

for their love and support throughout my life.

iii

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ........................................................................................................... iii LIST OF TABLES ......................................................................................................................... vi LIST OF FIGURES ...................................................................................................................... vii CHAPTER I INTRODUCTION .................................................................................................... 1

Problem Statement .............................................................................................................. 4

Research Objective ............................................................................................................. 5

Research Questions ............................................................................................................. 5

Assumptions ........................................................................................................................ 5 CHAPTER II LITERATURE REVIEW ....................................................................................... 6

Cations Substituted Magnetites........................................................................................... 7

Research Methodology ..................................................................................................... 10 CHAPTER III EXPERIMENTAL PROCEDURE ...................................................................... 11

Electrochemistry Setup ..................................................................................................... 11

Potentiodynamic Test........................................................................................................ 13

Analytical Techniques for Coatings Characterization ...................................................... 16

Fourier Transform Infrared Spectrometry ............................................................ 16

Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) .................................................................................................................... 17

X- Ray Photo Electron Spectroscopy ................................................................... 18 CHAPTER IV RESULTS AND ANALYSIS ............................................................................. 19

Design of Experiments ...................................................................................................... 19

Thickness and Morphological Evaluation ........................................................................ 19

Scanning Electron Microscopy/Energy Dispersive Spectroscopy of Coated Samples .... 22

X-Ray Photoelectron Spectroscopy Results ..................................................................... 33

Fourier Transform Infrared Spectroscopy Results ............................................................ 40

Corrosion Tests Data......................................................................................................... 46

Chromium Substitution ......................................................................................... 46

iv

CHAPTER V CONCLUSIONS AND RECOMMENDATIONS ................................................ 56 REFERENCES ............................................................................................................................. 58

v

LIST OF TABLES

Page

1. Physical, mechanical, optical and magnetic properties of magnetite ............................................. 3

2. Bath composition for preparation of different magnetite coated low carbon steel ............11

3. Design of experiments .......................................................................................................19

4. FTIR absorption bands for iron oxides and oxy-hyfroxides ..............................................40

5. Chromium substituted corrosion tests readings .................................................................51

6. Aluminum substituted corrosion tests readings .................................................................52

7. Magnetite corrosion tests readings.....................................................................................53

8. Averages readings for polarization resistance (Rp) with standard deviation ....................54

vi

LIST OF FIGURES

Page

1. Structure of magnetite ..........................................................................................................2

2. Experimental setup for electrochemistry ...........................................................................13

3. Potentiodynamic polarization plot for corrosion of an alloy in a corrosive solution .........14

4. Potentiodynamic polarization curve showing data for icorr calculations ..........................16

5. (a, b): Scale on rebar steels that are about 20-40 µm thick ................................................20

6. (a, b): SEM images of the edge of sample obtained with magnetite coatings ...................21

7. (a, b) EDS spectra for edge of a steel sheet, 15 minutes coated sample showing oxygen peak ....................................................................................................................................23

8. SEM image of chromium substituted magnetite coatings after 15 minutes ......................24

9. EDS spectra for chromium substituted magnetite after 15 minutes ..................................24



12. SEM image of chromium substituted magnetite coatings after 30 minutes. .....................26


14. SEM image of chromium substituted magnetite coatings after 30 minutes. .....................27



17. EDS Spectra for chromium substituted magnetite after 45 minutes ..................................29



20. SEM image of the aluminum substituted magnetite coatings after 15 minutes .................30

21. EDS spectra for aluminum substituted magnetite after 15 minutes ...................................31


vii




26. XPS spectrum for chromium substitution for 45 minutes .................................................35

27. Sputtering of chromium substituted magnetite sample of 45 minutes ...............................36



30. XPS spectrum for aluminum substitution for 45 minutes ..................................................38

31. Sputtering of aluminum substituted magnetite sample of 45 minutes ...............................38



34. FTIR spectra of Cr substituted Fe3O4 and γ-Fe2O3 .........................................................41

35. FTIR results of comparison of magnetite versus chromium substituted magnetite after 15 minutes ...............................................................................................................................42



38. FTIR results of comparison of magnetite versus aluminum substituted magnetite after 15 minutes ...............................................................................................................................44



41. Overlay of the plots for all the samples for 15 minutes .....................................................48



44. Comparison of polarization resistance values for different coatings .................................54

viii

CHAPTER I

INTRODUCTION

Aqueous corrosion is a phenomenon that occurs because of the presence of moisture.

Millions of dollars are lost every year because of the corrosion. Most of the loss is due to the

corrosion of iron and steel, though many other metals also corrode. It has been estimated that the

total cost of corrosion estimated is around $275.6 billion every year that include the direct and

the indirect cost of corrosion that accounts for 3.54% of gross domestic product [www.nace.org].

The direct cost of corrosion includes infrastructure, utilities, transportation, manufacturing and

government. The indirect costs of corrosion associated are cost of labor, equipment, cost

associated with the disruption of the supply of product and cost of loss of reliability. The four

basic methods for corrosion protection are protective coatings, cathodic protection, corrosion

inhibitors and using the materials resistant to corrosion. Coating the surface of the steel with the

help of magnetite deposition is one of the reliable methods.

Magnetite is an iron oxide mineral having magnetic properties with very distinguishing

characteristics. It is a member of spinel group having standard formula A(B)2O4. The A & B

represents different metal ions that occupy specific sites in the crystal structure. In magnetite

(Fe3O4), A cations are Fe+2 and B cations are Fe+3; two different metal ions in two different sites.

The arrangement causes a transfer of electrons between different ions in structured path

or vector. Magnetite coatings have varieties of applications such as steel coating in power plant

piping systems, petrochemicals, steelmaking, power generation stations, paint systems,

biotechnology, foundry, polishing compounds, cosmetics, polymers, construction, cement

manufacturing, fertilizers and biomedical applications.

1

Figure 1: Structure of magnetite

Silicon coated magnetite particles are used in water purification for the removal of Hg2+.

Magnetite has a great potential of removing the heavy metal ions from the polluted water with

the help of magnetic separation [1]. Magnetite particles are used in efficiently removing the

arsenic (III) and arsenic (V) from the water, the efficiency of which increases if the size of the

magnetite particle decreases [2]. Due to its high reductive capacity, magnetite containing Fe (II)

and Fe (III) is used to treat soil contaminants and ground water, which contains organic

compounds and hexavalent chromium that are harmful and toxic for living habitats [3].

The catalytic application of magnetite involves production of hydrogen through water gas shift

reaction [4].

2

Table1: Physical, mechanical, optical and magnetic properties of magnetite

Luster Metallic

Tenacity Brittle

Magnetic characteristics Ferromagnetic

Solid Density (gm/cm3) 5.1

Streak Black

pH 7

Transparency Opaque

Mohs hardness @20oC 5.5 – 6.5

Fracture Conchoidal

Specific Gravity 5.17 – 5.18

Color Black to Grayish

Crystal system Isometric

Particle shape Irregular

Due to its physical and chemical properties, availability and low cost magnetite has

several scientific and technological applications. Magnetite is used in ammonia synthesis through

Fischer-Tropsch reaction, which is important to get high value intermediates in chemical and

petrochemical industries. Magnetite is used as a catalyst in reactions such as hydrogenation,

ketonization, methane reforming and selective oxidation. Magnetite is also used as a catalyst

support for gold and palladium for various applications in addition to polymer waste treatment.

The magnetic nature allows it to be separated easily from the medium after the reaction. It is also

used as a catalyst for heterogeneous Fenton reaction [5].

Magnetite has applications in biotechnology, which includes selection of cells, stem cell

tracking differentiation and bio sensing. It also acts as a contrast agent in magnetic resonance

imaging to drug screening [6].

3

Magnetite is used as an iron oxide additive in glass and mineral wool. Magnetite also

finds application in heavy media separation, which involves coal washing, and scrap metal

separation, for coating industrial water steam tube boilers, for the production of synthetic fuels,

foundry applications where in it is used in moldable chill sand or anti finning additive. Magnetite

coating dissolution in power plant piping systems takes place through flow-accelerated corrosion

process that can cause catastrophic consequences.

The area of focus in this research is corrosion protection of low carbon steel by using

magnetite. Steel is the most commonly used metal that corrodes due to atmospheric corrosion.

Steel is selected for most of the applications because of its properties such as strength, toughness,

ease of fabrication and costs. Ferrous alloys have the property of resisting the corrosion by

forming a thin layer of oxide. Rebar steels used in continually reinforced concrete pavements

(CRCP) are coated with mill scale that contains mainly magnetite. Understanding characteristics,

formation, and transformation of Fe3O4 can contribute to reduction of materials damage and

waste as well as development of new products with enhanced qualities for different applications.

Problem Statement

Rebar steel used in construction of CRCP roads are coated with a scale layer that contain

different iron oxides (FeO, Fe2O3, and Fe3O4) constituents. Mill scale coated rebar steel is proven

to be vulnerable to corrosion in concrete leading to pavement cracking due to expansive

characteristics of the oxide layer. Another potential application of the enhanced magnetite

coating is for protection of cold-formed steels used in construction of residential and commercial

buildings.

4

Research Objective

The main objective of this research is to form magnetite coatings on low carbon 1018

steel sheets through a caustic aqueous solution with and without cations of Al and Cr. Al and Cr

cations substituted magnetite is expected to impart favorable corrosion resistance properties to

the magnetite coating by increasing polarization resistance of the low carbon steel. Steel sheets

coated with and without cations substituted magnetite were formed through a wet chemistry

process and their corrosion resistance was evaluated.

Research Questions

Specific questions for which answers were sought in this research are as follows:

1. Can Cr3+ or Al 3+ containing hot caustic baths substitute these cations in the matrix of magnetite that produces a uniform magnetite coating on low carbon steel sheets?

2. How does aluminum and chromium cation substituted magnetite coated steel sheets resist corrosion as compared with plain magnetite coated?

3. Which cation-substituted magnetite is best corrosion resistant as compared with plain magnetite coated?

Assumptions

1. The surface finish achieved across all the samples had no impact on corrosion resistance of the coated 1018 steel samples.

2. Differences in coating deposition temperature due to changing bath chemistries had no effect on corrosion resistance of the coated 1018 steel samples.

3. The 1018 steel samples are from same single sheet of material and samples chemistry is uniform.

5

CHAPTER II

LITERATURE REVIEW

The process of anticorrosion magnetite coating for the protection of 1018 low carbon

steel has been studied over the years. Different baths and methods are used to form magnetite

coatings. The features of artificial magnetite coatings in hot nitrate solution by the means of

electrochemical methods are studied by Flicker noise spectroscopy [7]. The accelerated corrosion

was characterized quantitatively under severe conditions exposed to hundred percent relative

humidity and daily showering. Also the tendency of steel with magnetite coatings to corrosion

was estimated by comparing the structural parameters using Flicker noise spectroscopy.

The preparation of magnetite by electro deposition in the form of nanostructure inside the

pores of polycarbonate membrane is studied over the years. The electrodeposited material

exhibits strain. The capacitance mechanism of magnetite electrochemical capacitor in aqueous

solution was studied by using the various techniques electrochemical quartz- crystal-

microbalance analysis & X ray photoelectron spectroscopy. The results observed indicated that

the low capacitance was the result of surface oxidation of the oxide electrode [8].

The corrosion study was also done in the field of civil engineering with the researches in

concrete reinforcement when it is exposed to various environments. The techniques of corrosion

potential monitoring, linear polarization method and electrochemical impedance spectroscopy for

the corrosion measurement of steel reinforcement in chloride atmosphere have been analyzed.

The corrosion rate values were higher for the rebar in concrete having high weight concrete ratio

[9]. The effect of chloride induced corrosion products at the rebar concrete interfaces and on the

crack surfaces at different loading conditions was studied. The study concluded that irrespective

of type of loading the corrosion products on the rebar steel in ordinary Portland cement concrete

6

had an average spread along the bar. The corrosion products built up from the crack into the

surface. Corrosion products built up from the cracks were responsible for the initiation of

cracking. The results differed in high performance concrete where in the branched cracking was

observed [10]. The mill scale properties of the rebar steel using scanning electron microscopy

and focused ion beam microscopy were studied. The effects of the chloride thresholds of the

underlying steel were studied. Also a crevice corrosion mechanism was proposed explaining the

reasons of rebar steel without mill scale having high de-passivation [11].

The corrosion behavior of steel bars galvanized using three different baths viz. Zn-Pb

bath, Zn-Ni-Bi bath and Zn-Ni-Sn-Bi baths was studied. The corrosion rate and the potential

were monitored with the bars exposed to 5% NaCl solution and tap water. The highest corrosion

rates were obtained from the coating of the bar coated in the Zn-Ni-Sn-bi bath [12].

The optimization of combined mechanical strength and corrosion behavior of steel rebar

steel is currently being studied in Egypt where the direct relationship could not be established but

the results indicated that cooling conditions and process parameters for thermo mechanical

treatment should be selected on the basis of corrosion requirements to produce the desired

mechanical properties [13].

Cations Substituted Magnetites

Magnetite is coated on the steel to protect it from corrosion. Magnetite containing the

chromium is prepared using the conventional co-precipitation method. The method involves

thermal treatment up to 270o C with O2 leads to the oxidation of Fe2+, which produces

chromium-substituted maghemite. At 600°C the cubic maghemite converts into hexagonal

7

hematite, which expels Cr from the iron oxide. When the concentration is low, Cr replaces Fe3+

and when the Cr content increases Fe2+ is substituted [14].

When Fe3+ ions are substituted by aluminum or chromium in octahedral site the

magnetite is gradually replaced by four characteristics bands of normal splines in Mossbauer

spectrogram. Experimental results have shown that when the aluminum substitutes chromium all

the bands shift towards low frequencies. The Fe2+ ions move into the octahedral site when there

is a gradual substitution of Al3+ ions by Fe3+ ions. When the substitution ratio is low the broad

absorption bands coincide with a continuous absorption specific to inverse spinel II-III that

contains mobile electrons having higher electrical conductivity [15].

Magnetite is prepared generally by wet methods or solid phase reactions. The

hydrothermal method is preferred as it involves low temperature, improved control of powder

homogeneity and uniform particles. The polycrystalline-substituted magnetite is generally

prepared by hydrothermal method [16]. Steel is also protected by various coatings such as zinc or

by aluminum. The steel to be protected for a longer time period is achieved by passivation of

bare steel. The overall passivation reaction is accelerated by addition of aluminum powder to the

zinc coating. The effect of deposition of corrosion products to electrochemical measurements in

presence of sodium bicarbonate solution has also been studied. Steel surfaces are protected by

anodic passivation by formation of compact film of magnetite overlaid by the other iron oxide

films [17]. The influence of chloride concentration on the surface of the steel has been also

investigated where the results suggested that the magnetite forms in large amount compared to

maghemite. The grain size of the magnetite is found to be large compared to maghemite. The

presence of Si, P, Ni, Cu and Cr affects the type of spinal phases to be formed in the typical

8

chemical composition of steel. The size of the grain depends upon the immersion time. Wider

distribution of range of average grain size is observed if the immersion time is longer [18].

The electrochemical behavior of iron oxides is also studied by immersing the steel in rust

converter. Oxide converter is one of the alternatives used for the protection steel surface having

some amount of rust. The results obtained suggested that electronic conductivity and cathodic

rate of reaction decreases. The presence of copper compound also represented an additional

cathodic reaction that accelerates the process of magnetite oxidation [19].

There are certain methodologies used for steel protection in addition to magnetite coating

process. Another coating over magnetite layer is formed for further protection.

Magnesium ferrite is formed over the magnetite coating at higher temperature in aqueous

solutions to increase the resistance to corrosion. The corrosion resistance can be improved

by formation of bilayer oxide coating, which increases the impedance twice as compared to

magnetite coatings [20]. The formation of passive films on the surface of the mild steel in

alkaline media is also investigated by application of anodic potential. The study involved

the formation of thin layers on the mild steel by the application of different anodic

potentials for characterizing the morphology, composition and electrochemical behavior.

The results indicated that oxides containing Fe2+ and Fe3+ composed the surface films. The

Fe2+ contribution disappear when the potential of the film formation is increased in passive

domain [21].

Nasrazadani [22] studied temporary corrosion protection of steel by deposition of

magnetite coatings containing aluminum and chromium cations using polarization resistance

measurement in caustic solution. This research is the continuation of the previous work, which

will emphasizes the study of chromium and aluminum cations substitution in magnetite films for

9

steel protection by studying chemical composition of top few mono-layers using x-ray

photoelectron spectroscopy and polarization resistance techniques.

Research Methodology

The research includes preparing magnetite coated 1018 steel samples along with cations

substituted magnetite and analyzing the results with the help of various tools like scanning

electron microscopy, energy dispersive spectroscopy, Fourier transform infrared spectroscopy

and studying the surface characteristics with the help of X-ray photo electron spectroscopy.

10

CHAPTER III

EXPERIMENTAL PROCEDURE

The 1018 low carbon steel samples of dimension of 1”x 2” were used. The surfaces of the

steel were cleaned with silicon carbide paper of grit sizes 120, 240 and 400.The samples were

polished up to 5µm alumina particle size. The samples were cleaned with acetone to make them

dirt and oil free. A coating bath was prepared by adding 100g of Na (OH), 25g of NaNO2 and

6.25g of NaNO3 . The magnetite coatings were prepared at temperature range of 140oC – 155oC

for time intervals varying from 15 minutes to 45 minutes. Exposing the samples in 3.5% NaCl

solution the Electrochemistry tests were conducted. The surface treatment is a science that can be

used to achieve the alteration in dimensions, improve the conductivity, increase or decrease the

friction, improve the resistance to corrosion and reduction of cost. [7].

The table 2 shows the bath composition for preparation of different magnetite coated low

carbon steel:

Table 2: Bath composition for preparation of different magnetite coated low carbon steel

SAMPLE Na (OH) (g) NaNO2 (g)

NH4NO3 (g)

Cr(NO3)3 (g)

Al(NO3)3 (g)

Magnetite 200 50 12.5 - -

Aluminum substitution 200 50 - - 12.5

Chromium substitution 200 50 - 12.5 -

Note: 250 ml of water used in all cases.

Electrochemistry Setup

A total of 27 steel samples with the dimensions of 1” x 2” were used. All samples were

cleaned with water and ethanol before using them for testing in order to remove any

11

contaminants or oil on the sample that might affect our testing results. The samples were cleaned

with silicon carbide paper of grit size 120, 240 and 400. The samples were polished up to 5µm

alumina particle size.

Corrosion tests were carried out on steel samples using a Princeton applied research

advanced electrochemical system (PARSTAT 2273). The PARSTAT 2273 potentiostat has five

leads that are used to connect to external cell and the computer. The five leads are as follows: the

white lead connects to the reference electrode, the green alligator clip connects to the working

electrode, the red alligator clip connects to the counter (auxiliary) electrode, the gray “sense”

lead connects to the working electrode, and the black alligator clip lead is ground, and be

connected somewhere to connect ground. A 3.5% NaCl solution was prepared using 35g of NaCl

crystals mixed with 1000 mL of water to use as the solution. 3.5% NaCl solution has the same

corrosive effect as compared to sea water. Corrosion test setup is shown in Figure 2.

Potentiodynamic polarization data was obtained in the form of potential vs. logarithm

current density curves in the range from -500 to 2000 mV using the PARSTAT 2273 computer

controlled potentiostat. The PARSTAT is controlled completely within the electrochemistry

power suite software. The scan height used was of 1 mV along with a scan rate of 0.1660 mv/s.

The step time used was that of 6.024 s plotting 1851 data points. Electrode potentials were

measured against a saturated calomel reference electrode. After corrosion tests, the surface of the

samples were examined using a scanning electron microscope (SEM).

12

Figure 2: Experimental setup for electrochemical tests.

Potentiodynamic Test

Potentiodynamic tests are run in order to evaluate corrosion rate of a metal in an

electrolyte. The potentiodynamic polarization technique is also used to gain a qualitative

understanding of a metal behavior (passive or active) in a corrosive solution. It also detects any

tendency of the substance to passivate. The potential range used to produce polarization plot is

13

often broad, ranging from -250 mV with respect to corrosion potential (Ecorr) to +1.5V or more.

Ecorr is an electrode potential resulting from a simultaneous action of more than a single

reduction or oxidation reactions, while the net electrode current is zero (Figure 3).

Figure 3: Potentiodynamic polarization plot for corrosion of an alloy in a corrosive solution.

The potentiodynamic technique scans the corrosion process in real time consisting of an

anodic region and a cathodic region. Tafel plot, which is a plot of log I versus E are used to

measure Tafel slopes (βa = anodic Tafel slope and βc cathodic Tafel slope). The graphs vertical

axis consists of the potential of the metal and the horizontal axis consists of the logarithm of

absolute current. The sharp point is the point where the current changes signs as the reaction

14

changes from anodic to cathodic, or vice versa. Overvoltage in anodic and cathodic regions are

defined by the following relationships:

ηa = βa log ia

ηc = βc log ic

Tafel plots were generated for bare samples, as well as for magnetite-coated samples and

Tafel Fit analysis was performed on each plot. This Tafel Fit analysis provided us with all the

corrosion parameters needed to calculate the corrosion rate of the sample being tested. The

corrosion rate parameters were obtained by using the electrochemistry power suite software

built-in Tafel Fit analysis. The parameters obtained were corrosion potential (Ecorr), corrosion

current (Icorr), and the polarization resistance (Rp). The same potential range was selected for all

the samples for the comparison.

Normally a classic Tafel analysis is performed by extrapolating the linear portions of a

log current versus potential plot back to their intersection. The intersection is Icorr or corrosion

current and Ecorr or corrosion potential.

The electrochemistry power suite software can calculate all the corrosion parameters. The

parameters can also be calculated manually. For example, one can calculate the Icorr manually by

using the Stern-Geary [23] equation

Icorr = βaβc / 2.3Rp (βa+βc)

Where, Rp is the polarization resistance.

The same equation can be used to calculate the Rp value by solving for Rp. The corrosion

rate (CR) can also be calculated manually by using the equation

CR = Icorr*K*EW/dA

15

where, K is the constant that defines the units for the corrosion rate, EW is the equivalent weight

in grams/equivalent, d is density in grams/cm3, and A is the sample area in cm2. Figure 4 shows a

typical potentiodynamic polarization curve for a representative sample used in this study.

Figure 4: Potentiodynamic polarization curve showing data for Icorr calculations.

Analytical Techniques for Coatings Characterization

Fourier Transform Infrared Spectrometry

Coated steel samples were characterized for phases present after coating process was

completed using Fourier transform infrared spectrophotometer (FTIR). FTIR identifies which

16

phases Fe3O4 γ-Fe2O3 or their cation-substituted varieties are present. An infrared spectrum

represents a fingerprint of a sample with absorption or transmission bands, which correspond to

the frequencies of vibrations between the atoms bonding them in a material. The coatings

obtained after different exposure time periods in a caustic bath were tested utilizing this

technique. The advantages of this technique are; it is simple, very fast (produces spectrum in

seconds), easy to use with minimal sample preparation. According to prior results of Nasrazadani

and his co-workers it is anticipated to observe very broad bands at 570 cm-1 for Fe3O4 whereas

the medium broad bands having IR peaks ranging 430 - 700 cm-1 (centered around 600 cm-1) are

expected for γ-Fe2O3. The oxides will be detected in case of substituted magnetite.

Potassium bromide (KBr) was used to mix with oxide powder samples in a ratio of 100

mg KBr to 2 mg of sample. The mixtures were pulverized in a mortar and pestle set and pressed

in a stainless steel die under 10000 psi compression for 3-4 minutes under continuous air suction.

The spectrometer collects a blank background spectrum collection to be subtracted from the

spectrum of the sample being studied. The sample is then placed in the sample holder and its 32

scans of the sample spectrum is generated at 2 cm-1 resolution. Either transmission or absorbance

data can be collected to plot resulting absorption bands that is the oxide fingerprint.

Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)

SEM and EDS techniques were used to study oxide morphology (SEM) and detection of

substituted elements (EDS) namely Cr and Al. Energy dispersive spectroscopy makes the use of

X ray spectrum emitted by a solid sample bombarded with a focus beam of electron to provide

the analysis. All the elements of technological importance having various atomic numbers can be

detected using this technique. The system involves two types of analysis viz. semi-quantitative

17

analysis and qualitative analysis. Semi-quantitative analysis involves the determination of the

concentration of the elements present, whereas the qualitative analysis involves the identification

of the lines in the spectrum. It is a fairly fast technique used to identify the concentrations of

materials within a given sample.

X- Ray Photo Electron Spectroscopy

X ray photoelectron spectroscopy (XPS) is quantitative technique used for measuring the

elemental composition, chemical and electronic state of elements that exists within a material.

Unique feature of XPS is its surface sensitivity and characterization of a few top monolayers in a

given sample. The XPS spectra are obtained by irradiating the material with X-rays while

measuring the kinetic energy and number of electrons that escape from the top within 1-10 nm of

the material that is being analyzed. The technique is used for studying the surface chemistry of

the material. Thin films formed on the surfaces of the samples are analyzed for a few

monolayers. In this technique, kinetic energy (KE) of the ejected photoelectron is measured and

set equal to the difference between incident electron beam energy (hv) and binding energy (BE)

according to the following equation [24].

KE = hv -BE

In this study physical electronics with Model 11-066 gun control at 5.4x10-8 torr, using

Al X-ray source (X-ray photon characteristic energy, hv = 1486.6 eV). Ar sputtering at 4 kV with

probe size of 3x3 mm2 was utilized.

18

CHAPTER IV

RESULTS AND ANALYSIS

Design of Experiments

Table 3 shows the detailed action plan for the experiments conducted on the samples. The

samples will be evaluated using various techniques as mentioned.

Table 3: Design of experiments to be performed

SAMPLES

CORROSION RESISTANCE EVALUATION CORROSION RESISTANCE EVALUATION POTENTIODYNAMIC

POLARIZATION (PASSIVATION)

POLARIZATION RESISTANCE

(Rp) XRD FTIR SEM/EDS XPS/

AES

Uncoated Samples No Low Steel

Phase No Morphology No

Steel + Magnetite

coating May be Medium Fe3O4

Fe3O4 or γ-

Fe2O3 Morphology Yes

Magnetite + Al

substitution May be High Spinel Detect

Oxides Check for Al Al Detection

Magnetite + Cr

Substitution May be High Spinel Detect

Oxides Check for Cr Cr Detection

Thickness and Morphological Evaluation

As a potential application of these coatings, thickness of scale in currently applied rebar

steels were measured and mill scale on a typical rebar was about 20-40 µm thick as shown in

Figure 5 (a-b). The thickness of the coating deposited on the edge of the samples is shown in

Figure 6 (a, b). Steel samples are coated with magnetite coatings for different time durations and

thickness of the coatings are measured using SEM. Figure 6 (a-b) shows measured thickness

values on SEM micrograph of each sample.

19

(a)

(b)

Figure 5 (a-b): Scale on rebar steels that are about 20-40 µm thick

20

(a)

(b)

Figure 6 (a, b): SEM images of the edge of sample obtained with magnetite coatings

21

Scanning Electron Microscopy/Energy Dispersive Spectroscopy of Coated Samples

Steel sheets coated with magnetite films containing cations of aluminum or chromium

were analyzed using EDS. Examples of results obtained for steel and plain magnetite coated steel

is shown in Figure 7 (a) and (b) respectively. Figure 7 (a) shows peak belonging to only iron (Fe)

as expected and the magnetite coated sample (coated for 15 minutes) showed both Fe and

oxygen peaks (Figure 7. b). It is expected that cation containing magnetite films show aluminum

or chromium peak showing effectiveness of the coating process. EDS is a semi-quantitative

technique and is useful for providing information on presence or absence of elements in a given

sample and quantitative data it produces can only be treated as an estimate of the chemical

composition.

(a)

22

(b)

Figure 7 (a, b): EDS spectra for edge of a steel sheet, 15 minutes coated sample showing oxygen peak

Figure 8 shows SEM micrograph of Cr substituted magnetite coating on steel surface at

low magnification indicating the morphology of the coated surface. Surface fine scratches

formed in the grinding step of sample preparation shows up even after the magnetite coating due

to low thickness of magnetite film. Corresponding EDS spectrum of the sample is shown in

figure 8 indicating presence of contaminants such as Na, Si, and Ca. Source of contaminant

elements are impurities in chemicals used to make caustic bath. Cr presence in the EDS spectrum

indicates effectiveness of the wet chemistry process in formation of Cr substituted oxide.

23

Figure 8: SEM Image of chromium substituted magnetite coatings after 15 minutes

Figure 9: EDS Spectra for chromium substituted magnetite after 15 minutes

24

Another example of coated steel in a Cr containing bath for 15 minutes deposition

time is shown in Figure 10. Figure 10 shows SEM micrograph indicating presence of surface

film fracture cracks. Crack formation may have been due to deep scratches in the bare steel.

These sites, if left uncovered could potentially act as anodic sites leading to excessive

corrosion rates. Corresponding EDS spectrum for the sample coated for 15 minutes is shown

in figure 11 showing elements constituting Cr, Fe, O and other impurity elements.

Figure 10: SEM image of chromium substituted magnetite coatings after 15 minutes.

Figure 11: EDS spectra for chromium substituted magnetite after 15 minutes

25

Samples coating for longer duration of 30 minutes showed stronger Cr peaks in their

EDS spectrum while morphology of the films remain similar. SEM and EDS results of a

typical sample coated in Cr containing bath for 30 minutes are shown in figures 12 and 13

respectively. Figure 12 shows a low magnification SEM micrograph of the sample indicating

scratches formed in grinding step. Figure 13 shows a strong Cr peak that is indicative of

higher Cr concentration in this sample. Similar trend was observed in other samples exposed

to longer deposition times as compared to cases where exposure times of 15 minutes were

used. Figures 14-19 shows other examples where higher Cr substitution was obtained in

longer coating durations of 30 and 45 minutes. SEM images (Figures 14,16, and 18) show

nearly similar surface features and their corresponding EDS spectra (Figures 15,17,and 19)

show relatively strong presence of Cr constituents in these samples.


26


Figure 14: SEM image of chromium substituted magnetite coatings after 30 Minutes.

27


Figure 16: SEM image of chromium substituted magnetite coatings after 45 minutes

28

Figure 17: EDS Spectra for chromium substituted magnetite after 45 minutes


29

Figure 19: EDS spectra for chromium substituted magnetite after 45 minutes.

Figure 20: SEM image of aluminum substituted magnetite coatings after 15 minutes.

30

Figure 21: EDS spectra for aluminum substituted magnetite after 15 minutes.


31



32


The SEM and EDS spectra for the chromium-substituted and aluminum-substituted

samples are shown in the above pictures. The EDS analysis shows the presence of chromium and

aluminum in the spectrum for all the samples of various time duration of 15 minutes, 30 minutes

and 45 minutes. The presence of chromium in the EDS spectrum shows that the chromium ions

get induced in the surface of the coating of the magnetite protecting it from further corrosion.

The presence of aluminum is not as strong as compared to chromium. As specified at the start of

the research study the aim was to check for the presence of the chromium and aluminum ions in

the magnetite coating the chromium and aluminum ions were detected in the EDS spectra.

X-Ray Photoelectron Spectroscopy Results

XPS is a highly surface sensitive technique that is capable of providing chemical

composition information from top few monolayers of the thin film materials. In this study, it

is important to know the surface chemistry of films that are a few micrometers thick and

commonly used chemical analysis techniques such as EDS for bulk materials will not be able

33

to provide surface information. In this technique, binding energy of a characteristic

photoelectron (core electron) of an element released from top layer atoms of a material under

study is measured and used for elemental identification [24]. Intensity of such photoelectrons

is used to perform quantitative analysis provided a calibrated system is available. A useful

feature in this analysis is to sputter layers by layers from top surface of a sample and perform

chemical analysis of each layer to gain elemental concentrations at different depths from

surface.

The photoelectron spectroscopy utilizes the photo-ionization and analyses the kinetic

energy of the photoelectrons to study the composition of the electronic state of the surface

region of the sample. XPS uses the soft x-rays having an energy level of 200-2000ev for

examining the core levels. In XPS the photons are absorbed by an atom in a molecule or solid,

which leads to the ionization and emission of inner shell electron. The number of emitted

photoelectron as a function of kinetic energy is measured using the electron energy analyzer

thus recording the spectrum. The difference in the energy between the ionized and the neutral

atom is called the binding energy of the electron, which is given by [24]:

KE = hv - BE

Figure 26 shows XPS spectrum of a sample coated in a bath containing Cr cations for 45

minutes. EDS results for these samples was discussed earlier and indicated that 45 minutes

coated samples show higher intensity of Cr concentration therefore 45 minutes exposed samples

were selected for XPS analysis. Cr coated sample for 45 minutes confirmed EDS analysis by

showing corresponding Cr peaks related to Cr-2p electrons in Cr2O3 according to standard tables

of binding energies provided by handbook of photoelectron spectroscopy [25].

34

XPS peaks belonging to other elements of Fe, O, and C are presented as expected. C-1s

peak is normally intense in all spectra due to the fact that hydrocarbon back stream into specimen

chamber is the main cause for such a high intensity at the very top few surface layers. Peak

related to Cr did not show up in the top layers due to surface contaminants such as C covering

coated steel sample. Figure 26 shows reduction of C peak and increase in intensities of O-1s, Fe-

2p. Cr-2p peak that was masked by carbon layer on top surface appeared in subsequent layers

and persisted up to 41 minutes of plasma sputtering shown in Figure 26. Appearance of both Cr-

p3/2 and Cr-p1/2 are solid proofs that spinel phase of magnetite did show Cr as a substituted

constituent. Figure 28 shows Fe atoms masked by the contaminant layers but upon the first

layer sputtering the spectrum revealed presence of Fe in the matrix of oxide layers. Fe-3/2 and

Fe1/2 peaks remained un-shifted as the sputtering continued. Fe-1/2 peak located at 711 ev

belongs to spinel phases (Fe3O4 , γ-Fe2O3). Figure 29 show O-1s peak that stayed nearly at the

same energy level after sputtering the first few monolayers of the top surface indicating

uniformity of the oxide layers at deeper sections.

Figure 26: XPS spectrum for chromium substitution for 45 minutes

1200 1000 800 600 400 200 0

Sputtering 10min (21min)



Sputtering 1min

Intens

ity [a

.u.]

Binding Energy [ev]

C-1s

O-1sCr-2p

Fe-2p

Surface

35

Figure 27: Sputtering of chromium substituted magnetite sample of 45 minutes


595 590 585 580 575 570

Cr-2p1/2


Intens

ity [a.u

.]

Binding energy [eV]




Sputtering 1min

Surface

Cr-2p3/2

730 725 720 715 710 705

Intens

ity [a.u

.]

Binding Energy [eV]

Fe-2p1/2





Sputtering 1min

Surface

Fe-2p3/2

36


Results of XPS analysis for Al substituted samples are shown in figures 30-33. Figure 30

shows an overall survey scan of a steel sample coated in Al containing bath for 45 minutes

showing presence of major elements of Fe, Al, O, along with contamination peak (C). Spectra of

the sample sputtered for 1 and 5 minutes are shown for comparison. Unlike the Cr substituted

samples, Al concentration appeared very low in this sample. Al-2p peak disappeared after only 5

minutes of sputtering indicating light substitution of Al in the matrix of spinel phase (figure 31).

Both Fe and O peaks appear stable as sputtering continued for 5 minutes (figures 32-33).

540 538 536 534 532 530 528 526

Int

ensity

[a.u.

]

Binding Energy [eV]





Sputtering 1min

Surface

O-1s

37

Figure 30: XPS spectrum for aluminum substitution for 45 minutes

Figure 31: Sputtering of aluminum substituted magnetite sample of 45 minutes

1200 1000 800 600 400 200 0

Int

ensity

[a.u.

]

Binding Energy [eV]

Surface

Sputtering 1min

Sputtering 5min (6min)C-1s

O-1s

Al-2p

Fe-2p

82 80 78 76 74 72 70

Intens

ity [a.u

.]

Binding Energy [eV]

Surface

Sputtering 1min


Al-2p

38



730 725 720 715 710 705

Intensit

y [a.u.]

Binding Energy [eV]

Fe-2p1/2

Fe-2p3/2

Surface

Sputtering 1min


540 538 536 534 532 530 528 526

Inten

sity [

a.u.]

Binding Energy [eV]

O-1s

Surface

Sputtering 1min


39

Fourier Transform Infrared Spectroscopy Results

FTIR is a well-established technique for phase identification of iron oxides [26-29].

Table 4: FTIR absorption bands for iron oxides and oxy-hydroxides [26-29].

Oxide/Oxy-hydroxide of Iron

Major absorption bands (cm-1)

α-Fe2O3 470cm-1, 540 cm-1 γ-Fe2O3 390 cm-1, 600 cm-1 Fe3O4 390 cm-1, 570 cm-1

α-FeOOH 800 cm-1, 900 cm-1 γ-FeOOH 1018 cm-1

According to table 4, major absorption bands of spinel phases are around 570 cm-1 to 650

cm-1. Specifically, magnetite phase exhibits a band at 570 cm-1 and γ-Fe2O3 phase has a band at

600 cm-1. Manjana et al. [30] used FTIR to characterize Cr substituted magnetite and showed

FTIR spectra of their samples as shown in Figure 34. As indicated in these FTIR spectra,

substitution of Cr in spinel phases broadened the characteristic bands in both Fe3O4 and γ-Fe2O3

phases. Another important observation in these spectra is a distinct difference between Cr2O3

spectrum and that of Cr substituted magnetite. Chromium when introduced in the magnetite

structure forms Fe3-xCrxO4 structure [31]. The analysis was performed at low temperature

ranging from 130 °C to 145 °C where the chromium content increase substitutes the Fe2+ ions.

40

Figure 34: FTIR spectra of Cr substituted Fe3O4 and γ-Fe2O3 [30]

Figures 35-40 show FTIR spectra of Cr, and Al substituted magnetite phase that formed

on steel samples during deposition process. In all cases, FTIR band shifted from 570 cm-1 to

about 600 cm-1, the bands were observed with some degree of broadening. The FTIR absorption

41

band shift from 570 cm-1 to 600 cm-1 was due to phase transformation from magnetite phase

(Fe3O4) to maghemite (γ-Fe2O3). Band broadening in FTIR spectra is due to Cr3+ or Al3+

substitution that distort crystal structure of spinel phases causing higher degree of scatter in the

absorption energy by the iron oxide compounds. All Cr3+ substituted phases retained their spinel

phase nature (cubic crystal structure) and did not transform into Cr2O3 that is the phase known to

protect stainless steels. FTIR absorption bands at 1634 cm-1 and 3448 cm-1 are known to belong

to water vapor from variety of sources ranging from atmospheric moisture to moisture gained by

the sample during sample preparation steps. Other contaminant bands are located at 1380 cm-1,

2843cm-1, and 2917cm-1.

Figure 35: FTIR comparison of magnetite (lower spectrum) versus chromium substituted

magnetite (upper spectrum) after 15 minutes

571.

62

1380

.70

1634

.05

2843

.59

2917

.14

571.

62

1380

.70

1634

.05

2843

.59

2917

.14

453.

12

604.

31

1380

.70

1634

.05

2843

.59

2913

.05

3448

.35

8

10

12

14

16

18

20

22

24

26

28

30

32

34

36

38

40

42

44

46

48

%T

ransm

ittan

ce

500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

42

Figure 36: FTIR comparisons of magnetite (lower spectrum) versus chromium substituted


Figure 37: FTIR comparisons of magnetite (lower spectrum) versus chromium substituted


449.

04

571.

62

1380

.70

1634

.05

449.

04

571.

62

1380

.70

1634

.05

461.

29

608.

40

1384

.79

1638

.14

2847

.67

2913

.05

42

44

46

48

50

52

54

56

58

60

62

64

66

%T

rans

mitt

ance

500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

571.

62

1384

.79

1638

.14

571.

62

1384

.79

1638

.14

465.

3850

2.16

587.

97

1380

.70

1634

.05

2847

.67

2913

.05

40

42

44

46

48

50

52

54

56

58

60

62

64

66

68

70

72

%T

ransm

ittan

ce

500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

43

Figure 38: FTIR comparisons of magnetite (lower spectrum) versus aluminum substituted




571.

62

1380

.70

1634

.05

2843

.59

2917

.14

571.

62

1380

.70

1634

.05

2843

.59

2917

.14

596.

14

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

%T

rans

mitt

ance

500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

449.0

4

571.6

2

1380.7

0

1634.0

5

449.0

4

571.6

2

1380.7

0

1634.0

5

612.4

9

52

54

56

58

60

62

64

66

68

70

72

%T

ransm

itta

nce

500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

44



The FTIR analysis showed the shape of the spectrum against the substitution ratio. The

results showed the shift of the frequency bands from 570 cm-1- 600 cm-1. The broadening and

intensity of high frequency bands depends on the nature of the cations. The bands are seen to be

broadened or widened compared to the magnetite bands. In both the cases of aluminum and

chromium substitution in comparison with the magnetite shows a shift widening of 570-600 cm-

1 peaks indicates expansion of lattice parameters of spinel phases. The inverse spinel to normal

spinel conversion is observed by gradual sharpening of both high frequency absorption bands. In

both the aluminum and chromium substitution addition of Cr3+ and Al3+ cations in magnetite

567.5

4

1638.1

4

567.5

4

1638.1

4

600.2

3

38

40

42

44

46

48

50

52

54

56

58

60

62

64

66

%T

ransm

itta

nce

500 1000 1500 2000 2500 3000 3500 4000 Wavenumbers (cm-1)

45

structure the frequency bands are shifted towards the lower frequency and are widened and

narrower in comparison with the magnetite frequency bands.

Corrosion Tests Data

Chromium Substitution

In linear polarization method or polarization resistance method the polarization resistance

(Rp) was measured in the presence of an electrolyte (3.5 % salt solution) with the help of

corrosion potential (Ecorr). The polarization resistance was further mathematically converted to

measure corrosion rates (CR). A plot of E vs log (I) was plotted in the presence of (Ecorr) is

generated by increasing the potential at a fixed rate of 0.166 mv/s and the output current was

measured. The polarization resistance (Rp) is defined as the slope of potential (E) plotted on Y

axis versus the current (log (I)) in the vicinity of corrosion potential Ecorr. When there is an

increase in the potential and the current is measured E is the independent variable and I is the

dependent variable [32].

The corrosion rate is inversely proportional to Rp. For calculating the Rp value for

magnetite, chromium-substituted magnetite and aluminum-substituted magnetite the respective

values were substituted in the Stern Geary equation [23] for calculating the value of Rp.

icorr = (1/ 2.303 Rp) ( βa βc / βa + βc)

Figures 41 through 43 show polarization resistance plots for samples coated with

magnetite, Cr substituted, and Al substituted samples when coated for 15 minutes (figure 41), 30

minutes (figure 42), and 45 minutes (figure 43). Polarization plots of all coated samples (figures

41-43) were compared with those of bare steel in each sets of plots. Results indicate bare steel

exhibits the most negative (lowest) Ecorr indicating most vulnerability towards aqueous corrosion

whereas, Cr3+ containing coatings showed the least negative (most positive) Ecorr corrosion

46

potentials indicating lower susceptibility to corrosion. Coatings containing Al3+ cations and those

with plain magnetite were between the two extremes (bare steel and Cr containing coatings)

indicating a moderate immunity towards aqueous corrosion. A similar trend was observed for

samples coated for all durations of 15 minutes (figure 41), 30 minutes (figure 42) and 45 minutes

(figure 43). Table 5 presents measured parameters needed for corrosion rate calculations

according to Stern-Geary model namely Tafel slopes (anodic, βa, and cathodic βc) along with

polarization resistance Rp. Three measurements under same conditions for each sample variety

were measured and average values of Rp were calculated and reported. The average Rp values

were then used to calculate corrosion rates (icorr) which were converted to thickness loss in terms

of mills per year (mpy). Tables 6 and 7 present similar data for Al3+ cations containing coating

(Table 6) and plain magnetite coating (Table 7). Average polarization resistance values measured

for all sample varieties are summarized in table 8 indicating all coated samples provide some

degree of steel protection with Cr3+ containing coating exhibiting the maximum protection (Rp

values range from 9.568 to 21.78 kΩ) followed by Al3+ containing coatings showing polarization

resistances ranging 2.115 to 2.723 kΩ. Plain magnetite coatings showed polarization resistance

ranging from 1.965 to 2.376 kΩ. Bare steels demonstrated the least polarization resistance of

1.73 kΩ as expected. Therefore, one may conclude Cr3+ containing coatings are most effective in

providing resistance toward aqueous corrosion containing 3.5% sodium chloride. Figure 43

compares polarization resistance offered by different sample varieties.

47

Figure 41: Overlay of the plots for all the samples for 15 minutes

48


49


50

Table 5: Chromium substitution corrosion tests readings

TIME PARAMETERS READING 1 READING 2 READING 3

15 MINS

Rp (kΩ) 10.97 13.109 5.808

E -302.136 -382.608 -337.711

icorr 0.07411 0.08323 0.1773

βc (mv) 3.891 4.468 3.528

βa (mv) 3.604 5.725 7.207 Corrosion Rate

(mpy) 0.03424 0.03845 0.08193

30 mins

Rp (kΩ) 36.28 6.214 22.85

E -311.164 -329.792 -337.136

icorr 0.0278 0.06996 0.01902

βc (mv) 3.679 10.722 6.22


(mpy) 0.01284 0.03232 0.008789

45 mins

Rp (kΩ) 14.16 8.817 5.728

E -359.949 -323.391 -311.09

icorr 0.03531 0.05362 0.123

βc (mv) 2.168 2.856 2.871


(mpy) 0.01632 0.02477 0.05683

51

Table 6: Aluminum substitution corrosion tests readings


15 MINS

Rp (kΩ) 3.594 2.96 0.082

E -388.078 -399.241 -423.501

icorr 0.2599 0.3933 137.6

βc (mv) 6.494 9.161 39.915

βa (mv) 3.211 3.79 75.19

Corrosion Rate (mpy) 0.1201 0.1817 65.38

30 mins

Rp (kΩ) 3.11 4.044 2.813

E -282.305 -378.429 -341.083

icorr 0.25 0.382 0.5776

βc (mv) 3.264 8.896 7.558

βa (mv) 3.961 5.917 7.396


45 mins

Rp (kΩ) 1.905 1.712 2.729

E -299.296 -320.027 -360.65

icorr 0.4829 1.146 0.4609

βc (mv) 5.178 10.007 9.815

βa (mv) 3.578 7.072 4.103


52

Table 7: Magnetite corrosion tests readings


15 MINS

Rp (kΩ) 2.133 2.382 2.614

E -269.039 -346.612 -360.361

icorr 0.8958 0.4811 0.3898

βc (mv) 8.966 5.396 6.162


(mpy) 0.4151 0.2222 0.1801

30 mins

Rp (kΩ) 3.586 2.152 1.802

E -391.148 -315.857 -301.183

icorr 0.3224 0.8648 0.5172

βc (mv) 6.562 6.962 3.542


(mpy) 0.149 0.4012 0.2389

45 mins

Rp (kΩ) 3.568 2.74 1.198

E -328.309 -389.625 -490.15

icorr 0.6088 0.6565 0.6604

βc (mv) 8.258 7.25 3.102

βa (mv) 12.653 9.652 4.407


53

Table 8: Averages readings for polarization resistance (Rp) with standard deviation.

Figure 44: Comparison of polarization resistance values for different coatings.

TIME BARE Rp (kΩ)

MAGNETITE Rp (kΩ)

CHROMIUM SUB. Rp (kΩ)

ALUMINUM SUB. (Rp (kΩ)

15 MINS 1.746 2.376(0.240) 9.962(3.753) 2.723(1.871)

30 MINS 1.746 2.513(0.925) 21.78(15.048) 3.322(0.642)

45MINS 1.746 1.965(1.202) 9.568(4.265) 2.115(0.540)

54

According to Kusnestov [7], reactions involved in hot nitrate process is as follows:

3Fe + 4H2O = Fe3O4 + 8Н+ + 8е- which becomes possible due to generation of excess hydroxide ions in the course of the reaction of nitrate reduction:

NO3- + 7H2O + 8е- ------ NH4OH + 9OH–

When cations of Cr or Al are introduced into the hot nitrate baths two possible ways exist for

their incorporation into reaction products. These cations may precipitate a completely new phase

such as Cr2O3 (in the case of Cr addition to reaction bath) that is known as a protective phase

protecting stainless steels, or become part of the oxide phase (magnetite) as substituting cations.

Based on FTIR results shown in figure 29-31, a structurally defective spinel phase formed as the

result of Cr addition. Otherwise an FTIR spectrum similar to what Manjanna et al. [30] found as

shown in figure 28 would have been seen. Furthermore, no distinct layer in SEM images of Cr

substituted magnetite coatings were seen indicating complete incorporation of Cr cations in the

matrix of magnetite to form a solid solution.

Comparing the amounts of Cr and Al incorporation into the magnetite matrix, observed

higher incorporation of Cr than Al can be explained by ionic radii differences between cations of

Fe3+ and those of Cr3+ and Al 3+. Ionic radii of Cr 3+ and Al 3+ are 0.63 Å and 0.51 Å respectively

as compared to that of Fe3+ that is 0.64 Å. This translates to 1.5% versus 20.62% atomic radii

differences of Cr3+ and Al3+ to Fe3+. Therefore, it is feasible for Cr3+ to form solid solution in

magnetite matrix than Al3+ if Hume-Rothery rules [33] are applicable to oxides as it is to metals.

T. Ishikawa et al. [34] showed metal ions Ti(IV), Cr(III), Cu(II) and Ni(II) impeded the

crystallization and particle growth of Fe3O4. Ishikawa’s results further points to possibility of

cation incorporation to cause atomic disorder rather than precipitation of new phases. Both Cr2O3

and Al2O3 phases are known to be protective of steel surfaces.

55

CHAPTER V

CONCLUSIONS AND RECOMMENDATIONS

This research was conducted to investigate the effect of cation substitution in the

magnetite with aluminum and chromium to study the protection of low carbon steel from

corrosion. The thin layer of magnetite was deposited on 1018 low carbon steel samples by

exposing it to different bath compositions. The 1018 low carbon steel samples for aluminum and

chromium substituted magnetite were also made for studying the effect of substitution in order to

explore alternative coatings which provides highest protection against corrosion.

The samples were coated with aluminum and chromium substituted magnetite by using

different bath chemistry and subjected to polarization resistance tests for measuring their

corrosion resistance in 3.5% salt solution. The samples of plain magnetite, chromium substituted

and aluminum substituted magnetite were prepared for various exposure time intervals of 15

minutes, 30 minutes and 45 minutes to find the effect of substitution on corrosion resistance. The

polarization resistance was calculated for all the set of samples using Stern Geary equation.

Average values of three polarization resistance measurements for all the samples were plotted.

The chromium substituted magnetite showed the highest values 9.662 kΩ, 21.78 kΩ and 9.568

kΩ of polarization resistance for the samples with exposure time durations of 15 minutes, 30

minutes and 45 minutes respectively indicating highest protection from corrosion. The bare

sample had the minimum Rp value of 1.746 kΩ compared with all the other sets of samples. The

chromium substituted magnetite showed the highest value of polarization resistance compared to

aluminum substituted magnetite and plain magnetite for all the samples of different time

intervals, which suggest that chromium substituted magnetite provides highest resistance of

corrosion as compared to plain magnetite and aluminum substituted magnetite.

56

It is recommended that the future studies investigate the effect of thickness of the film

coated over the surface of the steel and its effects on the corrosion protection ability. Also, these

results could be used further to compare the protection of low carbon steel from corrosion with

various other processes such as galvanizing.

The steel samples used for this research were flat steel samples. For further investigation

and applications in the real world the bent samples could be coated and the effects can be studied

by keeping the parameters same.

57

REFERENCES

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2. Vol. 345, issue 2, (2010)

3. http://ipec.utulsa.edu/Conf2005/Papers/Yean_Magnetite.pdf (abstract only)

4. https://www.novapublishers.com/catalog/product_info.php?products_id=27294 (abstract only)




8. Yu. I. Kuznetsova, D. B. Vershok, S. F. Timashev, A. B. Soloveva, P. I. Misurkin, V.A. Timofeeva, and S. G. Lakeev, “Features of formation of magnetite coatings on low-carbon steel in hot- nitrate solutions,” Russian Journal of Electrochemistry, 46 (2012)

9. Shih-Yu Wang, Kuo-Chuan Ho, Shin-Liang Kuo, and Nae-Lih Wu, “Investigation on capacitance mechanism for Fe3O4 electrochemical capacitors” Journal of the Electrochemical Society,Vol. 153 (2006)

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12. P. Ghods, O.B.Isgor, G.A. McRae, J.Li, G.P. Gu “Microscopic investigation of mill scale and its proposed effect on the variability of chloride-induced de-passivation of carbon steel rebar”, Corrosion Science 53 (2011)

13. T. Belleze, M. Malavolta, A. Quaranta, N. Ruffini, G. Roventi “Corrosion behavior in concrete of three differently galvanized steel bars”, Cement & Concrete Composites 28 (2006)

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