honours project 40069944

THE DESIGN AND

MANUFACTURE OF AN

EXPERIMENTAL RIG FOR

CORROSION RATE

MEASUREMENTS USING

IMPEDANCE ANALYSIS BEng (Hons) Mechanical Engineering

Joyce, Steve 40069944

Supervisor – Dr Mike Barker

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http://staff.napier.ac.uk/services/CorporateAffairs/marketing/Pages/Logos.aspx&ei=5PYSVdfLConbaPuOgIAL&bvm=bv.89184060,d.d2s&psig=AFQjCNFqS7-6cS3MCUdRvslz7AxXK7l7-g&ust=1427392609737360

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ABSTRACT The following report highlights the methods used within the Bio-Logic software and hardware

in order to measure corrosion rates using Electrochemical Impedance Spectroscopy. The

main objectives are to research the methods to an appropriate level and design an

experimental rig around the appropriate set up based upon research. With the successful

design of the rig, manufacturing will take place in order to conduct corrosion rate testing upon

coated and uncoated samples. The uniform corrosion rates recorded were of a satisfactory

level and highlighted the effectiveness of the methods for use in industrial applications.

Further investigation was taken into Electrochemical Noise Analysis and localized corrosion

procedures where the results offered no effective conclusion. The limitations and problems

were then discussed and the full report concluded with reference to the original objectives of

the project.

Word Count 10506

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CONTENTS Abstract ................................................................................................................................................... 1

Table of Figures ....................................................................................................................................... 5

Table of Graphs ....................................................................................................................................... 5

Table of Tables ........................................................................................................................................ 5

Table of Equations .................................................................................................................................. 6

Symbols/Abbreviations ........................................................................................................................... 6

Acknowledgements ................................................................................................................................. 7

1. Introduction .................................................................................................................................... 8

2. Literature Review ............................................................................................................................ 9

2.1. Basic Corrosion Mechanics ..................................................................................................... 9

2.2. The Use of Coatings With Respect To Corrosion .................................................................. 10

2.2.1. Ceramic Coatings........................................................................................................... 10

2.2.2 Polymer Coatings .......................................................................................................... 10

2.2.3. Metallic Coatings ........................................................................................................... 11

2.2.4. Composite Coatings ...................................................................................................... 11

2.3. Coatings Tested ..................................................................................................................... 12

2.3.1. Nickel ............................................................................................................................. 12

2.3.2. Titanium Nitride ............................................................................................................ 12

2.3.3. Silicon Carbide ............................................................................................................... 12

2.4. Processes of Selected Coatings ............................................................................................. 13

2.4.1. Ni ................................................................................................................................... 13

2.4.2. TiN ................................................................................................................................. 13

2.4.3. SiC .................................................................................................................................. 13

2.5. Corrosion Due To Salt Water ................................................................................................ 14

2.6. Corrosion Testing Settings .................................................................................................... 15

2.7. Corrosion Testing Methods ................................................................................................... 15

2.7.1 Physical methods .......................................................................................................... 15

2.7.2. Electrochemical Methods ............................................................................................. 16

2.8. Electrochemical Impedance Spectroscopy History ............................................................... 16

2.9. Electrochemical Impedance Spectroscopy In Industry ......................................................... 17

2.10. Recent Advancements In The Industry ............................................................................. 17

3. Design of experimental rig ............................................................................................................ 18

3.1. Problem Analysis ................................................................................................................... 18

3.2. Design Brief ........................................................................................................................... 18

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3.3. Product Design Specification ................................................................................................ 18

3.4. Concept development ........................................................................................................... 21

3.4.1. Initial concepts .............................................................................................................. 21

3.4.2. Selection Table .............................................................................................................. 23

3.4.3. Design evolution ........................................................................................................... 24

3.5. Detailed design ..................................................................................................................... 27

3.6. Modularity............................................................................................................................. 29

3.7. Manufacturing ...................................................................................................................... 29

3.8. Product specification ............................................................................................................ 29

4. Methodology ................................................................................................................................. 30

4.1. Electrochemical Impedance Spectroscopy Basics................................................................. 30

4.2. Hardware and Software Used ............................................................................................... 30

4.3. Test Set Up and Parameters ................................................................................................. 31

4.3.1. Test Parameters ............................................................................................................ 31

4.3.2. Electrodes...................................................................................................................... 31

4.3.3. Testing Solution............................................................................................................. 32

4.3.4. Sample preparation ...................................................................................................... 32

4.4. EC-Lab Software Practical Applications and Procedures ...................................................... 32

4.4.1. Linear Polarization ........................................................................................................ 32

4.4.2. Generalized Corrosion .................................................................................................. 35

4.4.3. Constant Amplitude Sinusoidal microPolarization ....................................................... 36

4.4.4. Zero Resistance Ammeter ............................................................................................. 37

4.4.5. Cyclic Potentiodynamic Polarization ............................................................................. 39

4.4.6. Multielectrode Potentiodynamic Pitting ...................................................................... 40

4.4.7. Multielectrode Potentiostatic Pitting ........................................................................... 41

5. Results ........................................................................................................................................... 42

5.1. Corrosion Rates ..................................................................................................................... 42

5.1.1. Linear Polarization Corrosion Rates .............................................................................. 42

5.1.2. Further Corrosion Rate Results ..................................................................................... 43

5.2. Polarization Resistance through Generalized Corrosion ...................................................... 44

5.3. Electrochemical Noise ........................................................................................................... 45

5.4. Pitting Potential .................................................................................................................... 46

6. Discussions .................................................................................................................................... 47

6.1. Design evaluation .................................................................................................................. 47

6.1.1. Final Evaluation ............................................................................................................. 47

6.1.2. Design for Manufacture ................................................................................................ 47

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6.1.3. Design for Assembly ...................................................................................................... 47

6.1.4. Other Uses..................................................................................................................... 48

6.2. Results ........................................................................................................................................ 48

6.2.1. Corrosion Rates ............................................................................................................. 48

6.2.2. Polarization Resistance ................................................................................................. 48

6.2.3. Electrochemical Noise ................................................................................................... 48

6.2.4. Pitting Potential .................................................................................................................. 49

6.3. EC-Lab Software use evaluation............................................................................................ 49

6.4. Project limitations ................................................................................................................. 50

7. Conclusions ................................................................................................................................... 52

8. Recommendations ........................................................................................................................ 53

8.1. Industry Applications ............................................................................................................ 53

8.2. Future work ........................................................................................................................... 53

9. Appendix ....................................................................................................................................... 55

9.1. Appendix - Design Portfolio ....................................................................................................... 55

10. References ................................................................................................................................ 58

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TABLE OF FIGURES

Figure 1 - Concept 1 .............................................................................................................................. 21

Figure 2 - Concept 2 .............................................................................................................................. 22

Figure 3 - Concept 3 .............................................................................................................................. 23

Figure 4 - Week 7 Design ...................................................................................................................... 24



Figure 7 - Exploded View of Final Design .............................................................................................. 27

Figure 8 - Final Design Assembled ........................................................................................................ 28

Figure 9 - Experimental Rig Set Up ....................................................................................................... 28

Figure 10 - VMP3 Hardware (Low Impedance Setting Far Left) ........................................................... 30

Figure 11 - TiN Samples, Worn Sample on Right .................................................................................. 31

Figure 12 - EC-Lab General Procedure Window .................................................................................... 49

Figure 13 - EC-Lab Characteristics Tab Window ................................................................................... 50

Figure 14-19 Complete Design Portfolio ............................................................................................... 55

TABLE OF GRAPHS Graph 1 – LINEAR POLARIZATION (E V T) .............................................................................................. 33

Graph 2– LP (log(I) v E) .......................................................................................................................... 33

Graph 3 - LP (I V E) ................................................................................................................................ 34

Graph 4 – GENERALIZED CORROSION (E V T) ....................................................................................... 35

Graph 5 – CONSTANT AMPLITUDE SINUSOIDAL MICROPOLARIZATION .............................................. 36

Graph 6 - CASP (I v t,f) ........................................................................................................................... 37

Graph 7 – Zero Resistance Ammeter (E v t) .......................................................................................... 38

Graph 8 - ZRA (E,I v t) ............................................................................................................................ 38

Graph 9 - Cyclic Potentiodynamic Polarization (E v t)........................................................................... 39

Graph 10 - CPP (E v log(I)) (Anderson Materials Evaluation, 2015) ...................................................... 40

Graph 11 - MPSP (E v t) ......................................................................................................................... 41

Graph 12 - AVERAGE CORROSION RATES OF SAMPLES ........................................................................ 44

Graph 13- Polarization Resistance of Samples ..................................................................................... 44

Graph 14– NOISE RESISTANCE OF SAMPLES ......................................................................................... 45

TABLE OF TABLES Table 1 - Selection Table ....................................................................................................................... 23

Table 2 - LP Corrosion Rates ................................................................................................................. 42

Table 3 - Further Corrosion Rates ......................................................................................................... 43

Table 4 - Polarization Resistance .......................................................................................................... 44

Table 5 - Electrochemical Noise ............................................................................................................ 45

Table 6 - Pitting Potentials .................................................................................................................... 46

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TABLE OF EQUATIONS Equation 1 - Stern-Geary (Icorr, nA) ..................................................................................................... 34

Equation 2 - Stern- Geary (Rp, Ω).......................................................................................................... 34

Equation 3 - Corrosion Rate (mmpy) .................................................................................................... 35

Equation 4 - Noise Resistance (Rn, Ω) ................................................................................................... 38

Equation 5 - Standard Deviation for Rn ................................................................................................ 38

SYMBOLS/ABBREVIATIONS EIS – Electrochemical Impedance Spectroscopy

I – Current

E – Potential

A – Amps

V – Volts

mm,cm – millimetre, centimetre

cm2 – area by cm

cm3 – volume by cm

OCV – Open Current Voltage (Eocv, Eoc)

Icorr – Corrosion Current

Ecorr – Corrosion Potential

Ewe – Working Electrode Potential

Ece – Counter Electrode Potential

Ba – Anodic Tafel Coefficient

Bc – Cathodic Tafel Coefficient

Rp – Polarization Resistance

Rn – Noise Resistance

Epit – Pitting Potential

Ni – Nickel

TiN – Titanium Nitride

SiC – Silicone Carbide

mA,nA – milliAmps, nanoAmps

CAD – Computer Aided Design

LP – Linear Polarization

GC – Generalized Corrosion

CASP – Constant Amplitude Sinusoidal micro-Polarization

VASP – Variable Amplitude Sinusoidal micro-Polarization

ZRA – Zero Resistance Ammeter

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EN – Electrochemical Noise

CPP – Cyclic Potentiodynamic Polarization

MPP – Multielectrode Potentiodynamic Pitting

MPSP – Multielectrode Potentiostatic Pitting

CPT – Critical Pitting Temperature

CM – Corrosimetry

HV – Victor’s Hardness

ACKNOWLEDGEMENTS Module Leader - Martin Askey

Supervisor – Dr Mike Barker

Alan Davidson

2nd Supervisor – John Sharp

Laboratory - Callum Wilson

Workshop - Brian Black

Dave Baxter

Sample Source - Mark Docherty

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1. INTRODUCTION The materials science laboratory within the university requires the investigation of impedance

analysis in order to measure corrosion rates of materials. The report will focus on the use of

such an application with regards to the marine industry where material selection is key to

factors such as; safety, performance, economy and durability. Establishing such parameters

is key material selection with respect to design of components, maintenance of materials, and

establishing failure causes. This report will outline the use of the VMP3 potentiostat hardware

and EC-Lab electrochemical software, produced by Bio-Logic, in terms of corrosion testing for

the marine industry. This will be done through the Electrochemical Impedance Spectroscopy

process and its procedures. By first understanding all corrosion mechanisms including the

relevant electrochemical reactions, and the use of coatings as an aid to reduce the effect of

corrosion, an outline of EIS procedures will be determined and explored within the practical

section of the report. An in depth look into seawater corrosion will highlight the types of

environments that the marine industry is required to protect its corroding components from.

The determination of the coating types to be examined will allow the research of their specific

uses and particular corrosion effects within the marine environment, as well as their application

techniques, highlighting the theoretical procedure of each application and performance of

each coating. Research will explore the industry behaviour for corrosion testing based on the

type, either physical or electrochemical, as well as common practise for testing environments

based upon the specific industrial requirement of the investigation and test. A short section

will also highlight the advancements of EIS uses with corrosion within industry. A methodology

based around researched procedures, as well as those available with the testing software, will

be defined. The relevant process, results format and analysis, of each procedure will be

outlined for the laboratory to reference in future work. Based on the procedures outlined by

the methodology, a suitable rig will be designed. This will take into account important aspects

including safety, functionality and the ease of manufacture in order to produce an appropriate

rig within the time frame constraints of the project. Experimental work will be undertaken in

order to highlight the approachability of each procedure, and results recorded. With reference

to the researched behaviour of the relevant coatings within the marine industry, the procedures

will be critically discussed according to their performance in offering suitable results. The full

project will be further critically discussed based on the overall performance with reference to

the project proposal and outline created at the start of the project offering a conclusion to the

effectiveness of the project.

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2. LITERATURE REVIEW

2.1. BASIC CORROSION MECHANICS Corrosion plays a very important role in engineering design and comes in many forms varying

substantially in different environments. The effects of corrosion are also substantially varied.

In some cases, only cosmetic degradation is seen for example in the tarnishing of silver. In

other cases, however, corrosion causes damage to the material's properties such as with the

corrosion in metals within electrical circuits. This corrosion will soon lead to the degradation

of the electrical conductivity of the material or could simply allow the connections within the

circuit to break making the entire circuit redundant. There are nine types of corrosion -

General Attack Corrosion caused by a chemical or electrochemical reaction resulting

in uniform corrosion over the entire exposed surface.

Localized Corrosion – i) Pitting occurs when a small hole or cavity is formed on the

surface of a material, this then creates a localized galvanic reaction with the

surrounding material.

ii) Crevice corrosion occurs with a stagnant micro-environment where the material in

the crevice is depleted of oxygen or subjected to acidic conditions.

iii) Filiform corrosion occurs under painted or plated surfaces where water breaches

the coating.

Galvanic Corrosion occurs when two different metals are located together in a

corrosive electrolyte. One material will become the anode and the other the cathode.

The higher corrosion of the anode (or sacrificial material) will allow slower corrosion of

the cathode material.

Environmental Cracking is the combination of environmental conditions such as

chemicals, temperature and stress which in turn can create stress corrosion cracking,

corrosion fatigue, hydrogen-induced cracking and liquid metal embrittlement.

Intergranular Corrosion is the chemical or electrochemical attack on grain boundaries

within a material. This is often due to impurities in the metal which are more

concentrated around the grain boundaries.

De-alloying occurs when a single material within an alloy is corroded independently.

This can result in a porous material such as copper when brass is de-zinctified.

Fretting Corrosion is present in materials that are subjected to repeated wearing,

weight or vibrations. Commonly found in transportation and rotational/impact

machinery.

High Temperature Corrosion can be caused by high temperature oxidization,

sulfidation and carbonization as well as in the low melting point compounds formed

during combustion.

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2.2. THE USE OF COATINGS WITH RESPECT TO CORROSION Many widely used materials are unstable in the atmosphere and return to their original ores or

to a similar metallic compound. The use of coatings is highly beneficial to engineers as it

protects materials with superior properties, such as steel and its strength, from degradation

due to this environment. This in turn can significantly increase the lifespan of a component

and the materials it consists of. Coatings protect the base material by establishing a boundary

between itself and its environment (Suzuki, 1989). As long as the coating itself is resistant to

the environment, this boundary will work. Thus ceramic, polymer, metallic and some

composite materials are used, each with unique benefits.

2.2.1. CERAMIC COATINGS Ceramic coatings are applied to metals to protect them from oxidation and corrosion at room

temperature, as well as at elevated temperature (Davis J. R., 2001). They can vary from use

in typical everyday products such as those in kitchen where porcelain enamels protect from

the heat and chemicals typically seen in those surroundings, to high performance applications

such as silicate glasses and oxides. Hot-Corrosion coupled with increasing levels of erosion

are the problems anticipated in industrial and marine gas turbines (Narnedra B. Dahotre,

1999). For example, Silicate glasses are prepared from glass powders and through their

resistance to extreme heats, have been found to be extremely beneficial in such applications,

as well as in aircraft, turbine and heat exchanger applications. These glasses are usually

applied through a spray-sinter process. Various oxides such as alumina, zirconia and

chromium oxides provide similar thermal protection while chromium also offers great wear

resistance aspects and alumina can offer great abrasion and corrosion protection. These

oxide coatings are commonly applied by flame spraying or plasma spraying.

2.2.2 POLYMER COATINGS Polymer coatings have been an important part of corrosion protection for decades, most

notably in the painting of steel and iron for both aesthetic and corrosion resistance reasons.

Through the years however polymer coatings have become a keen part of research and

development programmes for many electronic companies, offering both safety in some cases

as well as corrosion protection where it has been recognized that conductive polymer coatings

on a steel surface stabilizes the potential of the substrate in the passivation region and thus

protecting it from corrosion (Marijana Kraljic, 2003). Current research is tending towards the

development of multi layered polymer coatings that could drastically decrease the corrosion

rates of both the components as well as the deterioration of the coating.

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2.2.3. METALLIC COATINGS Polymer coatings have a downside where the surface connection (especially in simple painting

terms) is bad and blistering occurs. This requires the use and development of longer lasting

coatings for use in many building environments where life span of building components is key.

Galvanisation is now widely used to protect many weathered components such as street light

poles. This zinc metal coating is the primary example for a metal coating protecting steel or

iron from oxidisation and rusting. Metals are commonly used in specialist engineering

applications where high temperatures or chemicals are present and alloys are constantly being

developed to deal with the ever increasing demands on materials in this time of substantial

engineering development.

2.2.4. COMPOSITE COATINGS Composite coatings are used to enhance the material properties of a coating by mixing two or

more different materials. Processes such as electrodeposition with a co-deposit allow

materials such as Nickel and Alumina to cover the surface of a substrate. The low hardness

and high ductility of nickel can then be manipulated by the volume of co-deposited alumina to

create a harder and tougher coating material. Composite coatings offer the opportunity to

achieve fantastic mechanical properties with simpler processes than that of plasma spraying

and other high cost procedures. The deposition matrix (usually metallic) can also potentially

offer electrical conductivity that can be useful within the electronics industry (Saha, Mohamed,

& Khan, 2011).

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2.3. COATINGS TESTED

2.3.1. NICKEL One such metal is nickel which, in relation to steel, offers better toughness, better strength at

high and low temperatures, and a range of special magnetic and electronic properties. Nickel

is resistant to many corrosives and is a natural for alkaline solutions therefore it is used in

most tough corrosion problems. Another important aspect of a nickel coating is it's resistance

to stress-corrosion resistance due to the nickel content within a stainless alloy exceeds 10%.

With the exception of use with sulphur-bearing gases, nickel offers a good base for alloys

required to operate at high temperatures. Many of these benefits can of course also be seen

in nickel coatings too. A correlation has been found in research work that supports the

conclusion that nickel coatings deposited with a lower Ph level have a lower corrosion rate.

2.3.2. TITANIUM NITRIDE Titanium is a reactive metal and relies on its natural protective film to protect it from corrosion.

It is extremely good with chemical resistance and thus can be used very well in resistance to

chloride-salt, hypochlorite’s, wet chloride and nitric acid solutions. It's resistance to crevice

and pitting due to salts is extremely good. Special care must be taken when using titanium in

industry however because contamination through preparation or contact with a corroding

metal has catastrophic effects on the titanium's structure.

2.3.3. SILICON CARBIDE Silicon Carbide coatings are used most commonly for their superior resistance to wear and

great hardness ratings. For example an electroless nickel material has a hardness of 1000HV,

while an electroless nickel and silicon carbide composite has a value of around 1300HV.

Corrosion resistance of silicon carbide compounds are however significantly lower than that

of electroless nickel coatings. The electroless nickel matrix contains a large amount of co-

deposited inhibitor which in turn reduces the passivity and therefore corrosion resistance of

the compound. Due to the particulate form of the coating thus exposing steel that corrodes in

galvanic form with the Nickel. This galvanic reaction around the phosphides, nickel and

particles means silicon carbide compounds are also subject to cracking over the coating.

(Davis J. R., 2000)

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2.4. PROCESSES OF SELECTED COATINGS

2.4.1. NI Electroless nickel plating is used with two of the coated samples examined in this report

including the co-deposition of SiC. Electroless Nickel plating is a chemical reduction process

in which a reducing agent is oxidised and Nickel ions are deposited onto the substrate (steel

sample) surface (Taheri, 2003). Typical coating processes for a steel substrate involve the

following steps, cleaning, de-oxidizing and autocatalyzing. Surface cleaning is critical to

ensuring effective coatings and usually involves the pre-treatment using a series of alkaline

cleaners that must be rinsed off with water multiple times to ensure no chemicals are adhered

to the surface, as well as de-greasing to remove oils. Surface oxidization and unwanted metal

is then removed through chemical attack using acid pickling solutions (Hajdu, 1990). The

most commonly used reducing agents are that of sodium hypophosphite and formaldehyde

which reduce metallic ions to the metal state. The first layer of nickel which is deposited acts

as a catalyst for the process. Since the reaction is therefore autocatalytic (Schlesinger, 2010),

a linear relationship between coating thickness and time is present. The deposition process

starts on the catalytic surface and works through diffusion of chemicals to the deposited

surface and the by-products, such as hydrogen, away from the surface. Electroless Nickel

coatings have many applications due to excellent mechanical, electrical and corrosion

resistance properties. Typical coatings can be applied to many substrate materials and onto

intricate components where such coatings offer uniform plating over edges and projections.

2.4.2. TIN The electroless nickel procedure was manipulated for the use of titanium nitride, where the

same basic principle is used except by depositing the titanium instead of the nickel on the

surface of the substrate.

2.4.3. SIC Co deposition of Silicon Carbide particles can be easily incorporated into the electroless nickel

solution where the deposition of nickel carries with it the ceramic particles, creating a

consistent concentration of particulate material coating on the steel substrate. The deposition

of finely dispersed particles in a metal matrix by electroless co-deposition processes led to a

new generation of composites. These composite deposits present particular chemical and

physical properties that each component, taken separately, does not possess (A Grosjean,

2000).

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2.5. CORROSION DUE TO SALT WATER For the purposes of this report we will look at the effect of seawater on uncoated and coated

materials. Sea water is slightly alkaline with a salt content of around 3.4%, this can cause

galvanic and crevice corrosion and breaks down the oxide film that all metals (except gold)

have when in air (Thomas, 1996). The degree of corrosion is determined by oxygen content,

temperature, depth and the velocity of flow a component may be moving through the water.

Other factors include specific conductivity, the content of calcium, magnesium and pollution

as well as biological activity and the possible treatment of the water. These can all be partially

defined by geographical location.

Natural Sea Water – where outside influences such as pollution are neglected and the

natural composition of sea water is used.

Brackish Coastal Sea Water – differences in oxygen, chloride and pollution will be seen

as well as changes in specific conductivity and levels of organic compounds.

Polluted Sea Waters – the combination of lower oxygen levels and the presence of

sulphide ions and ammonia can result in decreased Ph levels.

Stored or Recirculated Sea Water – changes in Ph and oxygen levels will occur over

time as well as change in biological activity due to storage and manipulation.

Synthetic Solutions – characterised by the absence of all organic, biological and

bacteriological species and thus will not reflect real life reactions with samples.

Corrosion in sea water will be seen in different ways with a mixture of both uniform and

localized corrosion. The first and most evident corrosion method is that of general uniform

corrosion due to the reaction of the surface with the environment. There are however many

other forms of corrosion found with seawater. These include bimetallic, crevice and erosion

corrosion as well as pitting, intergranar and selective attacks, stress corrosion cracking and

corrosion fatigue (IJsseling, 1989). The chemical reaction present with rust which includes all

ferrous based materials including the steel sample examined in this report is as follows,

Fe(OH)2+ H2O + O2→ 4Fe(OH)3 (Fontana, 1986).

Measuring corrosion rates within these conditions is especially difficult with the classic

methods of density and weight comparisons not giving much insight into the actual corrosion

reaction mechanics. With the development of Electrochemical Impedance Spectroscopy, we

can now look at the characteristics of the reactions as well as denote meaningful information,

most notably, a samples corrosion rate.

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2.6. CORROSION TESTING SETTINGS

Since corrosion plays such a major role in industry where safety, sustainability and economic

factors are all greatly affected, the measurement of corrosion is key to determining the

appropriateness of a design and selection of material. Testing can be found in three separate

environments, in laboratory testing, a semiworks setting, and in field tests. Laboratory tests

can be defined as small specimens with defined conditions tested to the best convenience

possible. These tests serve as screening tests to determine the appropriability of a material

for an application. Semiwork testing is the most desired setting where testing takes place in

a small scale set up, mirroring the environment of the intended large scale application. This

allows a great insight into how materials and components will act in reality and will in most

cases finalize the simulation of a plant and prove either its failure, or success. Plant testing is

commonly used to evaluate better materials or components based on its actual application.

These three settings can be sequenced in order to offer a logical evaluation from the material

selection, to testing and system monitoring (Fontana, 1986).

2.7. CORROSION TESTING METHODS

2.7.1 PHYSICAL METHODS Visual inspection is a fundamental and key corrosion observation. Usually quantifying this

observation comes from measuring the weight loss after a period of time, problems arise

however when accuracy is key and localized corrosion must be quantified. This localized

problem raises the question of whether material appearance or strength is most important to

the application. If appearance is of importance, the frequency of pits or crevices can be

measured using microscopes, the higher the frequency, the more irregular the surface.

Strength applications require the measurement of depth and diameter of the crevices, or

arguably, the crevice with the greatest of these attributes. Feeler gauges and ultrasonic

methods can easily measure shallow depths, and with small, isolated pits, the limited depth of

focus on visible-light microscopes can be exploited to measure the distance between the

surface and the pit bottom. Problems arise when long, narrow and deep crevices must be

examined. These can only be measured by metallographic section which in turn will destroy

the component.

16

2.7.2. ELECTROCHEMICAL METHODS Electrochemical corrosion testing methods offer the opportunity to derive values for both

uniform and localized corrosion rates as well as chemical reaction behaviour without the need

to destroy the sample. The determination of potential slopes and measurement of current let

us find corrosion rates as a penetration rate (mm/yr) value. Other methods are used again

through the determinations of potentials and resistances to find reaction behaviour as well as

localized corrosion causes and values. This report will go further into these measurements

based upon the procedures outlined by the VMP3 device upon which these experiments will

take place.

2.8. ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY HISTORY

Over the past few decades, Electrochemical Impedance Spectroscopy has become the most

powerful electrochemical technique for determining reactions such as corrosion. The basis of

EIS can be seen in the work on operational calculus by Heaviside, and the diffusion process

work by Warburg (MacDonald, Reflectrions on the History of Electrochemical Impedance

Spectroscopy, 2005). However, the results of Epelboin in Paris through the 1960’s forced EIS

to the forefront of corrosion analysis. Through a partnership between Epelboin and

SOLARTRON Instruments Ltd, the frequency response analyser (FRA) was developed and

allowed impedance to be analysed at frequencies as low as 0.1 mHz, much like the first

potentiostat developed two decades earlier. Since then, EIS has progressed to contribute

more to our understanding of corrosion reaction mechanisms than any other process. This

theoretical investigative work was used to develop techniques for deriving the impedance

functions used to represent complex reaction mechanisms. These included coupling between

charge transfer, chemical, and mass transfer processes. Theoretical work has since

developed practical algorithms for performing Kramers-Kronig transforms, where the

amplitude of a response can be broken down to represent both the real resistance, and

imaginary impedance functions (Harbecke, 1986), for assessing the viability of impedance

data by testing for compliance with the constraints of the linear systems theory, the definition

of alternate perturbation/response transfer functions, and the development of harmonic

analysis. Fundamental outstanding issues are still present however, such as the

determination of the extent to which transfer function analysis need to conform to the linear

systems theory constraints (MacDonald, A Brief History of Electrochemical Impedance

Spectroscopy, 2002).

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2.9. ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY IN INDUSTRY Electrochemical Impedance Spectroscopy is primarily used within the research areas of

material science and industry. With such a fragile test procedure where parameters must be

met with low levels of error in order to measure accurate readings, performing a test in the

middle of a stormy north sea is not ideal. The procedure is therefore commonly used in

laboratory, research based environments where parameters can be closely met. A high

investment is also required for companies wishing to use the procedure with purchases

including expensive hardware, software, rigs, electrodes (both reference and noble), solutions

and samples as well as the expertise to analyse results. EIS offers a great number of highly

accurate results including both uniform corrosion values, and localized corrosion types and

values. These results are invaluable to companies looking into material selection for

component design in harsh environments. Advanced procedures involve the control of

temperature and flow rate within the test solution which offer amazing detail into types of

corrosion and values present in components such as high temperature fluid pipes. One major

downfall of the procedure is the inability to examine samples without a testing solution contact

between the potential readings. Therefore, for instance salt spray corrosion cannot be tested

other than through surface examinations and physical changes such as weight loss. Based

on the limitations experienced, EIS is commonly used in semiwork and laboratory

environments for the evaluation of a material in a particular environment, or for research

applications evaluating the performance of new developments.

2.10. RECENT ADVANCEMENTS IN THE INDUSTRY Although Electrochemical Impedance Spectroscopy has limitations on where the procedures

can be conducted, recent developments are trying to incorporate the highly detailed and

accurate test into the field and real time scenarios. One such step can be seen with the design

of the ACM Instruments Field Machine. This brings the potentiostat hardware and software

required for EIS into the field in a portable format. This system is however still subject to the

same limitations found in the laboratory where samples will need to be cut and the testing

parameters monitored closely. This does however open opportunities for on-site consultancy

offering quick and accurate results to field problems (Instruments, 2015). Significant

developments have however been made in the procedural aspects of EIS. These include the

validation of electrochemical impedance spectra (Ehm), a new approach to investigating the

origin of the faradaic time constants of solid electrodes (Antono-Lopez, 2001), and detailed

analysis of EIS data for mass transfer controlled electrochemical systems (Pauwels, 2010).

Significant developments are also seen in polymer based investigations, to address specific

corrosion phenomena, and the application of Electrochemical Noise Analysis for the

evaluation of a number of different corrosion phenomena (Mansfield, Huet, & Mattos, 2001).

18

3. DESIGN OF EXPERIMENTAL RIG

3.1. PROBLEM ANALYSIS This project requires the design and manufacture of a laboratory rig in order to conduct

experimental work into Electrochemical Impedance Spectroscopy procedures. The rig

should primarily offer a stable environment in order to conduct accurate and reliable

experiments which can in turn highlight the methodology of a series of EIS procedures.

3.2. DESIGN BRIEF A single three electrode rig must be designed comprising of a working, counter and reference

electrode. Differing testing solutions must be considered in order to increase the adaptability

of the rig. The design project should be completed before the interim break in order to ensure

the experimental work can be completed within the full project time frame. Materials and

technologies are based upon the universities workshop limited scope including basic materials

and manned machinery.

3.3. PRODUCT DESIGN SPECIFICATION Environment

The rig embodiment should be designed around a dry, room temperature environment. The

experimental solution components should be designed around the safe containment of a

series of harsh, aggravating chemicals such as sodium chloride and Hydrochloric acid.

Life span

Due to the low expected use, the life span of the rig should be excessive, around ten years.

Maintenance

The university workshop should be able to conduct maintenance work if required, thus a

simplistic design is desired. Components should also be independent, allowing the

replacement of a single component rather than the whole rig.

Cost

Due to the high cost of the reference electrode (£85), and the budget of around £100, the rig

should cost around £15.

Production Quality

A rigid and stable rig is required, therefore appropriate tolerances must be used. The materials

used must also be of an appropriate standard to deal with the harsh experimental solution.

Manufacturing constraints

19

The university workshop determines the manufacturing constraints. Thus most in house

components must be manufactured using manual machines such as lathes, milling machines

and pillar drills. There is scope for some outsourced components such as standard nuts, bolts

and washers.

Size

The size of the rig should be approximately 100x100x100mm based on sample sizes

(30x40x3mm), while also leaving scope for unconventional samples to be tested. The rig

should also have an appropriate centre of gravity for stability due to the use of potentially

harmful solutions.

Weight

The weight of the rig should be somewhat high for the size of rig to offer stability due to, again,

the containment of potentially harmful solutions.

Appearance

The only notable aesthetic property of the rig is that of a clear containment tube. This should

allow a sight of all the electrodes and allow future users a clear view of the experimental set

up.

Materials

A clear material such as Perspex which also offers the chemical resistance to the experimental

solutions should be used for the rig containing tube. All embodiment materials should be

efficiently cost effective, strong, and offer a suitable life span for the rig.

Standards

BS308 (BS8888) and ISO TC/213 standards should be used for engineering drawings with

appropriate tolerance methods.

ISO TC 10 should be consulted to complete the design documentation.

Ergonomics

The rig should be designed around a suitable level of safety for the user due to the use of

harmful substances. The rig should also be designed to be set up and broken down very

easily and quickly offering experiments to be completed quickly. A simplistic design should be

established for the quick and efficient manufacturing of the rig within the time frame with

respect to the manufacturing constraints.

Quality and Reliability

The rig should be extremely reliable due to the use of potentially harmful solutions. Reliability

must also take into account offering a reliable testing environment throughout the rig's life,

through its cleanliness and geometrical aspects such as the area of sample examined.

Time scales

The design aspect of the project has a deadline of around 4-5 weeks, then manufacturing

must take place either before, or during the interim break in order to provide a suitable rig for

20

experimental use after the interim break. Testing of the rig will occur throughout the

experimental procedures.

Testing

Testing will comprise of using the rig in experimental procedures and monitoring its suitability

based on its effectiveness in providing a reliable environment, as well as safety parameters

such as any experimental solution leakage.

Safety

Primary safety concerns should revolve around the use of the experimental solution and it's

containment as well as filling and emptying. Further safety considerations should also be

taken around the embodiment design and any sharp edges involved in this. Any clamping

mechanism must also be evaluated for safety when in use.

Project constraints

The main constraint on the project is the time frame. The university workshop also inflicts

some constraints, however it will be more than adequate for the size of rig and the mechanisms

it will involve.

Documentation

Further documentation will include a product specification, outlining the scope of the rig and

its full intended application use. A design evaluation will also be included to determine the

success of the design project.

Disposal

The rig should focus on using recyclable materials in order to recycle the rig once it becomes

redundant. The use of standardized components will also allow certain components to be

used elsewhere without being recycled.

Competitors

Companies such as Bio-Logic offer corrosion testing rigs based solely on their hardware's

requirements. With the design of an independent rig, the scope for experimental procedures

is increased and the project should aim to offer this at a much lower cost than that of

commercial rigs.

21

3.4. CONCEPT DEVELOPMENT

3.4.1. INITIAL CONCEPTS Concept 1

The first concept is that of the commonly found corrosion test rig found in industry, featuring a

U-shaped containing tube, a clamping mechanism on the bottom, and holes at the top of the

rig for electrodes.

FIGURE 1 - CONCEPT 1

22

Concept 2

The second concept is derived from the first commercial inspired concept, however

incorporating simpler components and geometries.


23

Concept 3

The third concept portrays the very basic three electrode set up principle by offering the

solution, and three electrode connections in a very simplistic manner.

3.4.2. SELECTION TABLE TABLE 1 - SELECTION TABLE

Concept 1 Concept 2 Concept 3

Functionality 5 5 5

Safety 4 5 1

Manufacturability 2 4 2

Production Time 2 3 3

Cost 1 3 5

Ease of assembly 4 4 4

18 24 20

Based on the selection process, Concept 2 shall be developed further.


24

3.4.3. DESIGN EVOLUTION Week 7 – This first CAD visualization builds on the primary concept design and its set up

procedure. It also highlights questions around the control electrode and its seal to the

containing tube. The testing solution would also be difficult to implement due to a very

accurate volume having to be poured in to ensure both electrodes were touching. A possible

solution was to use the rig on its side, however this also gave the problem of pouring in the

testing solution in the first place. The reference electrode has not been taken into account and

with further research, it was clear that this design would not work with the two electrodes

creating seals on either end.

FIGURE 4 - WEEK 7 DESIGN

25

Week 8 – The control electrode was chosen to be in rod in form to allow an open ended

containing in order to pour the solution in. The number of clamps could then be significantly

decreased due to them not being required for the control electrode. With the overall clamping

nature of the rig, the working electrode clamps were decreased to one in order to simplify the

design, while maintaining an electrical connection. A cap was designed in order to hold the

control and reference electrodes.


26

Week 9 – The engineering drawing for all components were completed highlighting the

components required to be manufactured, and the ones to be outsourced. This full

manufacturing drawing portfolio (Appendices 1) was used to liaise with the workshop and

dictate the manufacturing process, while simplifying and optimising component geometries.


27

3.5. DETAILED DESIGN Week 11 – Rig was completed by workshop and the full completed design is as follows.

FIGURE 7 - EXPLODED VIEW OF FINAL DESIGN

28

FIGURE 8 - FINAL DESIGN ASSEMBLED

FIGURE 9 - EXPERIMENTAL RIG SET UP

29

3.6. MODULARITY The modularity of the final design can be split into three separate applications. That of the

end bases, used to contain the electrodes and provide relevant electrical connections. The

containing assembly comprising of the sample and the containing tube, used between the two

bases to safely hold the testing solution. Finally the clamping mechanism, used to complete

the rig and hold all the components together.

3.7. MANUFACTURING Outsourced materials comprised of the containing tube, threaded rod, screws, the nut and the

bolt. All other components where produced in house. The two bases were manufactured

using a milling machine and pillar drill to produce the square edges and holes respectively.

The containing tube material had to be manipulated with the use of a lathe to create a v-groove

on one end where the O-seal could be glued in. The O-seal was manipulated to size with the

use of an O-seal cutter. The clamp threaded rod was cut to size and tapered on the end, then

glued into the base hole. The actual clamp piece was produced with the use of a milling

machine throughout. The clamping rods where produced cut to size and with the use of a

lathe, the end holes were drilled. A thread tap was then used to produce the threaded ends.

The final electrode holding cap component was produced mainly with a lathe, with the holes

being drilled with a pillar drill.

3.8. PRODUCT SPECIFICATION The final rig can be used for three electrode electrochemical corrosion testing procedures. It

offers the safety required to hold dangerous testing solutions, therefore it offers great scope

for manipulating the testing parameters. The rig can be used with varying sizes of testing

samples with the use of an adaptable sample clamping mechanism.

30

4. METHODOLOGY

4.1. ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY BASICS Electrochemical Impedance Spectroscopy is conducted by manipulating potentials and

currents through electrodes in an electrochemical reaction and measuring the appropriate

responses using a potentiostat. This can offer values on applications such as battery capacity

and performance, super capacitor performance, photovoltaic and fuel cell characteristics and

corrosion.

4.2. HARDWARE AND SOFTWARE USED

The hardware used in this project was the Bio-Logic VMP3. The VMP3 is a research grade

multi-channel, multi-user potentiostat which includes 16 independent channels with a unique

counter electrode – ground connection offering the possibility of multielectrode experiments.

Each channel offers two analogue inputs and one analogue output for the control of external

devices such as rotating electrodes. With the corrosion application, the low impedance setting

is used which allows reading to be taken at 1 nA. The device is controlled by a PC USB

connection and the EC-Lab Software. The EC-Lab software includes all techniques used

across all of Bio-Logic's devices and offers the simulation, analysis and fitting of techniques

and their results.

FIGURE 10 - VMP3 HARDWARE (LOW IMPEDANCE SETTING FAR LEFT)

31

4.3. TEST SET UP AND PARAMETERS Electrochemical cells can be analysed using a variety of different set up's including two, three,

four or even more electrode set ups. These each provide their own benefits from the two

electrode set up requiring only a little investment, to the four electrode set up providing the

opportunity for the use of two measuring electrodes, and two stimulating electrodes. This

offers more accurate results compared to the two electrode system when monitoring multi-

electrode systems. With the corrosion experiment, the three electrode set up is suitable as it

determines the working electrode, the material in which the electrochemical reaction is

occurring. The counter electrode is used to close the current circuit within the cell, while also

measuring potential and current difference readings. The reference electrode is independent

of the electrochemical reaction due to its noble form and offers a known and unchanging

potential that can be used as a reference in the cell to dictate potential sweeps and control.

4.3.1. TEST PARAMETERS In order to conduct a primarily investigative experiment in order to assess the procedures used

within the EC-Lab hardware and software, the following parameters have been determined.

4.3.2. ELECTRODES Working Electrode – Samples – 2 x uncoated samples

2 x Nickel coated samples

2 x Titanium Nitride coated samples

2 x Silicon Carbide samples

2 x Nickel coated samples after wear resistance test

2 x Titanium Nitride coated samples after wear resistance test

2 x Silicon Carbide coated samples after wear resistance test

FIGURE 11 - TIN SAMPLES, WORN SAMPLE ON RIGHT

Counter Electrode – 1mm Pure 99.999% Silver Wire

Reference Electrode – Ag/AgCl Silver Chloride Electrode +0.210 V at 25C reference potential

32

4.3.3. TESTING SOLUTION An artificial seawater testing solution was selected in order to mirror the marine environment

this project covers. Around 3.5% by volume of sodium chloride was mixed with tap water in

order to simulate this substance.

The test was conducted under room temperature with negligible wind effect and solution

velocity.

4.3.4. SAMPLE PREPARATION Samples were prepared for analysis by surface inspection and cleaning if required using water.

Degreasing agent was used on an uncoated steel sample to remove oil. Samples were then

clamped into the rig ensuring the O seal stopped solution leakage. The mild steel clamp was

then applied with the electrical connection between itself and the washer. The solution was

then poured in to an appropriate level (around 15.7cm3, or around 50mm in height inside the

containing tube) and the two further electrodes placed and electrical connections made.

4.4. EC-LAB SOFTWARE PRACTICAL APPLICATIONS AND PROCEDURES This project uses various procedures offered by the hardware and software in order to

formulate accurate recommendations around the operation and effectiveness of the EC-Lab

system. These procedures measure both uniform and localized corrosion (Bio-Logic, EC-Lab

Software User's Manual, 2014) (Bio-Logic, Software Applications and Tecniques, 2014).

4.4.1. LINEAR POLARIZATION Background

This technique is particularly designed for determining the polarization resistance in corrosion

cells and the corrosion current. It takes potential steps around the corrosion potential in order

to plot current density vs potential, and the Log (current) vs potential curves.

33

The corrosion potential (Ecorr) is determined through an open circuit analysis of the cell. From

this the software induces a potential scan around the Eoc and current readings are then taken.

Visualization of results and Corresponding fits

Current reading can be plotted on a log (I) vs Ewe graph as follows.

The software assumes that the all electrochemical systems are tafelien, meaning the current

flowing in the electrode is only limited by the electron transfer and not by mass transfer. The

anodic and cathodic Tafel coefficients which represents the corresponding current values

within the system can be found from this visualization based upon the two linear regressions.

With the manipulation of the limitations of the two linear fits, the Tafel curve can be best fit in

order to mirror that of the results. The middle part of the graph where log (I) is at its lowest, is

also the position of the corrosion potential and the corrosion current.

GRAPH 1 – LINEAR POLARIZATION (E V T)

GRAPH 2– LP (LOG(I) V E)

34

The anodic region of the cell is determined as when a ferrous atom at the metal surface

dissolves into moisture film leaving a negative charge in the metal. The cathodic region is

determined as a depolarizer removes electrons from the metal. The larger the difference

between the anodic and cathodic potentials, the larger the corrosion current. From this

manipulation the software determines the two Tafel constants Ba and Bc, the corrosion

potential Ecorr, and finally the corrosion current Icorr based upon the manipulation of the

Stern-Geary equation,

EQUATION 1 - STERN-GEARY (ICORR, NA)

𝐼 = 𝐼𝑐𝑜𝑟𝑟 exp (𝑙𝑛10(𝐸 − 𝐸𝑐𝑜𝑟𝑟)

𝐵𝑐) − 𝐼𝑐𝑜𝑟𝑟exp(

−𝑙𝑛10(𝐸 − 𝐸𝑐𝑜𝑟𝑟)

𝐵𝑎)

The current readings are also displayed on a Current vs Potential graph. From this the

software uses an Rp fit in order to determine the polarization resistance of the material.

By determining the Tafel coefficients found previously, and the manipulation of the range, the

software calculates the inverse of the linear fir slope as the Polarization Resistance as the

user matches the Corrosion Potential with the one found in the Tafel fit previously. An Rp fit

is again calculated through the Stern-Geary relationship seen here,

EQUATION 2 - STERN- GEARY (RP, Ω)

𝑅𝑝 =𝐵𝑎𝐵𝑐

𝐼𝑐𝑜𝑟𝑟(𝐵𝑎−𝐵𝑐)𝑙𝑛10+ 𝐸𝑐𝑜𝑟𝑟

Analysis of results

From the corrosion current found through the software, the corrosion rate of a material can

then be calculated where

GRAPH 3 - LP (I V E)

35

EQUATION 3 - CORROSION RATE (MMPY)

𝐶𝑅 =𝐼𝑐𝑜𝑟𝑟𝐾𝐸𝑊

𝐷𝐴

CR is in millimetre per year (mmpy) or mill inches per year (mpy)

Icorr corrosion current (in A).

K constant that defines the units of the corrosion rate.

EW equivalent weight (in g/equivalent). Defined as the molar mass of the oxidized metal

divided by the number of electrons involved in the dissolution reaction. For instance, for the

corrosion of iron Fe → Fe2+ + 2e- EW = 55.85/2 = 27.925 g/equivalent.

D density (in g/cm3).

A sample area (in cm2).

Where K = 3272mm/ (A cm year), CR = mm/year (mmpy)

The Polarization Resistance is key to the corrosion characteristics of a material because it

measures the resistance to the flow of current in a cell caused by chemical reactions. Hence,

the higher the Polarization Resistance, the greater the resistance to corrosion reactions.

4.4.2. GENERALIZED CORROSION Background

The generalized corrosion technique is used for the measurement of uniform corrosion based

on the assumption that the anodic dissolution is uniformly distributed over the entire sample

surface. This technique uses a half potential sweep around the Eoc and again, the current is

measured as the potential changes.

GRAPH 4 – GENERALIZED CORROSION (E V T)

36

Visualization of results, Corresponding fits and Analysis

Similar to Linear Polarization, the Generalized Corrosion technique relies on the log (I) vs Ewe

graph to determine the Tafel parameters and corrosion current through a Tafel fit, as well as

the Current vs Ewe graph to determine the Polarization Resistance through a linear fit.

These results can then be analysed to determine the corrosion properties of materials through

the corrosion rate and the polarization resistance.

4.4.3. CONSTANT AMPLITUDE SINUSOIDAL MICROPOLARIZATION Background

Constant Amplitude Sinusoidal microPolarization is used to determine the corrosion

characteristics of a Tafelian system. A sinusoidal voltage is applied around the open circuit

potential at a low amplitude and frequency. Based upon the Fourier transform, which

decomposes the signal into the frequencies that make it up, the amplitude of the harmonics

can be examined to calculate the corrosion parameters. This technique is faster than the

Polarization techniques highlighted earlier, however accuracy can suffer due to the greater

voltage window examined. CASP does however offer a less damaging approach to the

sample than that of polarization techniques.

Visualization of results

From the Fourier transform graph, the harmonics of the signal can be seen. This is done

through the CASP fit option within the software and from this the software can calculate the

corrosion parameters.

GRAPH 5 – CONSTANT AMPLITUDE SINUSOIDAL

MICROPOLARIZATION

37

Analysis of results

As before the corrosion current found through the Fourier transform can be used to calculate

the corrosion rate of the material. All corrosion coefficient values are however not calculated.

4.4.4. ZERO RESISTANCE AMMETER Background

The Zero Resistance Ammeter procedure is made to perform electrochemical noise

measurements. Electrochemical Current Noise is the spontaneous current fluctuations that

occur between two electrodes held at the same potential due to chemical reaction behaviours.

The process consists of applying zero volts between the working and counter electrodes and

measuring the current and potentials against that of the steady state reference electrode. An

initial open circuit voltage procedure takes place first, followed by the ZRA, this is repeated for

a set amount of times.

GRAPH 6 - CASP (I V T,F)

38

Visualization of results and Analysis of results

Results can be visualized through the (E, I) vs time graph.

The software then uses the standard deviation technique in its ENA tool to determine the Noise

resistance. Where

EQUATION 4 - NOISE RESISTANCE (RN, Ω)

𝑅𝑛 =𝜎𝐸𝑊𝐸

𝜎𝐼 and 𝜎𝐼and 𝜎𝐸𝑊𝐸 is obtained from

EQUATION 5 - STANDARD DEVIATION FOR RN

𝜎𝑥 = √1

𝑁−1 ∑ (𝑥 − ẍ)2𝑁−1

𝑖=0 where 𝑥 = ẍ is the average of the parameter.

GRAPH 7 – ZERO RESISTANCE AMMETER (E V T)

GRAPH 8 - ZRA (E,I V T)

39

Electrochemical Noise constitutes events such as film rupture and discrete events such as

metal dissolution and hydrogen discharge with gas bubble formation and discharge.

Measuring this therefore gives a great insight into the level of corrosion behaviour that a

material is subject to.

4.4.5. CYCLIC POTENTIODYNAMIC POLARIZATION Background

Cyclic Potentiodynamic Polarization moves into the measurement of localized corrosion

mechanisms and is used to evaluate the pitting characteristics of a material. The potential is

swept around one cycle above the open circuit potential. A hysteresis loop is formed within

the log (I) vs Ewe graph which would be indicative of pitting. The size of the loop is then

related to the amount of pitting.

GRAPH 9 - CYCLIC POTENTIODYNAMIC POLARIZATION (E V T)

40

Visualization of results and analysis of results

The hysteresis loop can be seen in the Log (I) graph shown here.

GRAPH 10 - CPP (E V LOG(I)) (ANDERSON MATERIALS EVALUATION, 2015)

From this the pitting potential can be calculated using the Multi Pitting Statistics tool within the

software. This dictates the lowest positive potential in which the material will start to undergo

pitting. With this information, engineers can determine the appropriability of a materials use

based on the environment in which the material will be used.

4.4.6. MULTIELECTRODE POTENTIODYNAMIC PITTING The Multielectrode Potentiodynamic Pitting technique is very similar to CPP however it

incorporates more than one electrode. Using multiple channels within the VMP3, the software

can conduct an identical potential sweep across various identical electrodes to comprise a

range of results for one material. From this an accurate reading can be found with an

appropriate tolerance value.

41

4.4.7. MULTIELECTRODE POTENTIOSTATIC PITTING The Multielectrode Potentiostatic Pitting technique is very similar to that of the MPP technique

in regards to offering a range of results. In this technique however the potential is applied at

a constant value rather than being swept.

GRAPH 11 - MPSP (E V T)

42

5. RESULTS

5.1. CORROSION RATES

5.1.1. LINEAR POLARIZATION CORROSION RATES TABLE 2 - LP CORROSION RATES

From the above table highlighting corrosion performance through Linear Polarization, an initial

assumption can be made that the Titanium Nitride coatings performed the best, with Nickel

offering a similar level of corrosion protection. All coatings offered corrosion resistance,

however, Silicone Carbide coatings performed to a lesser extent than the Titanium and Nickel

coatings.

Sample10 Sample12 Sample1 – 4.9Ph Sample2 – 4.0Ph Sample8 Sample9 Sample5 Sample6

Icorr (nA) 253.6 237.39 30.6 26.12 11.37 16.05 171.59 118.07

Icorr (A) 2.5360E-04 2.3739E-04 3.0600E-05 2.6120E-05 1.1370E-05 1.6050E-05 1.7159E-04 1.1807E-04

(K EW)/(DA) 3389 3389 3389 3389 3389 3389 3389 3389

Corrosion Rate (mm/yr) 0.859 0.805 0.104 0.089 0.039 0.054 0.582 0.400

Sample1 – 4.9Ph Sample2 – 4.0Ph Sample8 Sample9 Sample5 Sample6

29.43 28.67 17.55 18.18 130.54 124.61

2.9430E-05 2.8670E-05 1.7550E-05 1.8180E-05 1.3054E-04 1.2461E-04

3389 3389 3389 3389 3389 3389

0.100 0.097 0.059 0.062 0.442 0.422

Icorr (nA)

Icorr (A)

(K EW)/(DA)

Corrosion Rate (mm/yr)

Coated

Coated – Worn

Nickel TiN SiC

Nickel TiN SiC

Uncoated

43

5.1.2. FURTHER CORROSION RATE RESULTS

TABLE 3 - FURTHER CORROSION RATES

With further inspection into the Generalized Corrosion and Constant Amplitude Sinusoidal

microPolarization techniques offered by the software, the above corrosion rates were

recorded. With comparison to that of the LP results, a close tolerance can be seen between

all methods offering accurate results with the software.

The above also highlights secondary experimental procedures used on worn coatings with

exposed steel. Across the board, higher corrosion rate results can be seen again reiterating

the accuracy of the software techniques.

Sample10 Sample12 Sample1 – 4.9PhSample2 – 4.0PhSample8 Sample9 Sample5 Sample6

Icorr (nA) 247.89 221.5 26.48 27.94 12.8 14.56 187.1 120.86

Icorr (A) 0.00024789 0.0002215 2.65E-05 2.79E-05 1.28E-05 1.46E-05 0.000187 0.000121

(K EW)/(DA) 3389 3389 3389 3389 3389 3389 3389 3389


Icorr (nA) 276.23 243.67 27.08 31.97 15.1 12.31 140.12 104.01

Icorr (A) 0.00027623 0.00024367 2.71E-05 3.2E-05 1.51E-05 1.23E-05 0.00014 0.000104

(K EW)/(DA) 3389 3389 3389 3389 3389 3389 3389 3389


Sample1 – 4.9PhSample2 – 4.0PhSample8 Sample9 Sample5 Sample6

Icorr (nA) 30.53 30.41 19.1 19.411 145.9 133.62

Icorr (A) 3.05E-05 3.04E-05 1.91E-05 1.94E-05 0.000146 0.000134

(K EW)/(DA) 3389 3389 3389 3389 3389 3389

Corrosion Rate (mm/yr) 0.103466 0.103059 0.06473 0.065784 0.494455 0.452838

Icorr (nA) 31.04 27.86 18.11 18.29 102.43 176.88

Icorr (A) 3.1E-05 2.79E-05 1.81E-05 1.83E-05 0.000102 0.000177

(K EW)/(DA) 3389 3389 3389 3389 3389 3389

Corrosion Rate (mm/yr) 0.105195 0.094418 0.061375 0.061985 0.347135 0.599446

CA

SPG

C

GC

CA

SP

Uncoated Coated

Coated – Worn

Nickel TiN SiC

Nickel TiN SiC

44

GRAPH 12 - AVERAGE CORROSION RATES OF SAMPLES

5.2. POLARIZATION RESISTANCE THROUGH GENERALIZED CORROSION TABLE 4 - POLARIZATION RESISTANCE

The corrosion characteristics and resistance of each coating can be reiterated with the

examination of the Generalized Corrosion. This again shows similar results where the TiN

and Ni coatings offer superior protection to that of SiC and uncoated samples.

GRAPH 13- POLARIZATION RESISTANCE OF SAMPLES

Sample10 Sample12 Sample1 – 4.9PhSample2 – 4.0PhSample8 Sample9 Sample5 Sample6

Rp (Ohms) 143 183 1348 1287 1573 1395 213 267

Uncoated Coated

Nickel TiN SiC

45

5.3. ELECTROCHEMICAL NOISE TABLE 5 - ELECTROCHEMICAL NOISE

With EN giving us an insight into the reaction characteristics of materials such as film ruptures

and discrete events such as metal dissolution and hydrogen discharge with gas bubble

formation and discharge, the EN results recorded above show a considerable difference

between all samples, even that of TiN and Ni, which show similar corrosion rate behaviour as

noted previously, except from the TiN and uncoated samples. EN values can be analysed

further through the root mean square of the recorded amplitudes in order to determine the

“fingerprint”, or type, of localized corrosion (Gaona-Tiburcio, Aguilar, & Zambrano, 2013).

GRAPH 14– NOISE RESISTANCE OF SAMPLES

Sample10 Sample12 Sample1 Sample2 Sample8 Sample9 Sample5 Sample6

Rn (Ohms) 24.06 26.75 84.89 118.9 30 23.68 56.76 72.73

Uncoated Coated

Nickel TiN SiC

46

5.4. PITTING POTENTIAL TABLE 6 - PITTING POTENTIALS

The Pitting Potential values recorded above offer no insight into the localized corrosion

characteristics of materials due to the obvious inaccuracies in the results. Even with the use

of multiple software techniques, the results show no similarity or correlation, therefore further

work is required to deduce the reason for such inaccuracies after the software previously

offered reliable results.

Sample10 Sample12 Sample1 Sample2 Sample8 Sample9 Sample5 Sample6

Epitting (V) 1.16 0.423 0.346 -1.1 0.283 0.108 -0.316 -0.108

Epitting (V) 0.677 -0.53 1.381 1 0.137 0.5 0.608 0.475

Epitting (V) 0.076 0.387 -0.355 -0.9 -0.16 0.238 -0.296 1.7

Uncoated Coated

Nickel TiN SiC

47

6. DISCUSSIONS

6.1. DESIGN EVALUATION

6.1.1. FINAL EVALUATION Based upon the Design Brief and Product Design Specification, the design has met all

requirements, parameters and constraints. The design offers a safe and stable environment

in order to undertake EIS procedures and although the solution used was not harmful, the

design offers scope into highly corrosive solutions.

6.1.2. DESIGN FOR MANUFACTURE Due to the simple geometry used within the liaising portfolio, there were no problems when

consulting the viability of manufacturing with the workshop. The clamp was simplified after

finding the easiest manufacturing option for the workshop, and the threaded rods were

changed to offer a more aesthetically appealing assembly. All materials were readily available

except the containing tube which took an extra few days to arrive and this minimally set back

the production time. Manufacturing was complete around three weeks before the interim

break and took only five days to complete which meant the rig was just waiting on the

electrodes to be sourced in order to be complete.

6.1.3. DESIGN FOR ASSEMBLY The rig is extremely easy to set up and use, with the only tool required as an Allen key. The

hardest aspect is the positioning of the seal with the sample, however, with the transparent

containing tube a top filling hole, this can be fairly easily done. Although this can be fiddly, the

balance between the restriction of the tube size based on usable sample areas, and that of

the space within the containing tube for electrodes, has been well established. Filling the

containing tube with the solution is also easy provided an adequate pouring vessel is used.

The overall design could be simplified with the use of only three clamping rods around the

containing tube. This would offer the same adequate clamping performance with a little less

assembly time. The other major improvement that could be made to the design is that of the

electrode cap on the top. This is inadequate for the reference electrode purchased with has

a 6mm diameter while the cap was cut at 5mm. A piece of card was then used to hold the

electrodes, however, this caused some fiddliness in the movement of the electrodes to VMP3

connection. With a more appropriate electrode cap, the electrodes could be effectively kept

in position and more easily off the walls of the containing tube.

48

6.1.4. OTHER USES The design has the scope to be used in the cathodic disbondment testing procedure where a

similar set up is required. A sample can be clamped into the rig and the electrolyte solution

added. The calomel electrode and anode can then be inserted into the solution from the

electrode cap, connections made, and the procedure carried out. This makes the working

electrode clamp redundant, however this can just be fixed away from the procedure and will

not affect results (Guo, 2006).

6.2. RESULTS

6.2.1. CORROSION RATES The results seen in the corrosion rate techniques show strong correlations between the

coatings highlighting the best and worst, as well as the effect of a worn coating. Based on

previous research, the results can be seen as accurate where TiN and its passive oxide film

performs the best followed by Ni. The poor corrosion resistance of SiC can be explained from

research, where the particulate form due to the co-deposition process, exposes steel and thus

creates a galvanic coupling reaction between that of the steel and the nickel. The effect of a

worn coating can also be highlighted as having a negative effect on the corrosion resistance

of coated materials.

6.2.2. POLARIZATION RESISTANCE Following from the results seen in the corrosion rates results, the results are expected to be

incredibly similar where a high corrosion rate has a low polarization resistance. This can be

clearly seen in the results and reiterates the effectiveness of EIS methods for measuring

corrosion rates.

6.2.3. ELECTROCHEMICAL NOISE The electrochemical noise analysis of each coating highlights a great variance in the reaction

activity of each coating. This is highlighted in the likes of the passive oxide film present in TiN

coatings that protect against harsh corrosion reactions, and thus shows a very low noise value.

The results do not highlight the reasons or mechanisms for current fluctuation due to chemical

reactions within the cell however. This would be key information to engineers in the field and

therefore this project fails to offer usable results in the electrochemical noise area of EIS

corrosion testing.

49

6.2.4. PITTING POTENTIAL The project also does not show any correlation into the pitting potential, and therefore localized

corrosion behaviour of the coated samples. This is due to the limitation of the low impedance

setting used within the experiments. The technique requires a high level of potential range in

order to reach the Pitting Potential. Due to the restrictions of the low impedance setting only

offering potentials to around 1.5V through the working electrode, the setting will simply not

work. A normal channel could be used with the VMP3 potentiostat in order to reach the

relevant potentials. Further research should also be undertaken in this case to effectively

analyse the results in order to not only the pitting potential, but also the types of localized

corrosion that these values can give an insight into.

6.3. EC-LAB SOFTWARE USE EVALUATION The EC-Lab is incredibly simple to use with great documentation highlighting the steps of each

technique that it offers. Upon first use of the software, it takes a little time to understand the

jargon such as OCV periods, which refer to the open circuit voltage procedure used to find

Eoc at the start of each procedure.

The above set up procedure is used for the generalized corrosion technique and as can be

seen, the main steps within the technique are split into their own boxes or sections. The only

hurdle for new users is the understanding of each acronym such as the different potentials

and the understanding of the scope parameters such as the I and E ranges. These scopes

become apparent when the software warns of current or potential overload within the

procedure. This can be easily overcome with the manipulation of the ranges, and the

procedure retried.

FIGURE 12 - EC-LAB GENERAL

PROCEDURE WINDOW

50

Within the cell characteristics tab of the procedure set up, the attributes of the sample and

testing apparatus can be determined in order to provide personalised results for each sample

such as the specific corrosion rate. The software also offers options for all suitable reference

electrodes with their corresponding potential values, which in turn are used within the

procedure as a known reference. Within the Advanced Settings tab, the electrode connections

are defined and the option for Counter Electrode to Ground is available for the suitable

procedures.

6.4. PROJECT LIMITATIONS Looking at the original project proposal and the corresponding aims, the project has been a

success. There were however some limitations to the work. The design of the rig was defined

around the workshops limitations including materials and machinery. Although not critical to

the success of the design, lower limitations would allow the scope of the experimental

procedures to be expanded into incredibly harsh environments as well as the use of

temperature control apparatus in order to perform Critical Pitting Temperature experiments to

find the temperature at which localized corrosion begins to occur. A key problem that occurred

during the use of the rig was the degradation of the counter electrode when used over an

extensive period of time. Coating material from the sample would separate from the coating

and become deposited onto the silver electrode. Due to the nature of a counter electrode

where it performs as measuring apparatus due to its noble form, the electrochemical reaction

occurring on its surface from this deposited material would disrupt the results and nullify the

procedure. This was fixed with the use of a three micron diamond lapping paste that would

be applied and subsequently break the deposited material from the electrode. A final polishing

would then be used to remove the paste and any debris from the surface of the electrode and

it would once again be ready for use. The average time for this polishing was 2 minutes and

FIGURE 13 - EC-LAB CHARACTERISTICS TAB

WINDOW

51

was performed after every change of sample, or every 45 minutes of operation.

Limitations within the software revolved around the time constraints of the project. The

Corrosimetry procedure could have technically been performed as it uses the same principle

as that of the Linear Polarization technique. Corrosimetry however measures the change in

polarization resistance over time and the test usually lasts for days or even months. This

would give a great insight into the changing characteristics of corrosion through a coating,

however, the time frame would not allow the individual inspection of each coating. The

procedures within this report also limit the insight into Variable Amplitude Sinusoidal

microPolarization. Similar to CASP, VASP uses a variable amplitude to find the corrosion rates

and polarization resistance of materials. With the use of three already defined experiments to

determine these material characteristics, it was decided that due to the time consuming and

complex nature of result gathering, VASP did not offer anything new to the overall results of

the project. The Depassivation Potential procedure was also neglected from the project. This

offers results on material pitting potential based upon a procedure incorporating both MPP and

MPSP. This was neglected due to the inability to compare the results with any other results

due to the inconclusive CPP, MPP and MPSP procedures as highlighted previously.

The only hardware limitation that occurred during the project was that of the low impedance

setting potential range when measuring the pitting potential. This could be easily rectified by

using a standard channel connection. The rig design was however based around the

significantly smaller low impedance connections and thus a middle connection between the

electrodes and VMP3 must be designed. This could however raise safety concerns where

high voltages are used with exposed metals and the grade of wire for the working electrode

would need to be examined for safety.

52

7. CONCLUSIONS The overall project has been a success. By first establishing the use of the impedance

methods within industry, scope for the project was defined and the measurement of coatings

in artificial sea water was chosen in order to relate to that of the marine industry. Due to the

highly aggressive environments experienced, corrosion measurement plays a major role in

design and material selection of components to be used in such environments. Through initial

research into the modes of corrosion and the effects, the scope of the project was further

defined by the selection of coating materials. These were Nickel, Titanium Nitride and Silicon

Carbide, all of which offer great wear resistance and superior hardness values than that of

steel which was defined as the substrate material. Worn coating samples were also analysed

to give an insight into corrosion behaviours of incomplete coatings. An uncoated steel

substrate was also to be examined in order to compare coatings results to a standard

performance mark. With the scope of the project well defined, research was conducted into

the process of impedance analysis. This highlighted the methods and results available with

current technology, as well as the particular methods that could be used within the hardware

and software provided by the materials science lab. Relating to the known methods of

analysis, a rig was then designed around a generic three electrode set up where the samples

were defined as the working electrode, and a reference and counter electrode were also

required. A testing solution was required to initiate the electrochemical reaction and thus the

design would need to incorporate the three electrodes with relevant electrical connections to

the VMP3, submerged within the solution. Through liaising with the university workshop, the

design was optimised and produced within time to start testing after the interim break. Testing

then took place for all samples through the seven procedures; Linear Polarization, Generalized

Corrosion, Constant Amplitude Sinusoidal microPolarization, Zero Resistance Ammeter,

Cyclic Potentiodynamic Polarization, Multiple Potentiodynamic Pitting and Multielectrode

Poentiodynamic Pitting. These offered results in corrosion rates and polarization resistance,

electrochemical noise measurements, and pitting potentials. With the corrosion rate and Rp

procedure aims offering good results, the project can be seen as a success. The aims of the

project were however expanded in order to incorporate further results in Noise measurement

and localized corrosion characteristics. Due to lack of research into the deduction of Noise

results, only the process could be highlighted and the project offers no conclusions about the

type’s corrosion mechanisms apparent in the experiment. The localized corrosion procedures

also failed to offer any conclusive results on material behaviour based predominantly on the

limitation of the low impedance setting used with the VMP3. With correlating results and more

advanced research, the procedures demonstrated in this project could be used in accordance

with a suitably safe and operational rig, to perform these localized corrosion experiments and

53

deduct meaningful results from them. Overall, the project has been a success highlighting the

use of impedance analysis in order to measure corrosion rates in coated samples by designing

and using a suitable rig for the experiments.

8. RECOMMENDATIONS

8.1. INDUSTRY APPLICATIONS The general corrosion rate experiments highlighted in this report offer a great deal of industrial

application. Experiments are done in a stable laboratory environment and offer great accuracy

in results. Through this investigative setting, the processes are ideal for research into new

material classifications, and testing materials for their suitability within industrial applications

such as from a guard rail on an oil rig in the North Sea, to the metal materials found on deep

sea divers uniforms. The accuracy of the results offers a great deal of safety and reliability in

material selection when designing new components and the speed in which these procedures

can be conducted offers the option to examine many different materials offering a greater

scope on possible material choices. As seen in the Silicon Carbide coating results, the

procedures can pick up important features of a compound such as a particulate form coating

exposing the substrate. This could be an invaluable piece of information in industry where

simple physical methods of testing, such as the mass change, would not pick up such features.

8.2. FUTURE WORK Future work within this area would be the re-examination of the Pitting Potential techniques

54

and an appropriate design around the electrode connections. Research into the relationship

between the pitting potential value and the size of the hysteresis loop could prove valuable in

determining the actual corrosion mechanisms occurring on a materials surface.

Corrosimetry procedures could be conducted in order to re-iterate the Polarization Resistance

values recorded in the uniform corrosion measuring methods highlighted within this project.

This would also offer an insight into how coatings operate in terms of corrosion protection over

an extended period of time, and maybe even to the time where a coating is completely

removed due to corrosion.

Future work on the design of the rig could be done to incorporate temperature management

within the experiment. This would then offer the opportunity for the measurement of the critical

pitting temperature discussed previously. This work could prove extremely useful in other

industries outside of marine applications, where heat is more predominant and effective. The

materials used with the rig would however need to be examined for suitability as they were

chosen based on room temperature operation.

The rig has the scope to be used within a cathodic disbondment experiment and although this

would offer no insight into the use of the VMP3 and its software, it could offer results on the

effects of corrosion between materials which is highly beneficial to those in industry.

55

9. APPENDIX

9.1. APPENDIX - DESIGN PORTFOLIO FIGURE 14-19 COMPLETE DESIGN PORTFOLIO

58

10. REFERENCES

A Grosjean, M. R. (2000). Surface and Coatings Technology. Some morphological characteristics of

the incorporation of silicon carbide (SiC) particles into electroless nickel deposits, 252 - 256.

Anderson Materials Evaluation, I. (2015, February 26). Electrochemical Qualification of Biomedical

Implantable Devices: Corrosion Case Study #2. Retrieved from Anderson Materials

Evaluation, Inc.: http://www.andersonmaterials.com/electro/electrochemical-qualification-

of-biomedical-implantable-devices-corrosion-case-study-2.html

Antono-Lopez, R. (2001). A New Experimental Approach to the Time-Constants of Electrochemical

Impedance. Electrichima Acta, 3611 - 3617.

Bio-Logic. (2014). EC-Lab Software User's Manual. Bio-Logic.

Bio-Logic. (2014). Software Applications and Tecniques. Bio-Logic.

Davis, J. R. (2000). Nickel, Cobalt, and Their Alloys. ASM International.

Davis, J. R. (2001). Surface Engineering for Corrosion and Wear Resistance. ASM International.

Ehm, W. (n.d.). ZHIT - A simple Relation Between Impedance Modulus and Phase Angle. 1.

Fontana, M. G. (1986). Corrosion Engineering. McGraw-Hill.

Gaona-Tiburcio, C., Aguilar, M. L., & Zambrano, P. R. (2013). Electrochemical Noise Analysis of Nickel

Based Superalloys in Acid Solutions. 523 - 533.

Guo, G. (2006). Laboratory and Field Tests of Multiple Corrosion Protection Systems for Reinforced

Concrete Bridge Components and 2205 Pickled Stainless Steel. ProQuest.

Hajdu, J. (1990). Surface Preperation for Electroless Nickel Plating. 193 - 206.

Harbecke, B. (1986). Application of Fourier's Allied Integrals to the Kramers-Kronig Transformation of

Reflectance Data. 151-152.

IJsseling, F. P. (1989). General Guidlines for Corrosion Testing of Materials for Marine Applications.

Literature Review on Sea Water as a Test Environment, 26.

Instruments, A. (2015, February 3). Field Machine Data Sheet. Retrieved from ACM Instruments:

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Laque, F. L. (1975). Marine Corrosion Causes and Prevention. New York: Wiley.

MacDonald, D. D. (2002). A Brief History of Electrochemical Impedance Spectroscopy. 1.

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Mansfield, F., Huet, F., & Mattos, O. R. (2001). New Trends in Electrochemical Impedance

Spectroscopy and Electrochemical Noise Analysis. The Electrochemical Society.

Marijana Kraljic, M. Z. (2003). The Effect of Polyaniline and Modified Polyaniline Coatings on

Stainless Steel Corrosion Protection. 1.

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Narnedra B. Dahotre, T. S. (1999). Intermetallic and Ceramic Coatings. Marcel Dekker.

Pauwels, L. (2010). Contribution to the Impedance Data Analysis of Mass Transfer Controlled

Electrochemical Systems. 1.

Saha, R., Mohamed, S., & Khan, T. (2011). Effect of Coating Parameters on the Electrodeposition of

Nickel Containing Nano-sized Alumina Particles. In N. Bansal, Processing and Properties of

Advanced Ceramics and Composites III (pp. 41- 43). John Wiley & Sons.

Schlesinger, M. (2010). Electroless Deposition of Nickel. 447- 458.

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*Some references are bibliographic and uncited within the text.

honours project 40069944

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