nickel in an alkaline environment
TRANSCRIPT
STRAY CURRENT CORROSION OF CARBON STEEL,
ELECTROPLATED NICKEL, AND ELECTROLESS
NICKEL IN AN ALKALINE ENVIRONMENT
by
Arthur Pismenny
A thesis submitted in conformity with requirements for
the degree of Master of Applied Science
Department of Metallurgy and Materials Science
University of Toronto
O Arthur Pismenny 2001
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Stray Curmnt Comlon of Carbon Steel, Eloctroplateci NI an&-N)tn-mA--
Master of Applied Science 2001 Arthur V. Pismenny
Department of Metallurgy and Materials Science University of Toronto
ABSTRACT
The stabitity of metal parts in electrolytic environment is a function of
extemal and intemal cell field bias. Therefore, in order to achieve long-term
integrity of electrolyser cell components, the alloys and plated metals used
should be evaluated with respect to stray current corrosion. For this purpose,
three different materials were tested in this study in specially designed
electrochemical cells. The materials were carbon steel, Ni-electroplated carbon
steel, and electroless Ni plated on carôon steel.
In this comparative study, the corrosion of the three materials was
investigated using both thermodynamic predictions (Pourbaix diagrams) and
potentiostatic and weight loss data obtained from immersion testing for the three
materials. Effects of turbulence, hydrogen gas, and temperature variation on
corrosion were also examined for the above materials.
Finally, electroplated Ni and electroless Ni plated carbon steel showed
excellent resistance to stray current corrosion under the specific electrolyser
conditions.
ACKNOWLEDGEMENTS -- - ---- - A- -
I am mostly grateful to my sr ipewison, Prof. D.W. Kirk and Prof.
S.J.Thorpe for their advice, support, and patience, and Stuart Energy System
Corp., for initiating and supporling this challenging yet enjoyable research
p roject . I would like to thank rny friends and colleagues Anson Sinanan, Paulo
Borges, and Daniel Lumanauw for al1 their help with the project, thought
provoking discussions, and al1 the great times we've had as a team.
Many thanks to Sal Boccia and Fred Neub for their inestimable help with
SEM and EDX.
I also owe special thanks to Teresa Miniaci and Fanny Strumas for always
being very helpful and approachable.
Finally, I would like to thank my farnily for their understanding, spiritual
support and faith in me.
Arthur Pismenny
--.- TABLE OF CONTENTS .......... . . . . M . .
ABSTRACT ........................................................................................ i ACKNOWLEDGEMENTS .................................................................... .Ji ... ................ ............................................... TABLE OF CONTENTS .... ..III
................................................................................ LIST OF FIGURES vi ................................................................................. LIST OF TABLES ix
............................................................................... LIST OF SYMBOLS x
.......................................................... 1 . INTRODUCTION .................... ., 1 ............................... 1.1 Alkaline Water Electrolysis and Stray Currents 1
1.2 Structural lntegrity of Metal Parts and Material .................................. Considerations in Commercial Electrolysers 4
............................................................................. 1.3 Objectives -6 ....................................... 2. BACKGROUND AND LITERATURE REVIEW. 7
2.1 Cost of Corrosion .................................................................... 7 2.2 Alkaline Water Electrolysis Reactions .......................................... 8 2.3 Stray Cuvent: Causes and Effects ............................................. 10
............................................... 2.4 Therrnodynarnic Considerations 11 2.4.1 Pourbaix Diagrams ..................................................... 12 2.4.2 Galvanic Series ......................................................... 15
2.5 Kinetic Considerations ............................................................ 15 2.5.1 Empirîcal Corrosion Rate Laws as an lndicator of Materials' Performance ...................................................... 16
2.6 Materials Selection ................................................................. 18 ..................................................... 2.6.1 Fe-based Materials 19
........................... 2.6.1 . 1 Passivity and Protective Films 20 2.6.1.2 Effects of Metallurgical Structure on Corrosion ...... 21
2.6.2 Ni-coated Metals ......................................................... 22 2.6.2.1 Electroless Ni ............................................... -22
.............................................. 2.6.2.2 Electroplated Ni 24 2.7 Potentiostatic and Polarization Methods to Measure
Weight toss and Conosion Rats ............................................... 26 .......................................... 2.7.1 Factors Affecting Corrosion 29
2.7.1.1 Effect of pH .................. .. ........................... 29 ........................ 2.7.1.2 Eff ect of Temperature variations 31
2.9.1.3 Eff ect of Turbulence ......................................... 32 2.9.1.4 Effect of Hydrogen ......................................... 32
.
.............................................................................. . 3 EXPERIMENTAL 33 3.1 Electrode Materials ............................................................... 34
.............................. .............. . 3.1 1 Electrode Preparation ... 34 3.2 Electrolyte .......................................................................... 36 3.3 Elect rochemical Cells ............................................................ 37 3.4 Experirnental Setup .............................................................. -39
iii
4 . RESULTS ............................................................... ........................42 ................................. 4.1 pH Measurements Before and After Testing 42
4.2 Thermodynamic Results .......................................................... 43 ........................... 4.2.1 Pourbaix Diagram for Fe-H20 at 70°C -44 ............................. 4.2.2 Pourbaix Diagram for Ni-H20 at 70°C 45
4.3 Potentiostatic Data ................................................................. 46 4.3.1 Potentiostatic Curves for Carbon Steel.
Electroplated Ni and Electroless Ni at E=+0.8V vs . SHE in KOH at 70°C .......................................................... 48
4.3.2 Potentiostatic Curves for Carbon Steel. Electroplated Ni and Electroless Ni at E=+0.54V vs . SHE in KOH at 70°C ........................................................... 50
4.3.3 Potentiostatic Cumes for Carbon Steel, Electroplated Ni and Electroless Ni at E=-O.01V vs . SHE in KOH at 70°C .......................................................... 52
4.3.4 Potentiostatic Curves for Carbon Steel, Electroplated Ni and Electroless Ni at E=-0.60V vs . SHE in KOH at 70°C ............... ....,. ................................. 54
4.3.5 Potentiostatic Curves foi Carbon Steel. Electroplated Ni and Electroless Ni at E=-0.9V vs . SHE in KOH at 70°C .......................................................... -56
4.3.6 Potentiostatic Cumes for Carbon Steel without and with Flow of Hydrogen Gas, E=+0.54V vs . SHE in KOH.70°C .............................................................. 58
4.3.7 Potentiostatic Cunres for Carbon Steel and Electroless Ni Under Turbulent Conditions, E=+O.BV vs . SHE in KOH,70°C .................................................. 59
4.3.8 Potentiostatic Curves for Carbon Steel at an Elevated Temperature. E=+0.8V vs . SHE in KOH at 80°C ................ 60
4.3.9 Potentiostatic Curves for Niplated samples. extended exposu re (time = 1 month) ........................................... -61
4.4 Surface Analysis ................................................................... -62 4.4.1 Surface Images Before and After Test: Carbon Steel.
................................ E tectroplated Ni, and Electroless Ni 63 ...................................... 4.4.la E=+0.80V vs . SHE ..... 63
4.4.1 b E=+0.54V vs . SHE ........................................... 64 4.4.1 c E=- 0.01 V vs . SHE .......................................... -65 4.4.ld E=- 0.60V vs . SHE .......................... ............ 66
........................................... 4.4.18 E=-0.90V vs . SHE 67 ............................................................... 4.5 Weight Loss Results 68
4.5.1 MPY Corrosion Rate Calculations for Carbon Steel . .......... Coupons, E=+0.8V vs SHE in KOH. at 70°C 70
-..- s DISCUSS~O~ ................................................................................. n 5.1 Initial Surface Analysis of The Coupons ...................................... 72
5.1.1 Scanning Electron MicroscopyM-Ray Analysis ................. 73 5.1 . 1 . 1 a Commercial 1020 Carbon Steel ..................... -73 5.1.1 . 1 b Electroless Ni ............................................... 75
5.2 Electrochemical Analysis ......................................................... 77 5.2.1 Commercial Carbon Steel ............................................ -77 5.2.2 Electroplated Nickel .............................. ...A 5.2.3 Electroless Nickel ....................................................... 90 5.2.4 Cornparison between Fe-based and Ni-plated coupons ...... 94
5.3 Effect of Turbulence ................................................................ 95 ............................................................. 5.3.1 Carbon Steel 95
5.3.2 Electroless Ni ............................................................. 96 5.4 Effect of Hydrogen Flow .......................................................... 97 5.5 Effect of Elevated Temperature ................................................. 98 5.6 Corrosion Rate Law for Carbon Steel. E=+0.8V vs . SHE at 70°C ..... 99
5.7 Validity of the Pourbaix Diagram as a Means of Predicting a Metal's Corrosion Activity ............ .. .................. 101 5.7.1 Limitations of Fe-H20 Diagram .................................... 101 5.7.2 Limitations of Ni-H20 Diagram .................................... 102
................................................. 6 . CONCLUSIONS .......................... .. 105 ..................................... 7 . RECOMMENOATIONS FOR FUTURE WORK 107
7 . REFERENCES .............................................................................. -108 APPENDICES .................................................................................. -112
................................................................... A . Conductivity of KOH 1 1 2 B . Thennodynamic Calculations for Pourbaix Diagrams ........................ 113
................ BI Thermodynamic Data Used in Calculations for Fe-H20 113 ................. 82 Thenodynamic Data Used in Calculations for Ni.H20 114
......................................... B3 Calculations of the Fe-H20 Diagrarn 115 84 Calculations of the Ni-H20 Diagrarn ......................................... 121
C . Electrolyte Calculations ............................................................. 128 Cl Cakulationof pH ................................................................. 120
O . Electrode Potentials .................................................................. 129 ......................... D l Hg/HgO Reference Electroâe Potential at 70°C 129
E . Titration Calculations ................................................................ 130 F . Density of Electroless Ni ............................................................ 132
LIST OF FIGURES .....
Figure 1.1 Scheme of an alkaline water electrolysis cell ............................... 2
Figure 1.2 Stray current drain inside a structural part .................................. 3
.................................................... Ffgure 2.3.1 lnduced potential gradient 10
Figure 2.4.1 Potential-pH diagram for H20 ................................................ 13
Figure 2.4.2 PotentiaCpH diagram for Fe.H20 ............................................ 14
........................................................ Figure 2.5.1 Linear reaction kinetics 17
.................................................... Figure 2.5.2 Parabolic reaction kinetics 17
................................................ Figure 2.5.3 Logarithmic reaction kinetics -18
............. Figure 2.6.2.1 EDX spectrum of a coupon with electroless Ni coating 24
Figure 2.6.2.2 Electroplating bath .......................................................... 25
Figure 2.7.1 Representative potentiostatic curve ........................................ 28
Figure 2.7.1.1 Effect of pH with respect to passivation/depassivation of Fe ...... 30
................................................... Flgure 3.1.1 Typical coupon geometries 35
.......................................................... Figure 3.3.1 Electrochemical cells 37
Figure 3.3.2 Schematic of an electrochemical cell ....................................... 38
Figure 3.4.1 A schemaüc drawinq of the experimental setup ........................ -39 Figure 3.4.2 Arrangement of the cells in a water bath .................................. 40
Figure 4.2.1 Diagrarn constructed for Fe-H20 system. 70°C ......................... 44
Figure 4.2.2 EpH diagram constructed for Ni- H20 system. 70°C .................. 45
Figure 4.3.1a Potentiostatic data at E=+0.8V vs . SHE. 8M KOH at 70°C. Hg flow.
no stirring; tirne of exposure =7days ......................................................... 48
Figure 4.3.1 b Position of the potential E=+0.8V vs. SHE with respect to the - .
regions of stability on the €pH diagrams for Fe/Ni in KOH ........................... 49
Figure 4.3.2a Potentiostatic data at E=+0.54V vs. SHE, 8M KOH at 70°C, Hz
flow, no stirring; time of exposure = 7days ............................................... ..50
Figure 4.3.2b Position of the potential E=+O.54V vs. SHE with respect to the
regions of stability on the €pH diagrams for FdNi in KOH at H20.. ............... -51
Figure 4.3.3a Potentiostatic data at E=-O.01V vs. SHE, 8M KOH at 70°C, Hz
flow, no stirring; time of exposure =7days.. ............................................... 52
Figure 43.31, Position of the potential E=-0.01 V vs. SHE with respect to the
.............. regions of stability on the EpH diagrams for FdNi in KOH at H20.. 53
Figure 4.3.4a Potentiostatic data at E=-0.6V vs. SHE, 8M KOH at 70°C,
HI! flow, no stirring; time of exposure =7days. ........................................... 54
Figure 4.3.41, Position of the potential E=-0.6V vs. SHE with respect to the
regions of stability on the E-pH diagrams for FeINi in KOH at H20.. ............... 55
Figure 4.3.5a Potentiostatic data at E=-0.9V vs. SHE, 8M KOH at 70°C, Hg
flow, no stirring; time of exposure =7days ................................................. 56
Figure 4.3.5b Position of the potential E=-0.9V vs. SHE with respect to the
................ regions of stability on the EpH diagrams for FeINi in KOH at H20 57
Figure 4.3.6a Potentiostatic curve for carbon steel, E =+0.54V vs. SHE,
8M KOH at 70°C, with no flow of Hg present in the cells. ............................ 58
Figure 4.3.6b Potentiostatic curve for carbon steel, E =+0.54V vs. SHE,
8M KOH at 70°C, with no flow of H2 present in the cells. ............................ 58
Flgure 4.3.7 Potentiostatic curves for carbon steel and electroless Ni
vii
---- *--- under turbulent conditions, E = +0.8V vs. SHE in 8M KOH at 70°C, 7 days.. .59
--
Figure 4.3.8a Potentiostatic curves for carbon steel at an elevated
temperature (80°C), E = +O.eV vs. SHE in 8M KOH, 7 days. ..................... 60
Figure 4.3.9 Potentiostatic cuwes for Electroplated Ni and Electroless
Ni, E=-0.01 V vs. SHE in 8M KOH, at 70°C ............................................. 61
Figure 4.4.1a Surface images before- and after test: carbon steel,
electroplated Ni, and electroless Ni, E=+O.BV vs. SHE.. .......................... 63
Figure 4.4.1 b Surface images before- and after test: carbon steel,
electroplated Ni, and electroless Ni, E=+0.54V vs. SHE.. ............................ 64
Figure 4.4.1~ Surface images before- and after test: carbon steel,
electroplated Ni, and electroless Ni, E=-0.01 V vs. SHE.. ........................... .65
Figure 4.4.ld Surface images before- and after test: carbon steel,
electroplated Ni, and electroless Ni, E=-O.6V vs. SHE ................................ 66
Figure 4.4.18 Surface images before- and after test: carbon steel,
electroplated Ni, and electroless Ni, E=-0.9V vs. SHE.. ............................. .67
Flgure 4.5.1 Paralinear corrosion rate law derived for carbon steel samples,
E=+0.8V vs. SHE, in 8M KOH, at 70°C ................................................... 71
Figure 5.1 Initial (before test) surface images of the coupons ...................... 72
............................. Figure 5.1.1 Carbon steel, direction of rolling.. A
Figum 5.1 .l b X-ray analysis of the electroless Ni-plated sample surface
composition.. .................................................................................... -75
Figure 5.2.1 Five potentiostatic cunres for carbon steel and the corresponding
after-test SEM images.. ...................................................................... .77
viii
Flgure 5.2.2 Five potentiostatic curves for Electroplated Ni and the - - C A - . - - - - - --- - .- - - - - - .
corresponding after-test SEM images.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84
Figura 5.2.3 Five potentiostatic cunres for Electroless Ni and the
corresponding after-test SEM images.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .90
Figure 5.7.2 Revised Ni-H20 diagram for 25OC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .IO3
LIST OF TABLES
Table 4.5a Weight loss measurements for E=+0.8 V vs. SHE at 70°C
in 8M KOH, Hp flow, time = 7 days.. . . .. . . . .. . . . . .. . . .. . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . .68
Table 4Sb Weight loss measurements for E=+0.54 V vs. SHE at 70°C
in 8M KOH, H2 flow, time = 7 days.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68
Tabk 4 . 5 ~ Weight loss measurements for E=-0.01 V vs. SHE at 70°C
in 8M KOH, H2 flow, time = 7 days.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -68
Table 4.5d Weight loss measurements for E=-0.6 V vs. SHE at 70°C
in 8M KOH, Hg flow, time = 7 days.. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69
Table 4.50 Weight loss measurements for E=-0.9 V vs. SHE at 70°C
inûM KOH, bflow, time = 7 days ........................ .....................I...........68
Table 4.5.1 Weight loss results used for the calculations of the MDD
corrosion rate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70
Table A1 . Conductivity of KOH at 70°C.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 12
Tabk BI. Themodynamic Data Used in Calculations and Resulting
Values of AG0, Ni-H20 Diagram at 70°C.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1 14
EDX
SO
SEM
= activity of species x
= aqueous solution
= area [cm2]
= heat capacity [J/K]
= molar concentration of species x
= electron
= potentiel M
= standard potential of an electrochemicat reaction [VI
= energy dispersive x-ray spectroscopy
= Faraday's constant [96487
= Gibbs standard free energy change [JI
= current density [~lcm*]
= exchange current density [A/cm2]
= current [A]
= rate constant for reaction i [crn/s]
= dissociation constant of water
= molality of ion x [moVkg]
= metal species
= molarity [moVL]
= universal gas constant [8.314 J/Kamol]
= standard entropy [J/mol*K]
= scanning electron microscopy
- - -
SHE = standard hydrogen electrode
t = time [days]
T = temperature [KI
a = specific conductivity [1 IRm]
P = density [g/mL]
Y* = mean molal activity
1.1 Alkaline Water Electrolvsis and Strav Currents - Alkaline water electrolysis is used to produce hydrogen and oxygen of high
purity ('). It is being developed for use with intermittent power sources such as
wind and solar. When DC electricity is passed between hivo electrodes (anode
and the cathode) immersed in water, hydrogen is produced at the negatively
charged cathode and oxygen is produced at the positively charged anode, as
shown in Figure 1.1. Most commercially available electrolysers are based on
alkaline water electrolysis. They have a monopolar or bipolar configuration. In the
first case, each separate electrode has only one polarity; in the latter case each
electrode has two different polarities, one on each side. Bipolar cells are more
efficient being more compact than monopolar cells. Monopolar cells, on the other
hand, are easier to manufacture. The main chemical reactions occurring at the
two electrodes are:
Anode: 40H' + 0 2 +2HD+4e- (1.1)
Cathode: 4H90 +48' + 2H9 + 40H' fl .2)
Total: 2H20 -+ 2H2 + O2 (1 *3)
Alkaline medium (25 - 30% KOH) is used in order to maximize the
efficiency of electrolysis (KOH conductivity is the highest in this range of
concentrations, see Table A l in Appendices), and also to lower the cost of
structural parts. The choice of the electrolyte is dictated by corrosion probiems,
which are much more serious in acidic media. Somewhat elevated temperatures
(65 - 70°C) are used to achieve better conductivity of KOH(~).
Cathode Permeable Anode Membrane
Figure 1.1 Schematic of an alkaline water electroîysis cell.
In commercial electrolysen a potential drop is obsewed in the electrolyte
from the cathode to the anode (monopolar construction) or from one end of the
--- --- -L - stack to the other (bipolar construction). This potential gradient can be passed
onto the electrofyser's structural parts via hydraulic paths so that one can actually
monitor a range of induced potentials distributed along the structural part, which
is referred to as stray current drain (Figure 1 -2)
Cathode
-
E
O
Anode
+
l S tray current drain
Figure 1.2 Stray current drain inside a structural part on which the potential
gradient between the cathode and the anode is irnposed via electrolyte.
- - -- The result of the drain is that one end of the metal structure becomes
- -
anodic with respect to the other one and there is a distribution of potential from
on end to the other.
Whether or not a metal will corrode is determined by the free energy
change, AG, for a given corrosion reaction M+ Mn++ ne-, where n is the number
of electrons (? A negative value for the free energy change indicates that a metal
can react spontaneously to form the corrosion product. The inherent reactivity of
a metal M can also be described in terms of electrochemical potential, E, since
AG = -nFE, where F is Faraday's constant. As discussed in Section 2.4, at a fixed
value of pH (a measure of acidity) there may be identified several regions of Ps
where the metal M can be immune, thermodynamically stable (forms a protectiw
film), or unstable (does not form a protective film). Thus, variations of the
potential within the metal c m result in metallic corrosion. This corrosion
sustained by extemal currents (carried by the ionic flow in the electrolyte) is
called stray current corrosion. This means that currents generated by an anode-
cathode couple can induce stray cufrents for a structural part nearby, provided
there is a conductive medium between the two. Thus, stray current is an
electrical current going through a path other than the intended one.
1.2 Structural lntearitv of Metal Parts and Material Considerations in
Commerctal Electrolvsers.
The consequence of one end of a part being anodic with respect to the
other is often manifested in severe degradation. The stray current ("leaked
current") may cause serious corrosion in a very short period of time. Typically, - - - - - - - - -
the "leakedn current uses the network of hydraulic circuits for the ionic current.
One can minimize the leakage of current by making the ionic current path for
stray current very long hence increasing the resistance of the electrolyte.
Altematively, one can select an appropriate corrosion-resistant material, after
stray current mapping has been performed and the range of potentials is known.
The first requirement for materials selection is a clear and detailed
definition of the conditions. Candidate materials are identified and screened
based on available mechanical and physical properties data and on their
cortosion resistance.
Some of the most important considerations in materials selection are low
capital cost, maximum operating reliability, and minimal product contamination (4).
In order to estimate the resistance of alternative rnaterials, corrosion tests
are usually conducted. There are several standard corrosion tests developed by
the American Society of Testing and Materials (ASTM)'~). Cyclic Voltammetry,
Anodic Polarization, AC Impedance, Immersion and Weight Loss Analysis are
some of the popular techniques. Also it is important to identify the variables for
the corrosion tests. Any variable (e.g. pH, temperature, flow rate, or potential)
that can influence a reaction between the material of construction and its
environment should be given consideration. Apait from experimental tools, it is
weful to employ theoretical models: thermodynamics can provide some
guidance to whether a metal can be corrosion resistant or not under specific
conditions such as temperature, pH, and potential.
----A- - 1.3 Obiectlves
The objectives of this project were as follows:
1. To construct an experimental set up for bench scale corrosion tests to
simulate stray current corrosion.
2. To evaluate different materials with respect to corrosion under different
applied potentials.
3. To make thermodynamic predictions of stray current corrosion via EpH
diagrams and to vefify the appropriateness of the diagrarns for the tested
materials.
4. To study the effects of different variables (temperature, flow rate,
hydrogen flow) on the corrosion of the materials under study.
5. To determine corrosion rates for the materials tested in bench scale under
electrolyser conditions (8M KOH at 70°C).
2. BACKGROUND AND LITERATURE REVIEW - --Lw-= - - - - - - - -
2.1 Cost of Corrosion
The economic impact of corrosion is one of the main motivations for this
project. In the United States alone approximately $300 billion per year is
attributed to losses due to metallic corrosion @). About 1/3 of these costs have
been estimated to be avoidable by more extensive use of corrosion-resistant
materials as well as by use of the available corrosion prevention techniques.
The electrolytic industry is a sector in which corrosion is a recognized
factor and is likely to have the same opportunity for corrosion avoidance. In this
project the focus is on corrosion of structural parts due to stray currents in
alkaline water electrolysers which are used to produce pure hydrogen. Structural
integrity of electrolysers is key for hydrogen producers because of the capital
costs associated with the units. The useful life of electrolysers is decreased by
corrosion, and corrosion products will contaminate the electrolyte and reduce
production capacity.
In the study by Battelle/NIST, ten elernents comprising the cost of
corrosion were identified @):
Replacement of equipment
Loss of product
Maintenance and repair
Excess capacity
Redundant equipment
Corrosion control
- - -- - - - - - a Technical support
Design
Insurance
Parts and equipment inventory
These costs Vary in relative significance and not al1 of them are easily
recognized; yet they al1 make up the total cost corrosion. With respect to stray
current corrosion of the highest priority are proper design in order to increase the
resistance of the stray current electrolyte path, and corrosion control by selecting
appropriate corrosion resistant materials (').
2.2 Alkaline water electrolvsis reactions
The electrochemical splitting of water was discovered in 1800 by
Nicholson and Carlisle ". Decomposition of water is a redox reaction. Using a
DC electrical current, the oxidation reaction occurs at the anode, and the
reduction reaction at the cathode. Water is oxidized at the anode (alkaline
conditions):
4 0 H + O2 + 2H20 + 48- EO = +0.401 V vs. SHE (2.2.1)
Water is reduced at the cathode:
4H20 +46 3 2H2 + 40H' E0 = -0.826 V vs. SHE (2.2.2)
In the decomposition reaction, the volume H2 produced is twice the volume of 02:
2H20 + 2H2 + O2 Ecdi = -1.227 V VS. SHE (2.2.3)
Although the theoretical cell potential is 1.227V, there are many additional - -
factors that must be taken into consideration so that an excess energy is
required. A cell potential, Ecdl, can be expressed as:
EC~II = Erev + qa + tlt + iln + qhw (2.2.4)
where E, is the theoretical thermodynamic decornposition voltage,
qa is anodic overpotential
qc is cathodic overpotential
q~ is the solution ohmic drop between the anode and the cathode
qhw is the ohmic drop of the hardware (circuits, equipment).
In the pioneering work, an acidic medium was used for water electrolysis,
but for modern technology, an alkaline medium (8M KOH) is preferred due to the
lowest resistance obsenred for KOH at this concentration, allowing higher
efficiencies and less significant corrosion, especially when one uses carbon
steels as structural parts.
Elevated temperatures (70-90°C) increase the efficiency of the alkaline
water electrolysis allowing higher electric conductivity of KOH, however, the
electrolyte also becomes more corrosive.
2.3 Strav curmnts: causes and effects. _rr_e_ - -___ =:-._ . . A - - - - - - -
Stray-current corrosion is corrosion resulting from direct current flow from
an extemal source through paths other than the intended circuit Stray-current
corrosion is different from environmental corrosion because it is caused by an
induced electrical current and is practically independent of some of the
environmental factors that influence other forms of corrosion.
Two mechanisms can cause this type of corrosion. The current may be
due to a direct connection to a power supply and will flow in the structure creating
an anode at the current drain end, or the current may be induced by a potential
gradient between operating electrodes and enters the structure via the electrolyte
(ionic flow), developing an induced potential gradient as shown in Figure 2.3.1.
Figure 2.3.1 The shaded bottom of the cell indicates the induced potential
gradient.
.-:,---- --- The stray current mechanisms mentioned + above can result in a very rapid
- -
corrosion, usually much more severe than corrosion caused by other
environmental factors. This is due to the fact that some of the applied potentials
E may provide the necessary free energy change AG for a corrosion reaction
M+ Mn++ ne-, where no surface passivating oxide is formed that would otherwise
lower the corrosion rate (see Section 2.8).
Each situation involving stray currents requires individual study. The best
way to prevent straycurrent corrosion is to eliminate or reduce the stray current.
If the situation permits, it is always better to act on the source, not on the
consequences. If a structural part affected by stray currents can be brought to
the same potential, or if there is a possibility of discharging leakage currents,
outside of the ceIl, then corrosion damage will be minimized.
2.4 Thermodvnamic considerations.
Thermodynamics has been widely applied to corrosion problems for a long
time ('). Use of therrnodynamic calculations allows prediction of conditions under
which a metal is stable and corrosion will not occur.
Thermodynamics makes it possible to describe equilibria as a funcaon of
the species and compounds present in the electrolyte, as well as the
environmental conditions, such as temperature and pressure.
However, thermodynamics does not forecast how fast ttie corrosion
reaction will occur. In order to obtain the rate of corrosion, it is necessary to
- -- *-=- examine - - polarization and - mixed -- potential behaviour ('O). The knowledge of
exchange currents may also be required.
A very useful thermodynamic tool referred to as E-pH (or Pourbaix)
diagram is one of the means of predicting a metal's corrosion activity.
2.4.1 Pourbaix diaarams
The development of diagrams showing thermodynamic conditions as a
function of potential (E) and concentration of hydrogen ions, i.0. pH was one of
the greatest achievements in the science of electrochemical corrosion ('). These
diagrams, known as Pourbaix diagrams, present a map of the regions of stability
of a metal and its corrosion products in aqueous environments. The EpH
diagrams determine conditions where
1) the metal is stable and will not corrode,
2) soluble reaction products are formed and corrosion will occur, and
3) insoluble reaction products are formed and passivity will occur.
The diagrams are obtained purely from thennodynamic calculations, i.e.
no corrosion experiments are needed.
Most of the original Pourbaix diagrams have been presented for pure
metals in pure water at 25°C. There have been many studies to extend the
diagrams, for example to include other ionic species and temperatures.
Figure 2.4.1 shows the €-pH diagram for H20 with no metal present. The
horizontal axis indicates the pH of the solution. The values of the ordinate (E)
--- - - range - - from strongly reducing solutions - with large negative potentials to strongly
oxidizing solutions with large oxidizing potentials. The two lines (a) and (b)
identify the region of stability of water as a function of potential and pH. Any point
with potentiai and pH being above line (b) corresponds to the region where water
is thermodynamically unstable (oxygen evolution). Below line (a) hydrogen gas is
liberated, identifying the other region where water is themodynamically unstable.
The region between lines (a) and (b) is referred to as the region of
thermodynamic stability for water.
l I I Uberation of oxygen
W .-.-.- --=---.---*- (b)
--.-.o.-.- .--- --.-.-.-.,, Stable J i 2 0
8 -0.5 Ci .W.-.---.---.- .--o.-.---.-.-. (a)
---O-- -.-.-.<'
l "1 Liberation of hydrogen I
Figure 2.4.1 Potential-pH diagram for H20, modified from ('*).
(a) O2 + 4H+ + 4 6 + 2H20
(b) 2H' + 20- -+ Hp
Despite the fact that most thetmodynamic calculations are based on pure
water, €-pH diagrams can be extended to many practical situations with more
complicated systems ('2! A simplified potential-pH diagram for iron is shown in -- ---- - - - L * -= - - - -
Figure 4.2.2. The diagram is a map that indicates the regions of immune,
passive, and corrosion behaviour for iron as a function of potential and pH. Four
distinct regions are shown. The region at the bottom of the diagram represents
the conditions where iron is immune and no corrosion occurs. Under these
conditions, iron is thermodynamically stable. The upper left and bottom right
F lgure 2.4.2 Potential-pH diagram for Fe-H20 at 25*C, showing areas of
immunity, passivity, and corrosion, modified from (12).
areas represent regions where iron corrodes. In both the region to the left -z-- - - L --
(oxidizing and acidic) and the small region to the right (reducing and highfy
alkaline), iron reacts to form soluble products, and corrosion continues. The area
in the centre represents a region of passivity for iron.
2.4.2 Galvanic series
Another thermodynamic tool that indicates the inherent tendency of a
metal to corrode or to react in an aqueous environment to produce metal ions is
the galvanic series, which ranks metals from the most noble to the rnost active in
a specific environment ('*). The metats are placed on the saries in the
descending order of standard electrode potentials. The wide range of potential
values indicates a wide range in inherent tendencies to corrode. The more noble
metals are at the more positive values. The more active rnetals (the ones that are
more likely to corrode) are at the more negative values.
As it has been already mentioned, thermodynamics has its limitations:
potential-pH diagrarns do not provide any information regarding reaction rates.
Rates, in turn, are important as a way of predicting corrosion with time. In other
words, knowing the corrosion rate, the corrosion resistivity of different materials
can be assessed in order to make the - best choice of material for a given
environment.
The study of the rate of chemical reactions involves reaction kinetics. The
factors driving these reactions are a complex interaction between the test
material and the environment. Some of the factors involved are:
potential
temperature
time
0 corrosive species and their concentration
chemisorption and dissociation characteristics
stress
2.5.1 Em~irical corrosion rate laws of electrochemical reactions as an
indicator of material's performance.
Corrosion rate laws describing aqueous corrosion can be most commonly
ctassified as one of three types, Le. linear, parabolic, or logarithmic ('4). It has to
be pointed out, however, that these are chemical reaction types, i.e.
electrochemical reactions may exhibit deviations from these rate laws.
Linear rate kinetics are most often associated with materials that do not
form protective scales or whose scales are highly porous or poorly adherent. The
rate equation which describes linear reaction kinetics (Fig. 2.5.1) is
X = klt (2.5.1)
where x is the oxide thickness per unit area and kl is the linear rate constant.
O 0.5
Time, t
Figure 2.5.1 Linear reaction kinetics, modified f rom (1 5).
Parabolic reaction kinetics describe corrosion w hem the rate-de ter mining
step is diffusion. This often occurs due to protective scales and intemal
penetration by such corrosive species as oxygen, carbon, or sulfur in certain
materials.
The rate equation describing parabolic reaction kinetics (Fig. 2.5.2) is:
2 = k2t (2.5.2)
where x is the scale thickness or mass gain per area, k2 is the parabolic rate
constant, and t is time.
Figure 2.5.2 Parabol ic reaction kinetics, modified f rom (1 5).
--- .AL------ . - Logarithmic reaction kinetics are most often encountered at the initial
- - - - - -
stages of oxidation of certain materials, when highly protective scales are
fomed, and intemal scale cavities or precipitates interfere with diffusion
mechanisms. The rate equation describing logarithmic reaction kinetics (Fig.
2.5.3) is:
x = iqw(t+to) + A (direct log law)
where A, ta and kr, are constants at constant temperature.
Figure 2.5.3 Logarithrnic reaction kinetics, modified from (1 5).
2.6 Materials selection for an alkaline environment
Selection of the proper material is not only a matter of the most corrosion
resistant material. For instance, carbon steels are the most common materials of
construction, not due to their corrosion resistance, but rather because of their
great mechanical characteristics, weldability, and low cost. W)
Although no single material is suitable for al1 applications, normally there
are a variety of rnaterials that could perforrn acceptably in a given environment.
Literature survey - - - - - - - - ? % - - - - - - -
('7-2') shows that common alkalis such as potassium hydroxide
and caustic soda (at pH values not exceeding 9.5) are not extremely corrosive
and can be handled in steel in most applications where contamination is not a
problem, as the corrosion product maintains a pH of approximately 9.5 at the
surface of the steel ('). Nevertheless, thes8 materials are susceptible to stress
corrosion cracking and severe uniform corrosion at higher concentrations (and
pH values) and tempe rature^.('^) .
2.6.1 Fe-based materials
Despite its relatively limited alkali corrosion resistance, carbon steel is still
the most widely used Fe-based material in the electrolytic ind~stry.''~) Carbon
steels contain up to 1 .O% C and alloy contents of generally less than 2% by
weight. Only the low-carbon, or mild, steels containing 0.08 to 0.28%C are
considered for resistance to corrosion. They are usually more corrosion resistant
than the medium-carbon (0.28 to 0.55%C) and high-carbon (0.50 to 1.0%C)
steels. Low-carbon steels are also much easier to weld and form, a common
requirement for structures and parts.
The susceptibility zone for carbon steel in an alkaline environment (KOH)
can be summarized in tenns of the potential-pH diagram. Previously (Fig. 2.4.2),
the E-pH diagram for iron was presented, and a triangle of active corrosion was
predicted in highly alkaline environments. If the conditions of the environment are
within this region, rapid uniform corrosion can occur. In some cases, either stress - - - - -
--A-- -
corrosion cracking or localized corrosion can be observed.
2.6.1.1 Passivitv and arotective films
Oxygen and other oxidizing agents can sornetimes hinder corrosion by
forming protective films. Metals, such as iron, rnay form oxide films and thus
becorne "passiven due to these films. The measured potential of these films is
closer to that of platinum, rather than the potential of the non-protected rneta~!~')
It was also found that these oxide films have a resistance to corrosion orders of
magnitude higher t han that of the unpassivated "active" metal. (2')
The nature of the protective oxide (its composition) is very much dictated
by the mechanism of the corrosion process (or the chemical reactions involved).
This in turn depends on the potential imposed on the corroding metal, in
combination with al1 other variables, such as temperature, pressure, the
concentration of the electrolyte and its flow rate. Once again, potential-pH
diagrams can be useful in predicting possible oxide species formation on the
metal surface.
A distinguished feature of the protective films that form on iron is that they
are, as a rule, very porous, lack adherence and generally are easily destroyed,
making the base metal active (depassivation). Among the main factors
facilitating the protective layer removal in carbon steels are electrolyte turbulence
and hydrogen gas formation. Hydrogen can react with the oxide in the passive
film and by removing it chemically or by physical expansion of the film, destroys .- ---- ---- - - - - - &
its passivity (21).
2.6.1.2 Effets of metalluraical structure on corrosion.
Many features in the microstructure of carbon steels affect their corrosion
resistance. Metals may be ptastically defomed in fabrication (cold-r~lling).(~~) In
highly worked steels the grains are deformed. Generally, in this condition, the
material is more reactive in electrochemical environments. Fu rt hermore, the
deformation of the grain structure manifests itself in preferential corrosion, where
different directions of a specimen (direction of rolling vs. transverse) have
different susceptibility to corrosion.
Secondly, depending on the temperature of working, the amount of
inclusions will Vary. resulting in higher or lower electrochemical reactivity (higher
concentration of inclusions yields higher tendency to react even in mild corrosive
environments).
Finally, lattice defects caused by a certain regime of steel working can
produce highly localized differences in electrochemical behaviour of carbon
steels. These defects may be vacancies caused by the absence of atorns in
lattices within grains (point defects), as well as large lattice disturbances, line
defects (dislocations). Thus, differences in submicroscopic characteristics of
carbon steels also must be considered.
Nickel and nickel alloys are extensively used to fight corrosion in alkaline
media!") Nickel is suitable under practically al1 conditions of concentration and
temperature. lt's also known for its ability to withstand a wide variety of severe
operating conditions involving high temperatures and high stress. However, the
production cost of nickel is very high making it an expensive material.
It was found that corrosion resistance to alkalis is almost directly
proportional to the nickel content of an a~loy.''~ Therefore, it would be
economically sound to use Ni-coatings on other metals, in order to combine the
exceptional corrosion resistance properties of pure nickel and the lower cost of
the substrate metal such as steel.
At present, two Ni-plating techniques are widely used: 1) Electroplating
(electrolytic) technique, and 2) Electroless nickel plating. (23)
2.6.2.1 Electroless Ni
Electroless nickel plating is otherwise known as chemical or autocatalytic
nickel p~ating.(~~) In contrast to the electroplatirig technique, chemical nickel
plating baths work without an extemal current source. The plating operation is
based on the catalytic reduction of nickel ions on the surface that is being plated.
Because it allows a constant metal ion concentration to bathe al1 parts of the
substrate, it deposits metal eveniy along edges and over irregularly shaped
objects which are difficult to plate evenly with electroplating. Thus, electroless 2 - -- % - --
nickel plating has the following advantages:
rn No need for complicated anode arrangements
A unique throwing power of the solution (property of the bath to provide
uniforrn coating)
Uniform speed of coating growth
The most widely used electroless nickel is deposited by the catalytic
reduction of nickel ions with sodium hypophosphite in acid baths at pH 4.5 or 5,
at a temperature around QO°C. Tight temperature control is mandatory, as rate
deposition constant k is directly proportional to the log T. Another requirement is
constant agitation in the bath to ensure the constant metal ion concentration in
the bath.
The deposits typically contain 3 to 13% phosphorous by weight
(Fig.2.6.2.1). The obtained alloy is dependent on the chemical composition of the
solution and the operating conditions. The phosphorus content significantly
influences its chemical and physicat properties in both the as-plated condition
and after heat treatment. Nickel coating with 3 to 7% phosphorus content is
known to have superb corrosion resistance in concentrated alkaline media.
These coatings also have good Wear resistance.
+RAW O - 20 kéU ,ive: 100 s Preret: 100's Remai n i ng: 0 s
9.829 keV FS= 8 K OS= 128 ch 250= ,MEMI : ELEGTRQLESÇ N I
10.0 > 180 c t r
Fig. 2.6.2.1 EDX spectrum of a coupon with electroless nickel coating.
Coatings with phosphorus content between 10 and 13% are very ductile and
have the highest corrosion resistance against chlorMes and mechanical stress.
2.6.2.2 Electro~lated nickel
Another common method for nickel coating is e~ectroplating.(*~) In
electroplating, negatively charged substrat0 is immersed in a plating bath next to
positively charged anodes (Fig. 2.6.2.2). Ions of nickel in the electrolyte bath are ---- ---r -a - = - -
reduced and plated on the surface of the substrate forming the coating. The bath
composition, the electrolyte velocity , current density, and temperature determine
the chemistry and properties of the coating. Typical electroplating baths are
composed of aqueous salts of nickel.
Wx = Metal ion with positive charge of x
Figure 2.6.2.2 Electroplating bath, modified frorn (24).
This technique, however, has a few disadvantages. Sornetimes
undesirable reactions may take place on the surface of the substrate (for
instance, reduction of H+ along with ~ i * + ions causing entrapment of hydrogen
bubbles which compromises the integrity of the deposited layer). The effect of
throwing power (distance between the anode and the substrate, at which plating
is uniform) is a difficult issue often resulting in non-uniform coatings when dealing
with complex geornetries of the substrate. Uniforrn plating, on the other hand, is
key to the long-term performance of a coated part. In addition, electroplated
--.4 ---- -- nickel coatings are found to have low adherence when substrate is bent or - A = - - - . . - - - - - - -
welded. As a consequence, this technique is inferior to electroless plating when
it cornes to plating complex shapes.
For either of the two techniques, quality of the surface of the substrate to
be coated has a direct impact on the uniformity of coating.
2.7 Potentiostatic and mtentiodvnamic bolarization) methods to measure
weiaht loss and corrosion rate
Although time consuming, long-term potentiostatic tests can be useful in
determining the resistance to corrosion.(2s) However, a series of a few separate
potentiostatic tests conducted at different (fixed) potentials allows determination
of the effect of the complex potential gradients that occur with stray-currents. In
order to assess the corrosion resistance of various materials with respect to the
potential range imposed on the material, it is possible to determine (map out) the
range of potentials, then subdivide the range into a few fairly narrow regions
(potentials) of interest and then to perform a series of immersion tests at fixed
potentials, corresponding to those above. From the iron-water EpH diagram, by
changing E at a fixed pH, temperature, and pressure, different areas of corrosion
activity can be studied. It its important to know how a material performs within a
range of Es, when dealing with stray-currents.
The materials of interest were machined into block fom before being
measured, polished, weighed and exposed to the corrosive medium (25). Surface
area has to be accurately rneasured in order to be able to normalize the -- -
corrosion data. A potentiostat is used to hold the coupon surface at a constant
potential. After a set period of time, the blocks are removed from the electrolyte,
the surfaces are carefully cleaned and weight losses are determined (ASTM
method 031). The time frame involved even in long-terni potentiostatic tests is
still usually very short compared to the projected life.
Weight loss is used to characterize the rate of unifon corrosion via ASTM
standard G1 (25':
CorrosionRate = ( K x W ) ( A x T x D)
w h ~ 8 K is a constant ( ~ = 2 . 4 0 ~ 1 0 ~ for a corrosion rate in mdd), T is time of
exposure (hours), A is area in cm2 measured to the nearest 0.01 cm2, W is
weight loss in g to nearest 1 mg, and D is density (@cm3), ASTM G1.
However, corrosion rate values obtained for a fixed exposure time provide
corrosion rates only for that time frame. To detemine the corrosion rate law for a
given electrochemical system (at a fixed potential), several rates obtained from
different time exposures are needed. Once the trend (linear, parabolic.
logarithmic, or para-linear) is determined, one can extrapolate the cuive in order
to assess possible corrosion damage after longer exposures than those
As well as direct weight loss measurements, potentiostatic curves (current - - - - - - - - - - -
density vs. time) can also be useful in detemining the amount of metal dissolved
due to corrosion. This may be done by integrating the area under the curve (for
example the c u w in Figure 2.7.1). assuming that the entire amount of the
passed current corresponds to metal dissolution only. Depending on the E-pH
regime, oxygen or hydrogen can be produced (at potentials outside of the
stability region for water, see Section 2.4.1) concurrently with the metal
dissolution, thus offsetting the potentiostatic cunre by the current due to the gas
evolution. In this case, in order to make an assessrnent of the corrosion current
density, the amount of the produced gas would be detemined in order to apply
Faraday's law to obtain the amount of charge transferred as a result of gas
formation only. Then, the latter amount would have to be subtracted from the
O 2 4 6 8 1, days
Figure 2.7.1 Representative potentiostatic cuive (current density vs. time).
overall area of the plot to yield the tnre metal dissolution current density.
2.7.1 Factors affectina corrosion at a tixed mtentlal -A4--u- 3. -: - - -- - - - - -.
The potentiostatic method of corrosion rate measurement allows gathering
information about the dependence of corrosion on extemal variables such as pH
change, temperature change, turbulence (flow vs. no flow), and presence of
hydrogen or oxygen. (28-30)
2J.l ,l Effect of DH
The effect of pH on the corrosion equilibria of Fe and Ni-based materials
at a given potential is best rationalized in tens of the corresponding EpH
diagrams (Sections 4.2.1 and 4.2.2). In the case of Fe-H20 systern, for instance,
one can infer that in highly caustic solutions (pH-14) one can passivate or
depassivate the surface by changing surface pH depending on the region in
question (Sections 4.3.1 through 4.3.5). Some of the reactions that are likely in
the aforementioned pKrange are (*%
Fe203 + H 2 0 + 28' = 2HFe02 - (2.7.1 .l)
+ 2H20 + 28 ' = 3HFe02 - (2.7.1.2)
HFeOz- + 2H20 = Fe04 "+ 5H?+ 4e- (2.7.1.3)
shown as lines 1, 2, and 3, respectively (Figure 2.7.1 .l) between the
corresponding phases (shown on the lefi- and right hand side of each equation).
Figure 2.7.1.1 Effect of pH with respect to passivation/depassivation of Fe in Fe-
H20 system at 70°C.
At a fixed potential pH may determine whether Fe will passivate (forming
Fe2O3 or or depassivate (foming ionic species such as HFe02- or
F~O?). For example, at pH = 13.6 and E= -0.9V vs. SHE, Fe304 is the
thermodynamicell y fawwreble spscies, end thus the surface would passivate.
Altematively, at the same potential but a different value of pH, 0.g. pH = 14.3, the
thermodynamically favourable species is now HFe02- , and thus the surface of
Fe would depassivate.
The same type of approach should be valid for Ni-coated materials, when
referring to the conesponding Pourbaix diagram for nickel.
--A----: - - .
2.7.1.2 Effect of temmratum variations.
Temperature can have a significant influence on the corrosion process,
since corrosion is an elsctrochemical reaction and reaction rates do increase with
incteasing temperature. (29) The dependence of the rate constant of a reaction on
temperature is expressed by the Arrhenius equation:
k = Ae --T (2.7.1.2)
where Ea is the activation energy of the reaction (in kJImol), R is the gas
constant, T the absolute temperature, and e the base of the natural logarithrn
scale. The quantity A represents the collision frequency and called the frequency
factor. The speed at which species collide (and thus the reaction rate) is
increased as temperature is increased since the latter is a measure of the kinetic
energy of the colliding species. Kinetic energy, in turn, defines how easily it is for
the reacting system to reach the activation energy. Typically, for chemical
reactions there is a doubling in the reaction rate per each 1 O°C increase.
When corrosion is controlled by the rate of oxygen diffusion, the corrosion
rate increases linearly with increasing temperature (*@. For open systems, where
T exceeds 80°C, the solubility of oxygen in water decreases, thus the corrosion
rate drops approaching zero at 100°C. On the other hand, in a closed systern
(oxygen cannot escape) the corrosion rate keeps increasing as the temperature
increases up to the point where the oxygen supply becomes lirniting.
The effect of turbulence is most drastically manifestet J in the case of
carbon steels, rather than nickel @'). The relative motion of the electrolyte (KOH)
initially increases the corrosion rate by supplying more oxygen to the surface, up
to the point when a protective oxide layer forms. However, when turbulence is
increased, the oxide film may be destroyed causing the corrosion rate to
increase.
2.7.1.4 Effect of hvdroaen Dresence
Hydrogen, especially when coupled with turbulence, may be very
detrimental to the integrity of passive layers as it readily reacts with oxygen in the
passive oxide film (" ).
Hydrogen cracking (26) is another serious issue when dealing with
structural carbon steel parts that are stressed (due to welding, for instance). In
this case cracking is caused by hydrogen atoms entering the metal interstitially
either through a corrosion reaction or by cathodic polarization.
3. WPERIM ENTAL - >
One of the objectives in this work was to set up a series of laboratory tests
that would simulate actual service conditions. The standards and test methods
recognized by technical societies such as ASTM and NACE International were
used as the basis for this work.
There are many variations in a laboratory immersion test. ASTM G 31,
Practice for Laboratory Immersion Corrosion Testing of metals, and the NACE
lnternational Standard Test Method TM-01 -69 are general guides on how these
tests may be performed. Normally, small pieces (i.e. 1 cm by 4 cm by 0.5 cm
thickness) of the candidate material are exposed to the test medium (electrolyte)
while kept at a fixed potential, and the loss of weight of the material is measured
for a given period of time. Immersion testing remains the best method of
evaluating materials and eliminating those materials that should not be used.
This technique is the quickest and most economical way to make a preliminaty
selection of the best candidate materials. However, one of the serious
disadvantages of this method of corrosion study is the assumed average-time
weight loss. The corrosion rate could be high initially and then decrease with
tirne, or vice versa. In other cases, the rate of corrosion rnight increase very
gradually with time, it could cycle, or there could be a combination of the above,
reemphasizing the fact that corrosion is complex and not easily modeled.
In addition to the full immersion test at a fixed potential, there are several
variations that could be used to accentuate certain types of exposure conditions.
-a- ---- -- For example, the solution velocity could be changed by introducing a stirring
device into the electrochemical cell.
3.1 ELECTRODE MATERIALS
The following three materials were evaluated:
1. Commercial cold-rolled Carbon Steel 1020 (0.1 7-0.23% C).
2. Electroplated Nickel (commercial nickel deposited electrolytically on a
carbon-steel substrate, layer thickness at least 250 pm).
3. Electroless Nickel (306% P, nickel-phosphorous alloy deposited chemically
on a carbon-steel substrate, layer thickness at least 250 pm).
3.1 .l ELECTRODE PREPARATION
One of the most important parts of corrosion testing is the preparation of
the specimen to be tested. Test specimens should have sarne physical
characteristics of the material that will be used in the industrial application. This
necessitates consideration of a number of conditions, such as: composition
(elemental chemistry), mechanics (strength, hardness, etc.), form (sheet, plate,
etc.), sire and shape (surface area-to-corrodant volume ratio, end-grain
exposure), metallurgical treatment (heat treatment, welding, cold-working, etc.),
surface finish (saw-cut, ground and polished, electropolished, etc.), and cleaning
procedures (passivated, degreased water-was hed , etc.).
-- - - . In this work, al1 sample - - preparation = - - - - - procedures -- were divided into two
categories: those applied to carbon-steel ~amples and those applied to Nickel-
plated samples. The Carbon Steel samples had to be pre-cut, ground, polished,
and then cleaned. The nickel-plated specimens were commercially made and
required no furt her preparation, except ultrasonic cleaning (degreasing) with
methanol and acetone.
Figure 3.1.1 illustrates the three coupon shapes used in the
electrochemical tests. The surface areas of the samples had to be different to
ensure that the corrosion currents produced at fixed potentials would not exceed
the limit (1 A) set by the available potentiostat.
Figure 3.1.1 Typical coupon geometries (true size)
Once the samples were cut, they were spot-welded to 10 cm nickel rods.
Then surfaces were ground smooth and flat. Otinding was performed using
successively finer abrasives (240, 400, and 600 grit paper). During grinding, the
sample surface was kept cool. Cooling was provided by the use of water.
-- - -- * -A As soon - as grinding using 600 grit paper - was completed, mechanical
polishing was used. The polishing operation is very similar to grinding, but the
size of the pofishing abrasives is considerably smaller than the size of grinding
abrasives. Each carbon-steel sample was first rough polished using 6pm
diamond compound. The sequential principle was applied again by using 3pm
and then 1 pm diamond compound. An ultrasonic bath was used for cleaning
after polishing. Finally, samples were ultrasonically rinsed in water, followed by
ethanol, methanol, and acetone and blown dry to avoid staining. A
nonconductive epoxy paint (~mercoat~ 90HS) was used to mask the samples in
order to ensure insulative integrity. After cleaning, the test specimens wers then
weighed on an analytical balance to an accuracy of *OS mg, and dimensional
measurements to 0.001 cm were performed to permit accurate calculations of the
exposed surface area.
Prior to electrochemical testing, al1 samples were stored in a desiccator.
. - 3.2 ELECTROLYTE
The electrolyte solution was prepared by dissolving ~nalar@ grade KOH in
deionized water. An analytical scale was used to weigh the necessary amount of
KOH to be dissolved in chemically pure water. Before- and after test
concentrations of the electrolyte were determined by titration (Section 4.1 and
Appendix). All experiments were done using 8M KOH to simulate the service
conditions. The electrolyte solution was pre-electrolyzed for 24 hours using Pt- -------- --- - -- - - -- ---- . . - - - -
electrodes at 200mA/cm2 to purify the electrolyte even further.
3.3 ELECTROCHEMICAL CELLS
Three electrochemical cells were made from Plexiglas in order to be able
to monitor electrode processes during the tests. Each cell was cylindrical
(Figures 3.3.1 and 3.3.2) with the following dimensions: 10(D) cm x6(H) cm. The
top of each cell had 6 apertures for 1) the working electrode, 2) reference
electrode, 3) counter electrode, 4) gas inlet, 5) gas outlet, and 6) stirring device
(not shown). Each cell had to be sealed to prevent the electrolyte from leaking.
Evaporation tosses were visually monitored and a constant level of the electrolyte
was maintained during every test.
Figure 3.3.1 Electrochernical cells
PLATIN UM COUNTER ELECTRODE W m O
EFERENCE ELECTRODE
REFERENCE ELECTRODE RESERVOIR
Figure 3.3.2 Schematic of an electrochernical cell
Figure 3.4.1 shows the schematic drawing of the experimental setup.
MULTISCAN + 3 POTENTIOSTATS DATA ACQUISITION
w u AM
(C-STEEL, ELECTROPLATED NI, ANDELECTROLESSNWLATED ELECTRODES IN HI11 KOH, 7OC) A 4 f
I
FLOW M ETERS
Figure 3.4.1 A schematic drawing of the experimental setup.
The three cells were connected to three potentiostats and a multiscan
(lotech@ 1200), the latter was linked to the data acquisition systern so that one
could monitor three corrosion currents simultaneously. The software installed to
the potentiostats (Hokuto Denko LtdB, HAB-151) was ernployed to maintain a
fixed potential applied to the materials.
In order to regulate deaeration of the electrolyte in the three cells with
argon or to maintain a constant inflow of hydrogen, flowmeters were used. The
flow rate used was -30 mumin.
The three cdls were placed in a water bath to maintain constant A . . - - - . . - - -
temperature. Figure 3.4.2 shows an actual arrangement of the three cells in a
water bath.
Figure 3.4.2 Arrangement of the cells in a water bath.
3.5 SURFACE ANALYSIS
Since corrosion is essentially a surface phenornenon, surface techniques
can be used to provide useful information in regard to surface corrosion
mechanism.
Before and after each test, every sample was examined using SEM-EDS.
A specimen is scanned with a high energy (20KeV) electmn beam in a raster
pattern which causes the ejection of a nurnber of particles including secondary
electrons, backscattered electrons, and X-rays. Secondary electrons (wit h P m - - 3 --.. ...z- -- - - - A- -
energies less than 50 eV) are only detectable if they are generated in the top
surface of the sample; this causes the secondary electron output to be
dependent on the surface topography and therefore gives an image that is very
similar to that seen with an optical microscope.
Element identification is provided by analysis of the emitted characteristic
X-rays with an Energy Dispersive Spectrometer (EDS).
4. RESULTS - . - - --- .L-
This section presents the theoretical and experimental results. First, the
results of the pH measurements before and after testing will be presented and
the Pourbaix diagrarns corresponding to the evaluated metals will be shown.
Next will be the potentiostatic data, followed by the SEM analysis. Finally, the
results of the weight loss measurements will be provided. All relevant
calculations and tables are listed in the appendices.
4.1 DH Measurements of the electrolvte /KOHl More and after test.
The pH measurements before and after test were done to estimate the pH
change during the testing period (7 days). This is an important consideration
before trying to apply EgH diagrams in order to make therrnodynarnic
predictions.
Acid-base titration was used to calculate the pH of the electrolyte (KOH)
before and after test (for calculations see Section E of the appendices). The
titrant was a standardized 0.1 M solution of H2S04; phenolphthalein was used as
an indicator. The 8M KOH (i.8. before-test electrolyte) was diluted to make a
0.08M solution, each aliquot of the after-test KOH was also diluted 1 00 times.
The experimentalty determined before-test value of pH was in agreement
with the theoretically determined pH for 8M KOH at 70°C, being equal to 14.3.
The titration of the after-test KOH yielded pH=14.6, calculated for 70°C.
In this section the Pourbaix diagrams for Fe in H20 and Ni in H20 at 70°C
(343 K) are presented. The activity of the dissohred species was assumed to be
equal to 10"; the potential range was set to b8 from -1.5 V to +1.5V vs. SHE at
70°C. Equilibrium potentials of oxidation/reduction reactions were calculated for
possible chemical species encountered during the HER in aqueous 8M KOH
electrolyte at the above-mentioned temperature. The oxidation/reduction
reactions, as well as the corresponding potentials can be found in the
appendices.
AMhough the actual tests were conducted using calibrated Hg/HgO
electrodes, the results of the thermodynamic calculations presented here are
presented in terms of the SHE reference electrode potential, since at 343 K the
two potentials are ver- close E H ~ N = - 0.00058 V snE (please see Section
Dl in the appendices).
- - - -
4.2.1 Pourbaix diaaram for Fe - H2O at 70°C
Figum 4.2.1 Diagram constructed for Fe-H20 system at 70°C, al1 ions at an
activity of 1 O?
4.2.2 Pourbaix diaaram for Ni-H& at 7U°C
Figure 4.2.2 €pH diagram constructed for Ni-HP systern at 70°C, all ions are at
an activity of 1 o ~ .
4.3 Potentiostatic data - .-
This section presents the data obtained from a series of immersion tests
where the three evaluated materials were tested at fixed potentials (E = +0.8V,
E = +0.54V, E = -0.01V, E = -0.6V, and E = - 0.8V, al1 vs. SHE). Each set of
potentiostatic cuwes is shown along with the corresponding E-pH diagrams
demonstrating the location of the fixed potentials with respect to the regions of
thermodynamic stability on the Pourbaix diagrams, so that one can compare the
thermodynamic predictions with the actual electrochemical behaviour of each
material tested.
The first five sets of data present the potentiostatic behaviour of carbon
steel, electroplated nickel, and electroless nickel at the above-mentioned five
potentials in the absence of any additional external interferences; the other three
series of data introduce the effects of turbulence, absence of hydrogen flow, and
elevated temperature.
As seen on the E-pH diagrarns, of interest is the region corresponding to
pH114.3 for this is the pH calculated for 8M KOH at 70°C (see appendices). The
asterisks placed on the vertical line corresponding to the pH value of 14.3
indicate the fixed potentials under study. The choice of the five potentials is not
arbitrary: one could single out several (in this case five) different regions where
the materials under investigation could demonstrate dissimilar corrosion activities
depending, for instance, on such factors as the location of the potential with
respect- to the lines of oxygen and hydrogen evolution. Of importance is also - - z u e - + - -
whether or not the potential is held within a region where ionic species exist.
Electroless NI at E = ++.8V vs. SHE In KOH at 70°C.
Figure 4.3.1. Potentiostatic data at E = M.8V vs. SHE, 8M KOH at 7OaC, H2 ffow, no stirring; time of exposure = 7 days.
Figure 4.3.1 b Position of the potential E=+0.8V vs. SHE with respect to
the regions of stability on the EpH diagrams for FdNi in KOH at 70°C.
4.3.2 Potentiostatic Cunres for Carbon Steel. Electroalated Ni and -----__%A :_ _ j A _ _ _ - _ - - - -- --a ---- - - - -
Electroless Ni at € = +0.54V vs. SHE in KOH at 70°Cm
Figure 4.3.2a Potentlostatic data at E = +0.54V vs. SHE, 8M KOH at 7û°C, H2
flow, no stirring; time of exposure = 7 days.
Figure 4.3.2b Position of the potential E=+0.54 V vs. SHE with respect to
the regions of stability on the E-pH diagrams for Fe/Ni in KOH at 70°C.
Electroless Ni at E = -0.0iV va. SHE In KOH at 70°C
0.0002
0.0001 5 -Carbon Steel
0.0001
0.00005
P O
-0.00005
-0.0001 I I
t, dayr
3.OOE46
t, days
2.WE-W CY
Figure 4.3.3. E=-O.01V vs. SHE, 8M KOH et 70°C, H2 flow, time = 7 days.
Ti - Ekctroplated Ni
- - Electroless Ni
t
Figure 4.3.3b Position of the potential E=-O.01V vs. SHE with respect to
the regions of stability on the E-pH diagrams for FeMi in KOH at 70°C.
..--.. --- . - 4.3.4 Potentiostatic Cumes for Catbon Steel, Electro~lated Ni and
Electroless Ni at E = -0.6 V vs. SHE in KOH at 70°C.
0.006 -7
- Carbon Steel - Electroplated Ni - Electroless Ni
I I
t, days
Figure 4.3.4a Potentiostatic data at E=-0.6 V vs. SHE, 8M KOH at 70°C, Hp
flow, no stirring, time = 7 days.
Figure 4.3.4b Position of the potential E=-0.6V vs. SHE with respect to
the regions of stability on the EpH diagrams for FelNi in KOH at 70°C.
4.3.5 Potentiostatic Cuwes for Carbon Steel, Electro~lated Ni and
Electroless Ni a E = -0.9 V vs. SHE in KOH at 70°C.
4 1, days
t, days
Figure 4.3.511 E=-0.9 V vs. SHE, 8M KOH, 70°C, Hz flow, time = 7 days.
Figum 4.3.Sb Position of the potential E=-0.9V vs. SHE with respect to
the regions of stability on the EpH diagrams for WNi in KOH at 70°C.
--a--. . ..- 4.3.6 Potentiostatic Cuwes for Carbon Steel without and with Flow of
- -27- - -
Hvdroaen Gaa E = + 0.54V vs. SHE in 8M KOH at 70°C. 7 davs.
t, days
Figure 4.3.6a Potentiostatic curve for carbon steel, E = +0.54V vs. SHE,
8M KOH at 70°C, with no flow of H2 present in the cells.
Figure 4.3.Bb Potentiostatic curve for carbon steel, E = +0.54V vs. SHE,
8M KOH at 70°C, with H2 flow present in the cells.
--.a--.
- 4.3.7 Potentlostatlc Curves for Carbon Steel and Electroless Ni Under
Turbulent Conditions. E = + 0.8V vs. SHE in 8M KOH at 70°C. 7 dam.
4 t, days
1 0.9 -- 0.8 - ,
0.7 -
Figure 4.3.7 Potentiostatic curves of Carbon Steel and Electroless Ni with
Turbulent Flow added (25 rpm, E=+0.8 V vs. SHE, 8M KOH at 70°C, Hp flow,
time = 7 days.
E 0.6 0 p 0.5 - -- 0.4 - .
I
-
-Carbon Steel E lec t ro less Ni
. ...- - 4.3.8 Potentiostatic Cuwes for Carbon Steel at an Elevated - --- A - - - - - - - - --& -- -
Temmrature 180°C), E=+O.8V vs. SHE in 8M KOH. 7 davs.
t, days
Figure 4.3.8a Potentiostatic curve for carbon steel (E=+0.8V vs. SHE,
8M KOH at a higher temperature (80°C), no flow, time = 7 days).
Figure 4.3.8b Potentiostatic curve for carbon steel (E = +0.8V vs. SHE,
8M KOH at 70°C, no flow, time = 7 days).
4.3.9 Potentiostatic Curves of Nickel-Plated Sam~les, extended time frame
fl month) E=- 0.01V vs. SHE in 8M KOH.
10 15 20 25 30
t, days
3.00E-06
Figure 4.3.9 Potentiostatic curves for Electroplated Ni and Electroless Ni
( - Electroplated Ni 1
Samples, E=- 0.01 V vs. SHE in 8M KOH, at 70°C, H2 flow, time = 30 days.
This section presents SEM data obtained from the five series of immersion
tests where the three evatuated materials were tested at fixed potentials (E =
+0.8V, E =+0.54V, E =-0.01V, E=-0.6V, and E =-0.9V, al1 vs. SHE).
For each fixed potential and sample "before" and "aftef test SEM images
are presented. The magnification used for al1 samples during the SEM analysis is
XI KV.
4.4.1 Surface imaaes befom and after test: Carbon Steel. Electro~tated NI, ---LA- 3x4- . - - - .- -- - - - - - - - - " - - -- . - -- -------A- -
and Electroless Ni. E = +0.8 V vs. SHE.
Flgure 4.4.la SEM images before (top) and after (bottom) immersion test (E=+0.8V vs. SHE) of (left to right) Carbon Steel, Electroplated Ni, and Electroless Ni samples.
and Electroless Ni, E = +OS4 V vs. SHE,
Figure 4.4.1 b SEM images before (top) and after (bottom) immersion test (E=+0.54V vs. SHE) of (left to right) Carbon Steel, Electroplated Ni, and Electroless Ni samples.
4.4.1 Surface imaaes before and aRer test: Carbon Steel. EleJctm~latd Ni, . - - * .
_1_1___ - - A _ _ _ - .- _ ---L =_ - -- - - - -- r - - - - _ _ _ - = _ - ___-s-- _&_ -2 - - - - -
and Electroless Ni, E = 4.01 V vira SHEa
Flgum 4 4 . 1 ~ SEM images before (top) and after (bottom) immersion test (E=-0.01 V vs. SHE) of (Mt to right) Caibon Steel, Electruplated Ni, and Electroless Ni samples.
--- .- = - A - 4.4.1 Surface - - imaaes before and after test: Carbon Stem 1. Electro~latd Ni,
and Electroless NI. E = 4.6 V vs. SHEm
Figum 4.4.1d SEM images before (top) and after (bottom) immersion test (E=-0.6V vs. SHE) of (left to right) Carbon Steel, Electroplated Ni, and Electroless Ni samples.
Figure 4.4.1 e SEM images before (top) and after (bottom) immersion test (E=-O.9V vs. SHE) of (left to right) Carbon Steel, Electroplated Ni, and Electroless Ni samples.
4.5 Weiaht Loss Results.
Table 4.5a Weight loss measurements for E=+0.8 V vs. SHE at 70°C in 8M KOH, H2 flow, time = 7 days.
Average Initial Average Final Material
Weight, g Weight, g
Carbon Steel 3.4964 3.4696
Loss (9) or Gain
(m @dm 'lday)
Electroless Ni 3.8468 3,8478 +0.0010 7.0 a
Table 4.5b Weight loss measurements for E1t0.54 V vs. SHE at 70°C in 8M KOH, Hp flow, time = 7 days.
Material
Average Weight
Loss ( 0 ) or Gain
(+)I g -0.0272
+0,0051
+0.0013
Corrosion Rate,
mdd
(mg/dm2/day)
Average Initial
Weight, g
8.7545
9.201 1
9.3939
Carbon Steel
Electroplated Ni
Electroless Ni
Table 4 . 5 ~ Weight loss measurements for E=-0.01 V vs. SHE at 70°C in 8M KOH, Hz flow, time = 7 days.
Average Final
Weight, g
8.781 7
9.1 960
9.3926
Average Initial
Weight, g
12.8358
ai Oxide formation is assumed, Ni02 and Ni304, respectively.
Average Final
Weight, g
1 2.8340
Electroless Ni 1 3.4329
Average Weight
Loss (-) or Gain
(+Il 9 -0.001 8
0.2 13.4330
Corrosion Rate,
mdd
(mg/dm2/day)
3.2
+0.0001
Table 4.5d Weight loss rneasurements for E=-0.6 V vs. SHE at 70°C in 8M KOH, Hz flow, time = 7 days.
Material
Carbon Steel
Electroplated Ni
Table 4.50 Weight loss rneasurements for E=-0.9 V vs. SHE at 70°C in 8M KOH,
Average Initial
Weight, g
12.8465
Average Final
Weight, g
1 2.8438
Electroless Ni
H2f!ow. time = 7days.
1 3.01 45
Average Weight
Loss ( 0 ) or Gain
(+)# 9 -0.0027
1 3.5676
1 Electroless Ni 1 13.3937
Corrosion Rate,
mdd
(mg/dm2/day)
4.7
13.01 42
Material
Carbon Steel
Electroplated Ni
1 3.5679
Average Initial
Weight, g
12.81 51
1 3.1 522
-0.0003
Average Final
Weight, g
0.5
+0.0003 0.5
Average Weight
Loss ( 0 ) or Gain
(+), 9
Corrosion Rate,
mdd
(rng/dm2/day)
--- - - 4.5.1 MDD Corrosion Rate Calculations for Carbon Steel Coumns. E=+0.8V
vs. SHE in 8M KOH. at 70°C.
Table 4.5.1 Weight loss results used for
the calculations of the MDD corrosion rate
1 rime, 1 Mkl~n, 1 M m , 9 1 W, Q 1
Corrosion Rate = ( K x W ( A x T x D)
weeks 1
K = 3.45~1 oW3 ( inches per year) W = mass loss in g, A = area in cm2, T = time of exposure in hours, D = density in g/cm3, 7.86 for carbon steel
9 3.4964
Mdd = 0.696 x density x mpy
For Carbon Steel:
3.4696
CR1 = 35.1 9 (rnpy) or 192.50 mdd
CR2 = 83.76 (mpy) or 458.21 mdd
CR3 = 542.36 (mpy) or 2967.01 mdd
0.0268
C h = 1,100.22 (mpy) or 601 8.81 mdd
Corrosion Rate vs. Ti me
O 1 2 3 4 5 Time, weeks
Figure 4.5.1 Paralinear corrosion rate law derived for Carbon Steel
samples (E=+0.8V vs. SHE in 8M KOH at 70°C), based on four instantaneous
corrosion rates obtained h m 4 immersion tests with different times of exposure
(Ti=l week, T2=2 weeks, T3=3 weeks, and T4=4 weeks).
S. DISCUSSION - - - -- - - - - - - - -
5.1 INITIAL SURFACE ANALYSIS OF THE COUPONS
As introduced in the previous section, 'before tesr SEM images of the
three materials were collected (Figures 4.4.1 a, 4.4.1 b, 4.4.1 c, 4.4.1 d, and 4.4.1 e,
upper three images in each series).
High magnification of the sample surface (1KV) made it possible to see
the major differences in the surface morphology of the three materials.
As shown in Figure 5.1.1, the Carbon Steel sample seems to have no
distinguishable features after being metallographically polished.
Flgure 5.1 Initial (before test) surface images of the coupons (left to right: Carbon Steel, Electroplated Ni, and Electroless Ni).
Electroplated Nickel, on the other hand, has nodular surface features at
the same magnification. All the electroplated Ni-coated samples provided for
testing had nodules. The nodules would appear to be potential sites for
corrosion.
Unlike the Electroplated Ni samples, al1 initial Electroless Ni SEM images ---L - ---- - -- = - - - - 4 -- d --- - . - - - - - - - - - - - 9- - -
were rather featureless, presenting smooth surfaces, and had no indications of
nodules on the surface.
5.1.1 Scannina Electron Mlcrosco~v/X-Rav Analvsis
5.1 .la Commercial 1020 Carbon Steel
Prior to conducting electrochemical tests on carbon steel samples, the
microstructure of the steel samples was investigated. On hot rolled sheet that is
finished above the transformation temperature range, the cooling regime
determines the carbide characteristics and, to a lesser extent, the grain size.
Cold rolled sheets are generally annealed unless a hard temper is desired.
Figure 5.1.la demonstrates an SEM image of the longitudinal section of a
Carbon Steel sample that was metallographically polished and then etched with
4% nital. The image can be compared with literature images (Robert F Mehl,
'Microstnicture of Low-Carbon Sheet Steeln), taken at the same magnif ication
scale and similarly etched. The image shown in Figure 5.1.la has a - -
microstructure (approximate grain size and grain shape) indicative of a cold-
worked carbon steel sheet.
Flgum 5.1 .l Carbon Steel, direction of rolling
Given the assumed thermal histoiy, a prediction can be made with regard
to the extent of corrosion swceptibility that one might expect for the material.
Cold rolled low-carbon steels have higher concentrations of point- and linear
defects than hot rolled carbon steels. Additionally, larger iron carbide grain size in
cold rolled carbon steek contributes to more pronounced galvanic corrosion
between Fe and Fe&. Finally, the initial grain shape (affected by rolling) can
explain the final microstructure of the corrosion product, as can be seen on the
after test SEM images for Carbon Steel in Figures 4.4.1 b, 4.4.1 c, and 4.4.1 d.
5.1 .l b Electroless Ni -- L - - - - - -- -
K-RAY: O - 20 keU L i vo: 100s Prtsèt : 100's Remai n i ng: Q s
Figure 5.1.1 b X-Ray analysis of the Electroless Ni-plated sample surface
composition.
The X-Ray analysis of the Electroless Ni surface composition presented in
Figure 5.1 .l b allows an estimate of the atomic percentage of phosphorous in
nickel (based on the peaks' relative areas compared to a known standard). The
calculated phosphorous content of 4 0 % P in Ni should make it a material of
good corrosion resistance to concentrated alkaline media (*? The after test SEM - - - .-- -
images of Electroless Ni-plated samples (Figures 4.4.1 a through 4.4.1 8) indeed
indicate little change in surface microstructure for these samples.
5.2. Electrochernical Analvsis
5.2.1 Commercial Carbon Steel
VS. SHE
4.54 v vs. Sm
-0.01 V VS. SHE
. -- .:
-0.6 V VS. SHE
-0.9 V VS. SHE
Figure 5.2.1 Five potentiostatic cunres for Carbon Steel and the corresponding
after test SEM images.
StaRing the series of the fiue potentials is the potentiostatic curve for
Carbon Steel at +0.8V vs. SHE. Initially, there is a very sharp increase of current
density, followed by a rapid drop (within first 5 hours of the test). This portion of
the curve is indicative of quick iron passivation, whereby a protective iron oxide
layer is fomed and further corrosion is slow due to the protective layer. An oxide
layer can be seen on the specimen surface.
Another distinctive feature of the potentiostatic curve of Carbon Steel at
E=+0.8V vs. SHE is the relatively high current density after the surface was
passivated (about 100mA). This current density can be explained by the fact that
the above-mentioned potential is above the oxygen evolution line on the EpH
diagram, and therefore gas (O2) formation is expected. Indeed, th roughout the
test gas bubbling was observed, especially vigorous in the within the fint 5
hours, before the surface passivated. Consequently, the high residual current
density after passivation should be attributed to oxygen production, rather than
metal dissolution.
Towards the end of the testing period, as can be seen [rom the graph, the
current density beg ins to increase. This suggests slight srirface depassivation
(th8 protective layer is cracking, gradually exposing the base of the metal to the
corrosive media). This is also manifested in the SEM image. The final corrosion
product (as seen on the corresponding SEM image) indeed seems to have
cracks which may have contributed to the material's tendency to depassivation.
The corrosion rate for carbon steel at E=+0.8V vs. SHE is quite high, 188.8 mdd
over a 7-day period (Section 4.5). Therefore, any induced potential at +0.8V vs.
SHE caused by stray current would cause corrosion of carbon steel.
The second potentiostatic cuwe for Carbon Steel (E=+0.54V vs. SHE)
shows a very small initial current density (below 5 mNcm2). The Pourbaix
diagram seems to provide a clear explanation for this behaviour: the potential is
below the oxygen formation region, thus the observed current should now be
indicative of the true metal dissolution current. The absence of gas formation
was - ako - - - obsenred experimentally. The depassivation A trend is still present, as in
the previous case, and is also confirmed by the porous nature of the resulting
surface product (see the corresponding SEM image). The corrosion rate is
considerably high (64.5 mdd, Section 4.5). Thus, corrosion induced by stray
current at E=+0.54V vs. SHE would be severe enough to cause problems for
structural materials made of carbon steel.
The behaviour of the carbon steel sample at E = -0.01V vs. SHE shows an
order of magnitude less corrosion current density than at E=+0.54V vs. SHE. The
average value varied between O and 5x10" Ncm2. The magnitude of the current
is very low, as in the previous case, due to the absence of gas formation (the
potential is located in the middle of the "no gasn gap fomed by the lines a and b,
oxygen and hydrogen production lines, respectively. The overall corrosion
activity of carbon steel at this potential can be explained by the
thermodynamically predicted presence of the HFe02- species in the
corresponding region of the E-pH diagram. It is surprising, however, that the
values of current density are so low, since the Pouibaix diagram would predict
that HFe02 -species would be stable.
Also, at this potential carbon steel begins to show some cathodic
behaviour, i.e. negative values of current. The cathodic current spikes are most
likely caused by the local surface chemistry that makes the surface either
cathodic or anodic, depending on the corrosion product formed at a given time.
The film observed on the sample surface after the 7-day immersion at
-0.01V vs. SHE was very thin, almost invisible, unlike the porous films fomed on
the carbon steel samples tested at the two higher potentials. This indicates much
lower corrosion activity of carbon steel at this potential, which is also confirmed
by the low magnitude of the current (one order of magnitude lower than the
corrosion current density recorded at +0.54V vs. SHE). The corrosion rate is
considerably lower at this potential (3.2 mdd). Therefore, stray current induced
corrosion at E=-O.01V vs. SHE would not cause significant problems for
structural components.
The potentiostatic cuwe corresponding to the potential E=-0.6V vs. SHE
initially has about the same order of magnitude as the previous curve, however,
after a certain "incubation" period (in this case after the first 3-4 days) the carbon
steel sample begins to have short excursions to much higher current densities,
presumably due to breakdown of passivity leading to the formation of HFe02-
species, as suggested in the literature (12). Although some spikes of the
corrosion current density reach values, sirnilar to those at E=+0.54V vs. SHE, the
weight loss of the carbon steel sample at E=-0.6V vs. SHE is quite small, due to
the short lifetime of these higher current densities.
The corrosion product film closely resembles that observed on the carbon
steel sample tested at E=-O.01V vs. SHE in terms of the thickness and
appearance, which seems to be in line with the fact that both potentials lie within
the same region of the corresponding €pH diagram (in both cases HFe02 - is
the thermodynamically stable species).
. - - , At E = - 0.6V vs. SHE the current of the carbon steel sample changes from
- -
anodic to cathodic more frequently and to a greater degree than the current
observed at E = - 0.01V vs. SHE. At this potential there are still short duration
excursions of anodic current on the carbon steel surface. The SEM shows the
surface to be lightly attacked at inclusion sites. The corrosion rate is 4.7 mdd
(Section 4.5). Despite the occasional loss of passivity the corrosion induced by
stray current would not pose serious problem for structural members.
The carbon steel sample tested at the most negative potential, €=-OS V
VS. SHE in this study shows predominantly cathodic behaviour with excursions of
current density to values approaching -0.8 L4/crn2, accompanied by vigorous
bubbling during the test monitoring.
Frorn the Pourbaix diagram, -0.9 V vs. SHE is situated virtually on the line
of hydrogen production, given the pH of 8M KOH being equal to 14.3.
Consequently, the current spikes associated with the cathodic behaviour of the
electrode are obseived with the liberation of hydrogen, and explains the high
magnitude of the current density (-0.8 A/cm2). For the sake of comparison, a
similar current density, however, of the opposite sign was initially produced
during the first test (E=+0.8V vs. SHE), where oxygen was formed.
The SEM image of the surface after the test shows a nodular surface,
unlike the previous tests. This suggests that localized corrosion ocairred on the
surface of carbon steel at this potential. This is indeed supported by the weight
loss data (corrosion rate is 1.8 mdd based on a 7-day period, Section 4.5).
Considering the low weight loss, the corrosion induced by stray current at this .- - - A - - - -- - - -
potential would not cause significant problems for structural parts.
Figure 5.2.2 Five potentiostatic curves and the corresponding after test SEM
images for Electroplated Ni.
The series of five potentiostatic curves for Electroplated Ni presented in
Figure 5.2.2 starts with the data obtained at E=+0.8V vs. SHE. Since the
potential is above the oxygen formation line, higher current densities are
expected ÿust as in the case with the carbon steel sample data at the same
potential). Indeed, the magnitude of the current density is high (4.65A/cm2) and
almost matches that produced by the carbon steel samples tested at the same
potential. In cornparison with the cuwe for the carbon steel samples, the
Electroplated Ni cunre seems to indicate that the time needed for passivation is
longer (the magnitude of the current density decreases slowly throughout the -- --- - - -
test, dropping to -50 mAtcm2, whereas the carbon steel sample's current density
decreases after the first few hours of the test to a value of -1 O0 mA that is then
maintained for the rest of the test). The average weight gain (0.08% of the initial
weight, Section 4.5) was observed for the Electroplated Ni samples tested at
E=+0.8V vs. SHE). The weight gain may be attributed to the oxide film formed on
the surface. The corrosion rate was found to be 21.1 mdd. The current decrease
is attributed to the loss of activity for O2 evolution. The loss of activity may be due
to the surface passivation.
The SEM work carried out on the after test samples of Electroplated Ni
shows very little changes of the surface appearance compared with the before
test SEM images of the same material. The intergranular nodes seem to be
slightly more defined on the after test SEM images, though the weight loss
measurements do not support any noticeable losses of the material.
The second curve shown in Figure 5.2.2 corresponds to E=+0.54 V vs.
SHE. The cuwe follows the same trend the carbon steel counterpart did, Le. the
magnitude of current density reduces to values of 2-7 rn~/cm*. In the absence of
gas formation it is appropriate to assume that the produced current corresponds
to th8 current of metal dissolution. Further analysis of the cuwe shows that upon
surface passivation (after the first day) the current density did not undergo any
noticeable changes and stayed around the value of -2 mA/cm2 throughout the
test, indicating that the protective oxide film is also favourably stable at this
potential.
The SEM images of the after test surface show no significant changes,
similar to those for the specimens tested at E=+0.8V vs. SHE. Given that on the
corresponding EpH diagram for Ni the potential of +0.54V vs. SHE lies in a zone
where a protective layer of Ni304 is formed, high corrosion activity would not be
expected on the part of Ni. This is indeed supported by the weight loss data for
the Electroplated Ni samples tested at E=+0.54V vs. SHE, where there was a
gain of 0.06% of the initial weight, corrosion rate is equal to 12.1 mdd (Section
4.5). It is important to realize that although th8m is no oxygen evolution in this
regime (E=+0.54V vs. SHE), the anodic current shown for this potential in Figure
5.2.2 is not due to nickel corrosion only; there may be oxidation of impurities in
the electrolyte which offsets the current that would otherwise correspond to nickel
corrosion.
The third potentiostatic curve given in Figure 5.2.2 deserves special
consideration. The potential under study is E=-0.01 V vs. SHE and from the
corresponding E-pH diagram, it is situated in the middle of a region, where the
soluble species HNiOi is stable. Indeed, nickel corrosion for pHz14.3 would be
expected in the region of the diagram covering E=+0.3V to E=-0.7V vs. SHE.
The potentiostatic curves obtained at this potential for several samples do
not reveal any indication of a significant corrosion current. In fact, the magnitude
of the observed current density is considerably lower (values less than 1 pA/cm2)
than that recorded at E=+0.54V vs. SHE where a protective film was formed. The
lack of corrosion of Ni at E=-O.01V vs. SHE is also confirmed by the SEM post-
test work as well as in the lack of weight loss. The SEM image of the after test
Electroplated Ni surface does not seern to have any signs of active Ni dissolution - -- L .--- -
predicted by the corresponding Pourbaix diagram, nor does the determined
weight loss (only 0.005% of the initial weight, corrosion rate is 1.2 mdd) confirm
the validity of the thermodynamic predictions at this potential.
Since the exposure time was limited (7 days), it may be assumed that the
low corrosion current is due to kinetic restrictions. With this in minci the test was
repeated at the same potential for an extended time of 1 month (see the
potentiostatic cuwe in Figure 4.3.9). Despite the longer time of exposure, the
potentiostatic results obtained for the 7-day time frame were supported by the
potentiostatic data from the time-extended test for Electroplated Ni, Le. the
current magnitude was the same while the weight fosses were not appreciable
(0.008% of the initial weight, corrosion rate is 1.9 mdd).
The above observation may imply that the thermodynamic data made for
Ni in this region may be incomplete and that there is a solid phase species for Ni
which is stable.
The fourth potentiostatic curve (€=-0.6V vs. SHE) corresponds to another
potential located within the "activen region for Ni. Nevertheless, the current
density recorded for the Electroplated Ni samples tested at this potential is very
small, similar to the previous case. Of interest are the current spikes observed on
the cuive. Their magnitude (loo3 A/cm2) is higher than that of the spikes in the
previous case (only Io4 Ncm2) despite the fact that both potentials are in the
same region in ternis of corrosion activity. In both cases the spikes follow the
same mixed (anodidcathodic) pattern, howevet, at E=-0.6V vs. SHE hydrogen
formation line is very close and therefore if the local (near-surface) pH becomes 2 - - - -
slightly lower than 14.3, hydrogen could be formed, as the potential would now
intercept the hydrogen line thus producing higher currents due to gas liberation.
This hypothesis is supported since some bubbles were seen during the test at
this potential. The weight loss analysis dM not provide any significant weight loss
(only 0.002% of the initial weight, corrosion rate is 0.5 mdd, Section 4.5). The
after test SEM image of the surface (Figure 5.2.2) does not provide any signs of
corrosion, given the magnification scale. As in the previous case, neither the
weight loss analysis nor the SEM image cornparison yielded any indication that
Electroplated Ni was corrosion-active in the Pourbaix predicted reg ion of activity
for nickel.
The fifth potentiostatic curve for Electroplated Ni (E=-0.9V vs. SHE) shows
predominantly cathodic currents. Such behaviour was expected, considering that
the potential is just below the line of hydrogen formation and, therefore, the main
constituent of the cathodic behaviour will be formation of hydrogen. The latter
was easily observed throughout each test carried out at this potential. The
corresponding EpH diagram also predicts corrosion immunity for Ni at this
potential. Indeed, the weight loss results confirmed the thermodynamic
predictions for this case (there was an average weight gain of 0.005% of the
initial weight. corrosion rate is 1.1 mdd). The SEM studies show no visible signs
of corrosion on the surface of Electroplated Ni.
From the weight loss and SEM imaging data, Electroplated Ni does not
appear to be attacked at any of the potentials studied.
O 4.0002 -0.0004 4.-
4- - 4.001 2 4.0014 4.0018
O 2 4 6 a Time, dry8
Figure 5.2.3 Five potentiostatic curves and the corresponding post-test SEM
images for Electroless Ni.
Overall, the potentiostatic behaviour of the Electroless Ni samples (Figure
5.2.3) closely resembled that of the Electroplated Ni samples (Figure 5.2.2) at
each given potential not only in terms of the magnitudes of the cuvent density,
but also in ternis of the trends of passivation and depassivation. The post-test
SEM images of the Electroless Ni-plated samples also show no visible evidence
of corrosion, as in the case of the Electroplated Ni SEM images. F inally, the
weight loss measurernents made for the Electroplated Ni samples yielded
average - -- -- values of weight loss nearly matching those calculated for the
Electroplated Ni ~amples for each potential tested (se8 Appendix). All these
observations suggest that there is very little difference between the
electrochemical behaviour of Electroplated Ni and that of Electroless Ni.
However a few minor differences as well as important similar trends not
mentioned above need to be pointed out.
The first potentiostatic cuwe (E=+0.8V vs. SHE) shows higher values of
current produced at the initial stage, compared to the same cuwe for the
Electroplated Ni. This happens at a potential where oxygen is produced and the
higher current may mean a higher catalytic activity of the Electroless Ni-plated
surface, since the surface areas for both Electroless and Electroplated Ni were
the same. The higher current is not maintained and declines as did the
Electroplated Ni after a few days.
The second curve (E=+0.54V vs. SHE) shows a case where no gas is
fonned. The magnitude of the current density is higher for Electroless Ni than for
Electroplated Ni at this potential. However, the weight loss results indicate that
Electroless Ni gained less weight (0.01%) than Electroplated Ni did (0.06%). The
corresponding corrosion rates are 3.1 mdd and 12.1 rndd, respectively. This
suggests that there may be a link between the amount of product formed on the
surface of Ni (possibly oxides) and the catalytic acüvity of the resulting surface.
The higher the weight gain due to the surface product, the lower the catalytic
activity of the resulting surface, and vice versa.
The third potentiostatic curve (E=-û.01 V vs. SHE) represents a case
where Ni is expected to be active (HNi02- is stable, as suggested in the
literature), however, the corrosion current density is negligible (below 1 O-' ~ l c r n ~ )
and is mainly cathodic. This is consistent with the lack of corrosion for the
Electroplated Ni samples and suggests that the Pourbaix diagram for Ni-H20
may need to be revised. There was a resulting average weight gain (0.001% of
the initial weight, corrosion rate is 0.2 mdd, Section 4.5). The Pourbaix diagram
does not allow to justify the weight gain in this case since a solid species would
have to be stable, instead of the
H NiO2-.
Same trend is found for the potential E=-0.6 V vs. SHE. The cathodic
current density magnitude is higher than in the previous case (now in the order of
lo4 ~lcrn*) due to a higher driving force (the potential is more negative). As in
the case of Electroplated Ni, occasional Hz bubblinç was observed. The
calculated corrosion rate is 0.5 mdd.
The potentiostatic curve at -0.9 V vs. SHE shows the same trends
observed in the case of Electroplated Ni at the same potential: the current cycles
over a period of several hours to more and less active hydrogen evolution
regimes (the potential is close to the hydrogen line). An average weight gain
(0.004% of the initial weight) was found. The corrosion rate was found to be 0.9
mdd.
In summary, weight loss and SEM imaging data indicate that Electroless
Ni does not appear to be attacked at any of the potentials studied.
5.2.4 Com~erison between Fe-based and NC~lated cowons
While the differences in corrosion activity between the Electroplated Ni
and Electroless Ni plated samples are subtle, the differences between the
corrosion test results for the Ni-plated samples and the Carbon Steel samples
are evident, regardless of which one of the five testing potentials is considered.
All three methods of comparison (the potentiostatic curves, before and after SEM
images, and the weight loss measurements) are indicative of higher corrosion
resistance in the case of the Ni-plated samples. The difference becomes even
more apparent, when the long-term results are compared (Figures 4.3.9 and
4.5.1). While the two curves corresponding to the Ni-plated samples (Figure
4.3.9) are near '0' A throughout the testing period (30 days), the curve for Carbon
Steel (Figure 4.5.1) shows incteasing activity, following the paralinear trend given
the same time frame.
As far as the cornparison between the corrosion performances of the
Electroplated Ni and the Electroless Ni samples, one has to consider each tested
region (in terms of the potential) indhhdually. since the two materials may behave
differently depending on the potential. A good example is the region with E=-0.6
V vs. SHE (the fourth potential in the series). The Electroless Ni samples acted
cathodically throughout the test (Figure 5.2.3), while the Electroplated Ni samples -&&-A--- -a - J - -- - -
showed some mixed patterns (both anodic and cathodic currents were obsû~ed,
Figure 5.2.2). At this point the explanation for these differences is not cfear and,
perhaps, necessitates further study.
5.3 Effect of Turbulence
This section presents the discussion of the effects of turbulence on the
corrosion process. A turbulent flow was induced by a stirrer (30 rpm) to examine
effects on the corrosion activity of Carbon Steel and Ni-plated coupons at
E=+0.8V vs. SHE, at 70°C in 8M KOH. The addition of flow was regarded only as
an initial attempt to examine whether tuibulence has any significant bearing on
the corrosion activity of the two materials (Fe-based and Ni-based) at the given
conditions.
Figure 4.3.7 shows two potentiostatic curves (Carbon Steel and
Electroless Ni) obtained from a series of tests where the above-mentioned
turbulent flow was added. Aside from the addition of flow, the rest of the
conditions were kept the same as for the tests at E=+0.8V vs. SHE (Figure
4.3.1 a).
5.3.1 Catbon Steel
Cornparison of the potentiostatic data obtained for "no flow" and "turbulent
floM conditions for Carbon Steel shows a few differences. Firstly, the initial spike
of current density in the turbulent case is significantly higher (reaching values of
1Ncm2, compared to 0.7A/cm2 in the absence of turbulence). The higher
magnitude of the current density indicates that the critical current for passivation
is higher under turbulent conditions. Essentially, the flow facilitates the removal
of the protective film from the surface, exposing it to the corrosive medium.
Secondly, after the surface is film-protected, the current magnitudes for
Carbon Steel seem to be almost identical, with and without flow. This means that
once the surface is film-protected, the addition of flow makes practically no
difference, Le. the system is not controlled by flow. The small depassivation
trend near the end of the test is seen on both curves, with and without flow,
suggesting that the addition of turbulent flow has little bearing on the process
kinetics at these conditions.
Finally, the cornparison of the weight loss results (in both cases there was
the same loss of -0.8% of the initial weight after one week) showed no indication
of the turbulent flow influence.
5.3.2 Electroless Ni
There are a few differences in the corrosion behaviour of Electroless Ni
with and without flow. Figures 4.3.1 and 4.3.7a show the potentiostatic data for
Electroless Ni at E=+0.8V vs. SHE with and without flow, respectively.
The effect of turbulent flow is to decrease the time necessary for the
Electroless Ni surface to passivate, compared with the cuive obtained in the
absence of flow. The magnitude of the initial current density is only 0.3A/cm2 - -
(with turbulent flow), while in the absence of fluid How the maximum anodic
current density reaches values of 1Ahm2. The time required for the current
density to reach the minimum value (for both cases around O.l~lcm*) once the
surface is passivated is much less when flow is present (1 day vs. 5 days, in the
absence of turbulent flow). Less current indicates that passivation is more easily
achieved under flow conditions. This seems to be in agreement with the fact that
Ni-oxide films are veiy stable, unlike Fe-oxides, and when flow is added the influx
of oxygen is increased, which in tum facilitates faster surface passivation.
The comparison of the weight loss results, however, did not show any
significant difference for the flow and no flow tests. Thus the addition of turbulent
flow does not appear to have any significant influence on the corrosion
performance of Electroless Ni, since the weight losses were negligible in both
cases (see Appendix). There were no induced flow tests performed on
Electroplated Ni.
5.4 Effect of Hvdroaen Flow.
All tests were carried out with hydrogen flowing frorn an external source
through each testing cell, in order to simulate industrial conditions.
Some tests were mducted specifically to detemine the effect of this hydrogen
gas flow. Hydrogen is a reducing agent and might affect the ability of the metals
to fom their oxides. With this in mind Carbon Steel was tested at E=+O.S4V vs.
--A- -- - - SHE - -- . (the region where no gas is produced - - intemally). Figure 4.3.6a shows a
potentiostatic cuwe for Carbon Steel obtained in the absence of hydrogen at
E=+0.54V vs. SHE, whereas Figure 4.3.6b shows a potentiostatic curve for
Carbon Steel obtained under the same conditions but with hydrogen being added
(at a flow rate of -30 mumin) throughout the test.
The potential E=+0.54V vs. SHE is within the zone where water is
electrolytically stable and thus no gas is formed. The recorded current density
should correspond to the true metal dissolution current density. A comparison of
the Carbon Steel curves in the two figures shows that they follow the same
trends whether or not H2 is present in the corrosive medium. The magnitude of
the corrosion current density slowly increases forrn near-zero values to just over
0.005~fcm~ in both cases. The resulting weight losses are nearly identical
(4.3% loss of the total weight for both cases). These observations show that
the addition of hydrogen does not have any appreciable effects on the corrosion
behaviour of Carbon Steel at these conditions.
5.5 Effect of Elevated Temmratuie.
As shown in Figure 4.3.8, the effect of a temperature increase by 1 O0 was
examined for Carbon Steel at the potential where Carbon Steel proved to be
most active. An average potentiostatic cuwe was obtained at an elevated
temperature (+80°C), al1 other conditions kept the same (E=+0.8V vs. SHE, tirne
7 days in 8M KOH). Figures 4.3.8a and 4.3.8b show a comparison of the
potentiostatic cuwes for Carbon Steel obtained at 70°C, and 80°C, respectively. -- &L. L- - - - 2 - - - -
The cornparison yields at least three features that need to be discussed.
The initial spike, corresponding to the formation of protective film has a
slightly lower value (-0.64 IO.O1 Ncrn2) at 80°C than at 70°C (-0.68I0.01
Akrn2). This means that at the above conditions higher temperatures cause
faster Carbon Steel passivation, producing lower values of current.
Secondly, the magnitude of passivation current density after the i crlUmI is
considerably higher at 80°C, nearly twice the value of current density recorded at
70°C. This shows that the 10-degree temperature increase has a striking effect
on the rate of oxygen evolution (E=+0.8V vs. SHE is above the line of oxygen
formation).
The third feature of the potentiostatic curve at obtained at 80°C is a
noticeable depassivation trend towards the 5m day of immersion. The magnitude
of current increases even more, reaching values almost three times the current
obsewed at 70°C. The weight loss data also confinns this, since a 10% increase
in weight loss was found for Carbon Steel at 80°C when compared to the weight
loss &ta for Carbon Steel at 70°C (see appendix).
5.6 Corrosion Rate Law for Carban Steel, E=+0.8V vs. SHE at 70°C.
In order to be able to predict the effects of corrosion for Carbon Steel at
the potential where corrosion in this study was most severe (E=+O88V vs. SHE)
an attempt was made to derive a corrosion rate law (see Section 4.4.1, Results).
The results obtained for a series of experiments conducted at E=+0.8V vs.
SHE using different tirne frames (1, 2, 3, and 4 weeks) yielded a paralinear trend
(Figure 4-51). The corrosion was extremely severe: there was a 25% weight loss
for the sample immersed for 4 weeks at the applied potential of +0.8V vs. SHE.
The calculated rate of corrosion for the fourth sample was 1.1 inches per year,
which is an extremely high value. Most of the current flowing was due to oxygen
evolution. i.e. the flow of current was higher that could account for the weight
loss. Thus, given the vigorous oxygen evolution that was observed throughout
the testing period at this potential, the abnormally high magnitude of corrosion
rate may be explained by very low adherence of the protective film on the surface
of carbon steel. Upon examination of the post-test sample, the corrosion product
was indeed found to be very loosely adherent and appeared to be an array of
many layen, indicating that the protective film had a very short lifetime.
=- ---- - - - - 8.7 Validitv -- of the Pourbaix Diaciram as a Means of Predlctinci a Metal's
Corrosion Activity
As mentioned in the Background and Literature Review section, potential-
pH diagrams can be often times used to predict a metal's corrosion activity. It is
therefore important to discuss the validity of the Fe-H20 and Ni-H20 Pourbaix
diagrams with regards to their ability to predict the corrosion test results obtained
for Carbon Steel, Electroplated Ni, and Electroless Ni at 70°C for the potential
ranging from +OB V to -0.9 V vs. SHE.
$.7A Limitations of the Fe-HsO Pourbaix Diariram
A major expected limitation of the constructed Fe-H20 diagram is that it's
an idealized approach where it is assumed that the corrosion activity of carbon
steel would be the same as that of pure iron. However, as mentioned in Section
2.6, carbon steels may contain up to 1 .O% C as well as up to 0.6% Mn, 0.04% P,
and 0.05% S, al1 which could cause the corrosion activity of carbon steel to
deviate from that predicted for pure iron. In addition, the idealized Fe-H20
diagram obviously does not take into account any treatment methods used when
carbon steel is produced.
Nevertheless, as discussed in detail (Section 5.2.1), the corrosion activity
of carbon steel can be stil well rationalized in terms of the corresponding Fe-H20
diagram for al1 five potentials (E = +0.8V, E = +Oa54V, E = -O.OlV, E = -0.6V, and
E = -0.W vs. SHE). Therefore, the Fe-H20 diagram adjusted to 70°C does
---- -- - z correlate well with the experimental results, and thus the above-mentioned
limitations are not critical.
Agreement with the experirnental results for the passivity reg ions
(E=+0.8V and E=+0.54V vs. SHE, Sections 5.2.2 and 5.2.3) was found for the
constructed Ni-H20 diagram, but discrepancies were seen with the experimental
data for the regions where Ni is supposed to be active, i.e. E = -0.01V and E = - 0.6V vs. SHE). Even the long-terni (1 month) tests conducted on both
Electroplated Ni and Electroless Ni in the active region (E = -0.01V vs. SHE,
Figure 4.3.9) show extremely low values of current density (less than 1 @/cm2)
throughout the testing period.
As mentioned previously, the E-pH diagram can only indicate whether
corrosion is possible and it can provide guidance as to where the corrosion
driving force is high. No information about the rate itself can be inferred from the
diagram. Therefore, it may be that the corrosion is kinetically very slow in the
active zone,
Alternatively, there have been recent results in the literature proposing a
revised €-pH diagrarn for Ni-H20 These results suggest that many ions
originally written as HNi0; should be written as Ni(OH)x -". In addition, it was
reported that neither Ni304 nor Ni203 or their hydrated forms are the correct
compounds. Both electrochemical and x-ray diffraction data indicate that these
-- - species % - have to be revised in ternis of the solids NiO, (a$), and (y$) NiOOH.
The revised diagram for 25°C is shown in Figure 5.7.2.
Figure 5.7.2 Revised Ni-HzO at 25°C €-pH diagram. All ionic species are at
activity 1 O*, modified from @?
At present, the compensation for temperature is difficult to make in order
to reconstruct the revised diagram for 70°C, since the current database is limited
to room temperatures with respect to hydroxy species such as Ni(OH)3 :
Nonetheless, the revised Ni-H20 diagram would probably explain the low current
densities obsewed at E= - 0.01 V and E= 4.60V vs. SHE, since at an elevated
temperature (70°C) the Ni(OH)2, dolid region of stability would shift to the right
- =- . - bottom corner, A thus reaching the two potentials in question. This in turn, would
explain the low current densities since the stable species would now be insoluble
and thus passive Ni(OH)?, m, rather than soluble HNiOi.
8. CONCLUSIONS
(1) The experimental set up for bench scale simulation of stray current
corrosion under various conditions is possible and was successfully
demonstrated.
(11) Three different materials (Carbon Steel, Electroplated Ni, and
Electroless Ni) were evaluated with respect to corrosion under different
applied potentials (E = +0.8V, +0.54V, -0.01V, -0.6V, and 4 . 9 V vs.
SHE at 70°C in 8M KOH):
the corrosion of Carbon Steel was most active at E = t0.8V vs.
SHE, while high cathodic current density was observed at E = - 0.9V vs. SHE, but had low weight loss,
both Electroplated Ni and Electroless Ni showed very low
corrosion activities throughout the +O.BV to 4 . 9 V vs. SHE
range of potentials, as shown by weight loss data and the
potentiostatic current and SEM analysis,
(111) Thermodynamic predictions of stray current corrosion were made
using EpH diagrams and the validity of the latter was examined:
there is a good correlation between the thermodynamic
predictions and the experimental results for Carbon Steel for
ail five potentials;
both Electroplated Ni and Electroless Ni showed extremely
low values of corrosion in the regions of the Pourbaix diagram
for Ni where active species should be stable, Le. E=-O.01V
and -0.6V vs. SHE, which questions the validity of the
original Pourbaix diagrarn for Ni- H20.
(IV) Effects of elevated temperature, turbulent flow addition, hydrogen flow
on the comion of the materials under study were examined:
at an elevated temperature (80°C), the critical passivation
cuvent density was lower for Carbon Steel at E=+0.8V vs.
SHE; also, the oxygen evolution current density doubled as
compared to the data at 70°C.
for Carbon Steel, the critical current density for passivation is
higher under flow conditions at E= +0.8V vs. SHE,
for Electroplated Ni and Electroless Ni the critical current
density for passivation is lower under flow conditions at
E=+0.8V vs. SHE,
there was no effect of turbulence on the material weight loss
for ail three materials at E=+0.8V vs. SHE,
addition of hydrogen did not have any appreciable effects on
the conosion activity of Caibon Steel at E = +0.54V vs. SHE.
Corrosion rate was determined and a rate law was derived for
Carbon Steel at E=+0.8V vs. SHE in 8M KOH at 70°C.
The following subjects are suggested for future research:
1) lnvestigate the effect of turbulence on the corrosion of the three
materials under several different (lower) potentials.
2) Construct a revised Ni-H20 EpH diagram for 70°C in agreement with
the new species proposed, when the missing constants (aT and b~
used in the Correspondence Principle for elevated T's) become
availa ble.
3) lnvestigate the passivity breakdown for Electroplated Ni and
Electroless Ni using the scratch technique.
4) Study the electrochemically fomed oxide film on both Electroplated Ni
and Electroless Ni by XPS to identify the oxides.
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APPENDICES
APPENDIX A Conductivitv of KOH
Table Al . Conducbivity (a, cm m4) of KOH at 70°C @?
BI. Therrnodvnamic Data Used in Calculations and Resultinci Values of AG0. Fe-Hg0 Diwram at 70°C,
Tabk Bl. f hermodynamic Data Used in Calculations and Resulting Values of
AG0, Fe-H20 Diagram at 70°C
Data taken from CRC Handbook of Chemistry and Physics, 57& Ed., page D-62.
(a) R.L Cowan and R.W. Staehle, JECS, 1 18,557 (1 971).
82. Thermodvnamic Data Used in Calculations and Resultina Values of
Table BI. Therrnodynamic Data Used in Calculations and Resulting Values of
AG0, Ni-H20 Diagram at 70°C.
Data taken from CRC Handbook of Chemistry and Physics, 57m Ed., page D-64.
- - -- 63. Calculations for Fe-Hn) diaaiam. T=70°Cm al1 ions are at activitv of I O ~ .
Reaction (a)
AG^ = AG;)^^ - AG; )H+ = -1427 - 2 x (-91) = -1245 cal l mol ut 2S°C)
EL =- (-1245 x4* 184) J 1 mol = +0,0270 v vs, s H b e , 2eqlmo1 x96487Cleq
Equation for reaction (a)
E = E ' - 2.303RT 1 log -
2~ [ H + P E , = 0.0270 - 0.06807pH V VS. SHE MT
E , = (0.0270 - 0.0270) - 0.06807 pH = -O.Cl6807 pH V VS. SKE ma=
Reactlon (b)
= E0 - 0.068OîpH
AG; = Z A G ~ ~ AG^^ AG;+ =2(-57.7)-0-O=-115.4 kcallmol
AG' = FE'
---- - - . Reaction - . - (1)
Fe203 + H,O + 2e- = 2HFeOi
AG;^ = A AG; - (AG; ) Fe203 - (AG;*, ) = 2(-90844) - (- 178044) - (-577 16) = 54072 cal 1 mol
.: AEL = -1.172 V vs. SHE,., AEL =-le199 V VS. SHE,oy
E,, = - 1.199 - 0.06807 ~ o ~ [ H F ~ o ; ] = 4 . 7 9 1 V vs. SHETOec
Reactlon (2)
E = E ' - 2.303RT 1 log- = E' - 2.303 x 8.3 l4J ImolK x 343K
8F ' = E0 - O.06807pH [ ~ ' r 8eq 1 mol x 96487C 1 eq
x ~ I o ~ I H * ]
AG;, = AG; ) , + AG; ) ,, - (AG; - AG; ) = Y-274) + 4(-577 16) - (-2420 15) - - 8(-9 1) = -1 1057 cal 1 mol
;. = 4.060 V vs. SM&,
or
EL, = 4 . 0 8 7 V vs. S H E , ,
E, = -0.087 - 0.06807pH
Reaction (3)
AG; = 2 w ; ),@, + (AG; ),, - 3 w ; ) , O , - 2 w ; ),+ =
= 2(-242045) - 57716 - 3(-178044) - 2(-91) = -7432 cal lm01
.: EL = +O. 161 V vs. SHE,, or 0.134 V vs. SHE700,
und E = +O. 134 - 0.06807pH
= -24589 cal l mol
AGW E ; ~ = --= tO.533 V vs. SHE,., or + O S 0 6 V vs. S H E , , . 2F
;. Eu, = 0.302 - O. 102 1pH
AG;, = 3 ( q ),,,; + (AG; )H + - AG; ],fi - 2 W ; ),@ = 3(-90844) + (-9 1) - - (-2420 15) - 2(-577 16) = +a4824 cal / mol
/. EL = -1.839 V US* SHEsOc or - 1.866 V VS* SHE,ooc.
and E,, = -1.253 + O.034OXpH
:. AG;, = O - 45(3 1.21) + 309.6 - 343(0.9675) = -1427 cal / mol
AG;,, for Fe203 :
.: AG;, = -177100 - 4J(2lS) -1 169 - 343(3.6SOS) = -180.489 cal /mol
AG;,, for Fe,O, : 343
c , ~ T = 1112.38 + 1.62 x IO-~T - 0.38 x 1 0 ' ~ - ~ ] d ~ = 298
=12.3&45)+0.8~~10-~k343)~ -(298)']+0.38xLOs['-'1 343 298 =564
:. AG& = -242400 - 45(3S.O) + 564 - M(l.76 15) = -24401 5 cal /mol
From C.M. Criss and J.W. Cobble, J. Phys. Chem, 86,5385 (1 964), Table III on
p. 5389.
At 70°C, by linear interpolation, SOM=-1 .O5 caVmolK
sw-s, - - - i .os - (-5.0) cal C, 1: = = 28.09 -
ln(343 / 298) ln(343 / 298) molK
AG^ = AG;, - ATM, + Cp ]a {AT - 343 ln(3431298)) =
= 0 - 45(0) + 28.09[45 - 343 ln(343/298)]= -91 cal / mol
AG;,, for HFeO;
From Criss and Cobble, for acid oxy anions,
AG; = AGL - ATML + C, 1: {AT - 343 ln(343 / 298)) = -90844 cal / mol
Figure 83. €pH diagram for Fe-H20 at 70°C. Activity of ionic species is
assumed to be 10?
--- -- B4. Calculations for NiSI& diaaram. T=70°C. al1 Ions are a activitv of IO?
AG;, = -56690 cal 1 molK
AS, = 16.75 cal /molK
AG^ = -56690 - 35(16.75) + 1 8.O4(3S) - 343[18.04 ln(343 / 298)] = -573 12 cal l mol
343 343 Ic,&' = ((4.06 + 7 . 0 4 ~ ~O- 'T)~T = 4.06(35) + 3 . 5 2 ~ 10" (343' - 298') = 219.84 298 298
343 343
cPd In T = J(4.06~ -' + 7.04 x 1 0 " ) d ~ = 4.06 ln(343 / 298) + 7.04 x IO-' (35) = 0.6973 290 298
AG& = O
AS;, = 7.12
AG; = O - 337.12) + 2 19.84 - 343(0.6973) = -262 cal / mol
Frorn Criss and Cobble, p. 5389, Table III, for acid oxy anions: a(343)=-13.5, b(343) =1.380
AG^ = AG^ - ATM;, + C, 1: {AT - 343 ln(343 1 298)) = -83676 cal / mol
298 290
AG;, = -5 l3ûû cal 1 mol
AS;, = 9.08 cal / molK 343 343 I c , ~ = l(4.99 + 3 7 . 5 8 ~ ~ o - ~ T +3 .89x106~- ' )d~ 298 298
29% 238
AG, = - 1 12270 cal / mol
ASL = 2 1.5 cal / molK
= -224.55 cal / mol 343 343
I ~ , d l n T = 1 C, - = T(23.49T-1 + 18.60~10" - 3 . 5 5 ~ l ~ ~ ~ - ~ ) d l . = 4.7213 198 298 * 298
AG, = -1 12270 - 35(21.5) - 224.55 - 343(-0.7213) = -1 13007 cal /mol
AG;, = -5 1420 cal l mol
AS, = 12.68 cal lmolK 343 343
IcPdT = 1(16.60+ 2 . 4 4 ~ 1 0 - ~ ~ -3.88x10~T-')dT 298 290
343 I c , ~ T = 16.60(35) + 1.22 x 10-~ (343* - 298') + 3.88 x 106 (343-' - 298-' ) = 298
= -760.5 cal / mol 343 343 dT ! C , ~ I ~ T = 1 Cp - = 1(16.60T1 + 2 . 4 4 ~ 1 0 - ~ - ~ . ~ ~ x I o ~ T - ' ) ~ T = -2.422 298 298 T m
AG^ = -5 1420 - 35(12.68) - 760.5 - 343(-2.422) = -5 18 18 cal 1 mol
AG;, =-170150 callmol
AS;, = 35.66 cal ImolK 343 343 I c , ~ T = 1(30.84+ 1 7 . 0 8 ~ 1 0 " ~ - 5 . 7 2 ~ 1 0 ~ ~ - ~ ) d ~ 298 -%
343 343 I c , ~ l n 7 = 1 C, = 1(30.84T-' + l7 .08~10-~ - 5 . 7 2 ~ 1 0 ~ T - ~ ) d ~ = -2.369 298 298 298
AG; = -170150 - Xj(35.66) - 749.4 - 343(-2.369) = -171359 cal / mol
E = E ' - 2.303RT 1 log- = E' - 0.06807pH 2F [H* P
AG: = AGii + AG;:, - AG" - AG,, = O + (-57.3) - (5 1.7) - O = -5.6 kcalI mol
E = E ' - 2.303RT log 1 2 F 3 = E' + 0.03304 log - 0.099 12pH
'HN~O-2 a H'
AG: = AG;, + AG;,, - AG^^^.,^ - AG;, = -0.26 + 2(-57.3) - (83.7) - O = -3 1.2 kcal l mol
Reactlon (3)
AG" = -RT In K = -2.303RT log ; AG. = -2.303RT(6+ pH) ' H N ~ O - 2 4~'
AG; = AG;, + AG;$ - AG^^^^^ - AG;. -5 1.7 - 57.3 - (83.7) - O
=-25.3 kcallmol
---- - Reaction (4)
AG; = AG^^^ + AG^^^ - AG^^^^, - AG;, = 3(-57.7) - 5 1.3 - (-171.4) - O = -41 .O kcal lm01
Reaction (5)
E = E O - 2.303RT 1 log- = E' -0.06807pH 2F [HtP
AG; AG;,,,, AG;^, AG^^,, - 2 A ~ i . =2(-171.4)-57.3-3(-113.0)-O=
=-61.1 kcallrnol
Reaction (6)
AG; = AG^^^^^ + AG^^^ - AG;,. ou - AG;, = (-1 13.0) - 57.3 - 2(-5 1.8) -O =
= -66.7 kcal l mol
= E' - 0.03304 [3(-6) - pH] = E0 + 0.5947 + 0.03304pH
AG; = AG;,,; +AG;+ - AG, - 2 6 ~ ; ~ ~ = 3(-83.7) +O - (-171.4) - 2(57.3) =
= +34.9 ka1 l mol
Figure 84. E-pH diagram for Ni-H20 at 70°C. Activity of ionic species is
assumed to be IO?
- - C. Etectrolvte C
- .- - - alculations
Cl. Calculation of DH
Activity :
For KOH: son- = f (aKon)
KOH+ K++ OH'
K+ and O H have the same ion size parameter. Thus,
~ K O H = ~ K + ~ O H - = ( ~ H - ) ~ (1 )
K+ and OH- have the same molal concentrations, therefore
~ R O H = fm~+mon- = Y?(~oH-)~ (2)
where a& = molal ionic activity of hydroxide ion
a ~ + = molal ionic activity of potassium ion
a ~ c m = activity of KOH
m& = molality of hydroxide ion assurning cornplete dissociation of KOH
y* = mean molal activity
Rom (1) and (2) we can combine:
(aonJ2 =
01 m. = y&m&
For 8M KOH
8M KOH = 34 ~ t % KOH
At 70°C, y* = 3.37 (from 'Properties of Aqueous Solutions of Electrolytes", CRC
Press, (1992), p. 1323.)
m* = (3.37)(9.18 kglmol) = 30.94
At 25OC, y* = 5.7
----- - - - a ~ ( = (5.7)(9.18 kglmol) = 52.3
--
The dissociation constant of water at 25°C:
At25'C(298K), KHX)=1~14+-l~gK~20=14.0
At 70°C ( W K ) , K H ~ ~ = 1.51 3x1 + -log K ~ 2 0 = 1 2.8
H20 + H+ + OH
Kn20 = amo = an+ son- an+ = Knx> /--
pH = - log a ~ + = -log (KH~o /~on- )
For 8M KOH:
At 2S°C, pH = log (52.3)+ 14 = 15.7
At 70°C, pH = log (30.9) + 12.8 = 14.3
Activity of water:
Osmotic coefficient 0 of KOH at 25°C: 0 = 2.095
However, 0 = - 1000 ln [awy(vmMw)
2.095 = - 1 000 In [aJ((2)(9.000)(18.015))
a, = 0.507
b. E M r a d o Potmtiats
Dl. HdHrrO Retemnce Electrode Potential
Hg0 (s) + H20 + 26 o Hg + 20H, at 25°C
E ~ ~ 0 0 = EoHwm - (RTE) In [a W. / \I aw]
E ~ ~ 0 0 = +0.0976 - (8.3 1 4)(298)/(96487) In [32.59 1 d 0.50q
E = -0.00058 V vs. SHE, at 25OC
mole = volume x molarity
mole = volume , x rnolarity , At the end point moles SM = moles , Therefore, volume std x rnolarity ,M = volume , x molarity
Or Vsmx Mstd = Vxx Mx
Since VSM, Mstd and V, are known, Mx = Vstd x Mstd Nx
The reaction of titration is:
2 KOH + H2SOd + K2S04 + 2H20 (W
In this reaction, the number of moles of KOH is 2x the number of moles of
H2S04, thus moles KOH = 2 x moles sa.
For 8M KOH.
Table 4.1 shows the relation between the molarity, weight % and density (g/mL)
for KOH that was used to prepare the KOH aliquots for dilution.
Molarity (M) can be converted to molality (m), as
rnolality KW = moles KoH /(total mass of solution - mass of KOH)
So for 8M KOH,
Mass of the solution = density x volume = 1.323 g/mL x 1 O00 mL = 1323 g
Mass of KOH is 8 mol x 56.1 1 @mol = 448.88 g
Molarity, MoVL Density, g/mL Weight K
The mass of water is total mass - mass of KOH = 1323 g - 448.88 g = 874.12 g ------y-- - - - - -- ---
= 0.874 kg.
Now we can calculate the molality:
molality = 8 moles KOW0.874 kg water = 9.153 m
The pH is defined as: pH = - log a H+ = - log (KH& a
The dissociation constant of water at 25°C and 70°C
At 25°C (298U), Knzo = 10 -14 - log K "20 = 14
At 70°C (343K), KHm = 1.513~10 -13 * - log K "20 = 12.8
For 8M KOH = 34wt% KOH = 9.1 53 rn KOH
Molal ionic activity of hydroxide ion, a OH' = y* mon'
The mean molal ionic activity, y*, for 8M KOH is [321:
At 25OC, y* - 5.70
At 70°C, y* a 3.37
Using Eq. E l the molar concentration of the post-test KOH was found to be
10.5M. The molality of a 10.5M KOH is 11 .5.
For e 1 1.5 molal KOH, et 70°C, y* - 5.54 (frm 32)
aon- = ymon-
Therefore, am- = (5.54)(11.5 kg/mol) = 63.71
At7O0C(3431<), K n x , = 1 . 5 1 3 ~ 1 ~ 1 3 + - l ~ g K ~ ~ ~ = 1 2 . 8
pH = - log an+ = -log (KH~o IBOH- )
At 70°C, pH = log (63.71) + 12.8 = 14.6
--,.-, . ..- 2 --=. F. Densitv - - - - of - - Electroless - - Nickel -- -
Density of electroless nickel (EN) coatings depends on the interatomic
spacing of the metal lattice which is directly correlated to phosphorus content and
to a small extent CO-deposited impurities. An essentially linear decrease with
increasing phosphorus content is illustrated graphically below.
Effed of phosphorus content on EN depdt density