Galvanic corrrosion.doc

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The effect of protective coatings on galvanic

corrosion for bolted componentsKS. Yeung

FordMotor Company, Ml, USA

AbstractThe increasing demands for better fuel-efficiency have led the automobile

industry to use lightweight metals such as aluminum and magnesium in

conjunction with conventional metals such as steel. Galvanic corrosion is the

unfortunate result of this mixed metals usage. The automobile companies rely

primarily on proving ground testing to evaluate the severity of corrosion. Such a

procedure is costly and time consuming. The objective of this project is to

explore the feasibility of using computer simulation to predict galvanic

corrosion in automobile environments. The BEASY code has been chosen for

this project. There are two parts in the study. The first part is to validate BEASY

and the second part is to perform the bolted component corrosion simulations.This study shows that with as little as 2 mm of protective coatings, the peak

corrosion rate can be reduced by as much as 70%. The BEASY code is a

potentially useful tool to estimate the corrosion rate if an accurate representation

of a current density and electrical potential relationship - the polarization curve -

of the metals of interest can be experimentally determined.

Introduction

Corrosion is the destructive result of chemical reactions between a

metal or metal alloy and its environment and presents some of the most costly

problems to the automobile and other industries. The cost of corrosion in the

United States in 1975 alone was estimated to be $82 billion, or 4.9% of the

United States gross national product [ 11. Because of the recent demands for

better fuel-efficiency, more lightweight metals such as aluminum and

magnesium are utilized in conjunction with conventional metals such as steel.

This increasing mixed metals usage further increases the cost of corrosion in

addition to the corrosion costs that already exist.

Galvanic corrosion [2] is one type of corrosion and can occur when two

different metals are coupled in the presence of a corrosive solution. Therefore,

galvanic corrosion is the undesirable result of this mixed metals usage. The

Page 2

274

y Element Technologv XIVautomobile companies rely primarily on proving ground testing to evaluate the

severity of corrosion. Such a procedure is costly and time consuming. The

objective of this project is to explore the feasibility of using computer simulation

to predict galvanic corrosion in automobile environments. There are two parts in

the study. The first part is to validate an appropriate computer code capable of

performing galvanic corrosion. The second part is to predict galvanic corrosion


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of bolted components in automobile environments.

There are two types of simulation tools available, experimentally and

analytically based tools. The experimentally based tools include expert systems

and neural networks. They rely heavily on test data, previous experience and are

problem specific and very easy to use. Finite difference, finite element and

boundary element methods are the analytically based tools. Since galvaniccorrosion occurs on the surfaces of bimetals, the boundary element method

appears to be more appropriate than the other methods for corrosion simulations.

Most early corrosion simulations were carried out by the oil and gas

industries. Their primary structures of interests are offshore platforms and pipe

lines. They generally use the experimentally based tools. The marine industry

conducts limited numerical corrosion studies. The automobile companies have

done very little corrosion simulation. The galvanic corrosion in automobile

environments is quite different and is more difficult to simulate because the

bimetals are coupled with thin electrolytes.

Munn [3] was a pioneer in the modeling ofgalvanic corrosion. He

used the mathematical analogy between electrical and thermal conduction.

Consequently he was able to solve corrosion problems with a finite element

program [4] having heat transfer capability. He solved several problems

including the beaker and tank problems, which are used as benchmark problems

for validation.

In the beginning of this investigation, BEASY [S] was the only

galvanic corrosion software available. It is a boundary element computer code.

The main feature of the boundary element method is that only surface elements

are needed as compared to the finite element method where volume elements are

required. So the boundary element method offers a significant modeling time

advantage over the finite element method for corrosion studies.For validation purposes, we compare BEASY results with the results of

the beaker and tank problems in 131. We also compare BEASY results with

those from a computer code - ANSYS [6] using its heat transfer option. After

the BEASY code has been properly validated we then proceed with our bolted

connection simulations.

Because corrosion simulations and the concept of boundary element

method are relatively new in automobile applications, we first briefly review

galvanic corrosion formulation within the context of the boundary element

method. We then give a brief description between the corrosion problems and

the corresponding heat transfer problems. Some of the shortcomings of usingheat transfer to simulate corrosion are also stated. Subsequent sections cover the

results of the validation and the presentation of bolted connection galvanic

simulations. The final section offers summaries and conclusions.

Page 3

Boundary Element Technology XIV

275


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I

Boundary element (BE) and finite element (FE) methods

Both the BE and FE methods lead to an integral equation, which

consists of a volume integral and a surface integral in three-dimensional (3D)

,

space. In the FEM, both integrals are discretized, creating volume elements inthe interior and surface elements on the surface of a 3D model. In the BEM, the

volume integral is further converted to another surface integral [7] through the

so-called fundamental solutions. These fundamental solutions are nothing more

than the response functions at some locations called field points due to a unit

input such as a unit load or displacement at other locations called source points.

As a result, the original integral equation contains only surface integra1.s.

Therefore we need to generate surface elements instead of volume elements in

the BEM. It is very difficult and time consuming to generate volume elements.

Either too many elements are required or the aspect ratio of the elements is

excessive, introducing undesirable errors. It is, however, much easier to generate

two-dimensional (2D) surface elements. If it is a 2D model, then the surface

elements degenerate to line elements.

The primary unknowns, such as electric potentials and temperatures, to

be solved are at the nodes for both methods. The derived quantities, such as

current densities and heat flux, are evaluated at the centroid or the integration

points and then extrapolated to the nodes in the FEM. But the same quantities,

however, are directly calculated on the surface without extrapolation in the

BEM. So the BE results are more accurate than the corresponding FE results.

Since galvanic corrosion occurs on the boundary, the BE approach is much more

attractive than the FEM.

In the BEM, the transformation of the volume integral to a surfaceintegral depends on the existence of fundamental solutions. For most nonlinear

problems, a fundamental solution does not exist. In this case we need to mesh

the interior of the nonlinear portion of the model. Consequently BEM will lose

its attractiveness. Those interested in a more detailed comparison between the

BE and FE methods are referred to [7].

Galvanic corrosion formulation in BEASY

The governing equation for electrostatic simulation [7] is the Laplace

equation

M$=O

(1)

in the electrolyte domain In subjected to appropriate boundary conditions on the

boundaries ri where i =l, n. The unknown Q is the electrical potential on the

boundary and within the electrolyte. Typical corrosion boundary conditions are

given by the following equations

Page 4

Boundary Element Technology XIV

c


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.

4) = $0 on r1

w

q=qoonr2

m

+ = fa (q) on I& and

(2 1C

Q = fc (9) on r3c

cw

where q is current density and can be calculated from equation (3)

9

.

=-KA$

(3)

tc in equation (3) is electrical conductivity of the electrolyte and is assumed to

be a constant in this investigation.Corrosion is an electro-chemical process, but the Laplace equation (1)

contains no information about the chemical reactions of the corrosive metals.

The coupling of the electrical potential with the chemical reactions is through

the boundary conditions imposed upon the anode, equation (2~) and the cathode,

equation (2d). The relationship between the current density q and the potential Q

is called polarization in corrosion. The polarization is a complex function of

types of metals and electrolyte properties as well as temperatures and flow

velocities of the electrolytes. The accuracy of the galvanic corrosion simulations

depends on realistic polarization information obtained from experiments.

Polarization curves for different metals are depicted in Figure 1 [2].

Corrosion occurs at the anode. The anodic current density distribution

can be determined on the surface of an anode from BEASY. The corrosion rate

mpy, mil per year, can be obtained from the Faraday Law [8],

mpy = (129*w*q)/(n*d).

(4)

In equation (4), w is the atomic weight of the corroding metal, q (milliamp/cm2)

is the current density, n is number of electrons lost per atom, and d (g/ cm2) is

the density of the corroding metal.

Thermal analogy to corrosion

The differential equation that governs electrostatics in equation (1) also

governs heat conduction and many other phenomena. In equation (l), if weconsider Q, as temperature, q as heat flux we will have a heat transfer problem.

The material constant K becomes the thermal conductivity. The heat convection

boundary conditions are similar to the polarization corrosion boundary

conditions, equations (2c and 2d) with the following exceptions. The

polarization relates the potential $-the temperature in heat conduction-explicitly

as a function of current density q-the heat flux in heat conduction. The heat

convection boundary conditions are, however, imposed by the following


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equation [6],

q=h($)/$b*(+(t)b)

(5)

where h($)/eb is a pseudo film coefficient, Q is the temperature on the boundary

and h is the bulk temperature in the surrounding fluid. The above equation

becomes equation (6)9 - -h (9)

(6)

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277

Figure 1: Polarization Curves for

Various Metals and Alloys

Figure 2: Beaker & Bolted

Component Models

. WALLS

if b>> 9. The resulting heat convection is similar to the correspondingpolarization in equations (2c and 2d). Therefore we can solve corrosion

problems using a thermal analogy by applying the boundary conditions similar

to equation (5) and imposing

(h,= 9*

(7)

One of the shortcomings of using a thermal analogy is the condition

required by equation (7). Since we do not know how large a value of $t, we

should use, we have to use a trial and error procedure. This time-consuming

procedure requires repeated solutions of the same problem with different values

of $, until a converged solution in Q is found.

ValidationA beaker and tank problems are used to assess the accuracy of the

BEASY for galvanic corrosion simulations.

McCtierty [9] derived an analytic solution for a beaker with

electrolyte inside. The anode and cathode are located on the inner and outer base

of the beaker as shown in Figure 2. He assumed the following linear anodic and

cathodic polarization,

qa = Ouch

and

qc = o.04*(~c- 1)

Page 6278

Boundarv Element Techologv XIV

e

.

The electrical conductivity K was 0.04 mho/cm. The inner and outer radii of the

cathode were 0.5 and 1.0 cm, respectively. The height H of the electrolyte was

0.5 cm. An axisymmetric model was used.


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Figure 3 is a potential graph where we plot the potential on the surfaces

of the anode and cathode as a function of radii of the beaker from McCaffertys

analytic solution as well as from BEASY and ANSYS results. The BEASY

solution is in good agreement with both the analytic and ANSYS solutions.

The tank problem was investigated numerically and experimentally by

Munn [3], and is a demonstration of an iron-zinc galvanic couple. Figure 4describes the geometry of the tank where an iron plate and a zinc rod were

submerged in seawater. The dimension of the tank normal to the paper had a

unit length. Munn used an electrolyte conductivity of 0.04 mho/cm, the iron

polarization curve as described in Figure 1, and a zinc fixed potential of -1005

mV.

Munns experimental potential results for the tank problem are shown

in Figure 5. In this validation, the problem is first solved with BEASY. Figure 6

shows the BEASY results. The same problem is then analyzed with ANSYS

heat transfer option where an accurate solution for the corresponding corrosion

simulation requires convergent studies. To convert the corrosion problem to a

corresponding thermal one, it is necessary to set the bulk temperature @b equal

to a value larger than the surface temperature Q in equation (7). Figure 7

describes the convergence of the temperature as a function of the bulk

temperatures. For this tank problem, Figure 7 shows the minimum bulk

temperature must be equal to le+lO. The converged temperature or the potential

plot is given in Figure 8. BEASY results compare well with ANSYS

corresponding heat transfer results. There is also a reasonably good agreement

between the numerical solutions and Munns experimental results. Exact

agreement, however, cannot be expected due to errors introduced in obtaining

input data from the polarization curve in Figure 1.

Figure 3: McCaffi Beaker Problem ResultsFigure 4: Side View of Tank Model

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Element Technologv XIV

279

w

.I

Figures: Experimental Potential Results for the Tank Problem

Figure 6: BEASY Potential Result for the Tank Problem

Page 8

280Figure 8: ANSYS Temperature (Potential)

Figure 7: Tank Potential (Temperature)

R&,&s for the Tank goblem \

Convergence vs. Tbulk (ANSYS)

:I Bolted component simulations

The axisymmetric model for the bolted component simulations is


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shown in Figure 2. It is a cylinder with a metallic base. The electrolyte in the

cylinder is 3% salt solution and its height H is 12.7 cm. The base has a cathode

in the center and an anode surrounding the cathode. The inner and outer radii of

the cathode are 0.635 cm and 4.950 cm, respectively. The cathode is used to

simulate a fastener such as a steel bolt. The anode represents the component

being joined together. Around the cathode we install protective coatings aselectrical insulators of thickness equal to 0.0, 0.0859 and 0.1793 cm. We use

four different galvanic couples. The metals used for the bolt and for the

component are tabulated in the table below.

Cathode-Bolt

Fe

Fe

CU

Fe

c Anode-Component Mg Zn Al Al

The BEASY results for typical radial current density distributions for a

steel bolt and a zinc component with protective coatings of 0.0, 0.0859 and

01793 cm are compared in Figure 9 where Dr represents the thickness of

protective coatings. The figure demonstrates a peak anodic current density drop

in the zinc component from -0.630 mA/cm**2 without protective coatings to -

0.196 mA/cm**2 with 0.0859 cm protective coatings. This is a reduction of

69% in current density with just 0.0859 cm protective coatings.Figure

1Osummarizes the effect of protective coatings on reduction of peak current

densities for various galvanic couples. Figure 12 shows that it is very effective

to install thin protective coatings around a bolt to reduce corrosion rates.

Depending on different galvanic couples, with 0.0859 cm protective coatings the

reductions in corrosion rates range from 9% for Fe-Al to 69% for Fe-Zn.Doubling the protective coatings to 0.1793 cm will not significantly further

reduce corrosion rates.

Page 9

Element Technology XIV

28 1

Figure 9: Effect of Coating on Current Density of FeZn

0 . 0

0.s

1.0

1.62 . 0

2 . 5

3 . 0

3.5 4.0 4.5

6 . 0

Radial Distance


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Figure IO: Reduction of Peak Anodic Current Density

8 0

1

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

Protective Coating Thkkness (cm)

ConclusionsWe have compared BEASY results with two benchmark problems, the

beaker and tank problems. We also obtain a corresponding thermal solution for

those problems. The beaker problem has an analytic solution. These analytic

and thermal solutions compare well with BEASY results. Experimental results

are available for the tank problem. The numerical results agree very well with

each other and also are in reasonably good agreement with the experimental

results. We do not recommend thermal analogy to be used to solve

corresponding corrosion problems because this approach inherently involves a

Page 10

Boundarv Element Technologv XIVw

.

time-consuming trial-and-error procedure. We conclude that BEASY is a

potentially useful tool capable of performing galvanic corrosion.

In this study we use an axisymmetric model to study the effect of

protective coatings on corrosion rates of bimetal bolted components. Because of

a lack of experimental polarization information, we use polarization curves from

the literature. The simulation results thus obtained are useful only for

comparison purposes. We have found from this investigation that a small

amount of protective coatings installed around a fastener can reduce the peakcorrosion rate as much as 69% for some galvanic couples. Further increases in

thickness, however, will not substantially reduce the corrosion rate.

Acknowledgement

The author would like to thank Dr. John Bomback of the Material

Science Dept. for suggesting this project.

References

1

2.

3.

4.

5 .6.

7.

8.

9.

Economic Effect of Metallic Corrosion in the United States, National

Bureau of Standards Special Publication 5 l l-2, Battelle Columbus


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Labs, Washington, D.C., U.S. Government Printing Office, 1978.

Denny A. Jones, Principles & Prevention of Corrosion, 2nd Ed.,

Prentice Hall, NJ, 1996.

Raymond S. Munn, The Modeling ofGalvanic Corrosion Systems

Using Numerical Methods with Particular Attention to Boundary

Conditions of Nonlinear Polarization, Ph.D. Thesis, University ofConnecticut, 1986.

MARC General Purpose Finite Element Computer Program, MARC

Analysis Research Corporation, Palo Alto, 1972.

BEASY User Guide, Computational Mechanics Publications,

Southampton, U.K. and Boston, MA., 1995.

ANSYS Users Manual, Rev 5.0, 1994.

J. Trevelyan, Boundary Elements for Engineers: Theory and

Applications, Computational Mechanics Publications, Southampton,

Boston, 1994.

Robert G. Kelly, Patrick J. Morgan, John R. Scully and Glen E. Stoner,

Electrochemical Techniques in Corrosion Engineering, University of

Virginia, Charlottesville, VA, 1997.

E. McCafferty, Mathematical Analysis of Circular Corrosion Cells

having Unequal Polarization Parameters, Naval Research Laboratory

Report No. 8107, 1977.

Galvanic cell - Wikipedia, the free encyclopediaI am in. Are you?Donor:Peter ChangDate:December 10, 2009Amount:USD 35.00Donate Now

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us protect it. [Show]Wikipedia Forever Our shared knowledge. Our sharedtreasure. Help us protect it. Galvanic cellFrom Wikipedia, the free encyclopediaJump to: navigation, searchA Galvanic cell, named after Luigi Galvani, is an electrochemical cell that

derives electrical energy from chemical reactions taking place within the cell.It generally consists of two different metals connected by a salt bridge, orindividual half-cells separated by a porous membrane. It is sometimes called a"Voltaic cell", after Alessandro Volta, inventor of the voltaic pile, the firstelectrical battery. In common usage, the word "battery" has come to include asingle Galvanic cell, but a battery properly consists of multiple cells.[1][2]

Contents [hide]1 History2 Description3 Cell voltage4 Notation

5 Galvanic corrosion6 Cell types7 See also8 References9 External links

[edit] HistoryIn 1780, Luigi Galvani discovered that when two different metals (copper andzinc for example) were connected together and then both touched to differentparts of a nerve of a frog leg at the same time, they made the leg contract.[3]He called this "animal electricity". The voltaic pile invented by AlessandroVolta in the 1800s is similar to the galvanic cell. These discoveries paved theway for electrical batteries.[edit] DescriptionSchematic of Zn-Cu galvanic cellA Galvanic cell consists of two half-cells. Inits simplest form each half-cell consists of a metal and a solution of a salt ofthe metal. The salt solution contains a cation of the metal and an anion tobalance the charge on the cation. In essence the half-cell contains the metal intwo oxidation states and the chemical reaction in the half-cell is anoxidation-reduction (redox) reaction, written symbolically in reductiondirection as

Mn+ (oxidised species) +n e- M (reduced species)In a galvanic cell one metal is able to reduce the cation of the other and,conversely, the other cation can oxidise the first metal. The two half-cellsmust be physically separated so that the solutions do not mix together. A saltbridge or porous plate is used to separate the two solutions.The number of electron transferred in both directions must be the same, so thetwo half-cells are combined to give the whole-cell electrochemical reaction. For


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two metals A and B

This is not the whole story as anions must also be transferred from one

half-cell to the other. When a metal in one half-cell is oxidised anions must betransferred into that half-cell to balance the electrical charge of the cationproduced. The anions are released from the other half-cell where a cation isreduced to the metallic state. Thus, the salt bridge or porous membrane servesboth to keep the solutions apart and to allow the flow of anions in thedirection opposite opposite to the flow of electrons in the wire connecting theelectrodes.The voltage of the Galvanic cell is the sum of the voltages of the twohalf-cells. It is measured by connecting a voltmeter to the two electrodes. Thevoltmeter has very high resistance, so the current flow is effectivelynegligible. When a device such as an electric motor is attached to the

electrodes a current flows and redox reactions occur in both half-cells. Thiswill continue until the concentration of the cations that are being reduced goesto zero.For the Daniell cell, depicted in the figure, the two metals are zinc and copperand the two salts are sulfates of the respective metal. Zinc is the morereducing metal so when a device is connected to the electrodes, theelectrochemical reaction is

Zn + Cu2+ Zn2+ + CuThe zinc electrode is dissolved and copper is deposited the copper electrode. Bydefinition, the cathode is the electrode where reduction (gain of electrons)takes place, so the copper electrode is the cathode. The cathode attractscations, so has a negative charge. In this case copper is the cathode and zincthe anode.Galvanic cells are typically used as a source of electrical power. By theirnature they produce direct current. For example, a lead-acid battery contains anumber of galvanic cells. The two electrodes are effectively lead and leadoxide.The Weston cell was adopted as an International Standard for voltage in 1911.The anode is a cadmium mercury amalgam, the cathode is made of pure mercury, theelectrolyte is a (saturated) solution of cadmium sulfate and the depolarizer isa paste of mercurous sulfate. When the electrolyte solution is saturated thevoltage of the cell is very reproducible, hence its use as a standard.[edit] Cell voltageThe standard electrical potential of a cell can be determined by use of astandard potential table for the two half cells involved. The first step is toidentify the two metals reacting in the cell. Then one looks up the standardelectrode potential, E0, in volts, for each of the two half reactions. Thestandard potential for the cell is equal to the more positive E0 value minus themore negative E0 value.For example, in the figure above the solutions are CuSO4 and ZnSO4. Each


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solution has a corresponding metal strip in it, and a salt bridge or porous diskconnecting the two solutions and allowing SO42 ions to flow freely between thecopper and zinc solutions. In order to calculate the standard potential onelooks up copper and zinc's half reactions and finds:

Cu2+ + 2 e Cu: E0 = +0.34 V

Zn2+ + 2 e Zn: E0 = 0.76 VThus the overall reaction is:Cu2+ + Zn Cu + Zn2+

The standard potential for the reaction is then +0.34 V - -0.76 V = 1.10 V. Thepolarity of the cell is determined as follows. Zinc metal is more stronglyreducing than copper metal as shown by the fact that the standard (reduction)potential for zinc is more negative than that of copper. Thus, zinc metal willlose electrons to copper ions and develop a positive electrical charge. Theequilibrium constant, K, for the cell is given bywhere F is the Faraday, R is the gas constant and T is the temperature in

Kelvin. For the Daniell cell K is approximately equal to 1.51037. Thus, atequilibrium, a few electrons are transferred, enough to cause the electrodes tobe charged.[4]Actual half-cell potentials must be calculated by using the Nernst equation asthe solutes are unlikely to be in their standard states,where Q is the reaction quotient. This simplifies towhere {Mn+} is the activity of the metal ion in solution. The metal electrode isin its standard state so by definition has unit activity. In practiceconcentration is used in place of activity. The potential of the whole cell isobtained by combining the potentials for the two half-cells, so it depends onthe concentrations of both dissolved metal ions.The value of 2.303R/F is 0.19845x10 3 V/K, so at 25C (298.15 K) the half-cellpotential will change by if the concentration of a metal ion is increased ordecreased by a factor of 10.These calculations are based on the assumption that all chemical reactions arein equilibrium. When a current flows in the circuit, equilibrium conditions donot obtain and the cell potential will usually be reduced by various mechanisms,such as the development of overpotentials.[5] Also, since chemical reactionsoccur when the cell is producing power, the electrolyte concentrations changeand the cell voltage is reduced. The voltage produced by a galvanic cell istemperature dependent because standard potentials are temperature-dependent.[edit] NotationThe galvanic cell, as the one shown in the figure, are conventionally describedusing the following notation:Zn(s) | ZnSO4(aq) || CuSO4(aq) | Cu(s)(anode)----------------------------------(cathode)An alternate notation for this cell would be:


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Zn(s) | Zn+2(aq) || Cu+2(aq) | Cu(s)Where the following applies:

(s) denotes solid.(aq) means aqueous solution.The vertical bar, |, denotes a phase boundary.

The double vertical bar, ||, denotes a liquid junction for which the junctionpotential is near zero, such as a salt bridge.[6][edit] Galvanic corrosionMain article: Galvanic corrosionGalvanic corrosion is a process that degrades metals electrochemically. Thiscorrosion occurs when two dissimilar metals are placed in contact with eachother in the presence of an electrolyte, such as salt water, forming a galvaniccell. A cell can also be formed if the same metal is exposed to two differentconcentrations of electrolyte. The resulting electrochemical potential thendevelops an electric current that electrolytically dissolves the less noblematerial.

[edit] Cell typesConcentration cellElectrolytic cellElectrochemical cellLasagna cellLemon battery

[edit] See alsoAlessandro VoltaBattery (electricity)Bio-nano generatorElectrode potentialElectrosynthesisGalvanic seriesSacrificial anodeVoltVoltaic pile

[edit] References^ Merriam-Webster Online Dictionary: "battery"^ "battery" (def. 4b), Merriam-Webster Online Dictionary (2008). Retrieved 6August 2008.^ Keithley, Joseph F. (1999). Daniell Cell. John Wiley and Sons. pp. 4951.ISBN 0780311930.^ Atkins, P; de Paula (2006). Physical Chemistry. J. (8th. ed.). OxfordUniversity Press. ISBN 9780198700722. Chapter 7, sections on "Equilibriumelectrochemistry"^ Atkins, P; de Paula (2006). Physical Chemistry. J. (8th. ed.). OxfordUniversity Press. ISBN 9780198700722. Section 25.12 "Working Galvanic cells"^ Atkins, P., "Physical Chemistry", 6th edition, W.H. Freeman and Company, NewYork, 1997

[edit] External links


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GALVANIC CORROSIONT his is the html version of the file

Page 1Corrosion for EngineersDr. Derek H. ListerChapter 4: Galvanic Corrosion page4-1GALVANIC CORROSIONRemember

electrochemistry basics in aqueous solution,

metal dissolution is ANODIC

:M -> Mn+ + ne(e.g., Fe -> Fe 2+ + 2e)

and there are several possible CATHODIC reactions

hydrogen evolution (acids)2H + + 2e -> H2oxygen reduction (acids)O2 + 4H+ + 4e -> 2H20

oxygen reduction (neutral or base)O2 + 2H20 + 4e -> 40H"metal ion reductionM3+ + e -> M2+metal deposition

M+ + e -> M

More than one oxidation and more than one reduction reaction canoccurduring corrosion.

Page 2


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Corrosion for Engineers Chapter 4: Galvanic CorrosionDr. DerekH. Listerpage4-2MULTIPLE CATHODIC REACTIONS ARE IMPORTANT.

Thus, metals tend to dissolve more readily in aerated acids than in pure acids;

oxygen reduction AND hydrogen evolution can occur. Also, an "oxidizer" suchas ferricion as an impurity incommercial acids makes them much morecorrosive than pure acids - extra cathodicreaction.

Fe3+ + e - Fe2+

NOTE: corrosion in sea water or fresh water is usually governed by oxygen reduction;if water is de-aerated, it becomes much less corrosive because the main reaction:

O2 + 2H20 + 4e -> 40H"

can no longer occur.

Page 3Corrosion for EngineersDr. Derek H. ListerREMEMBER:e.g.addChapter 4

: Galvanic Corrosion page 4-3

the metal dissolution reaction (corrosion) must always be balanced by one or morereduction reactions:2Fe -> 2Fe2+ +4e

2Fe + 02 + 2H20 ->2Fe2+ + 40H" (ppt) 2Fe(OH)3

f- 1/2 02 + H20 + 2Fe(OH)2"rust" (oxidizes)and

Fe + Cu2+ -> Fe2+ + Cu (the "nail in copper sulphate" trick!); clearly, the iron wantsto be in solution more than the copper; the copper is more NOBLE than the iron; theiron is more ACTIVE than the copper.

Page 4

Chapter 4: Galvanic Corrosion page

4-4GALVANIC SERIESA metal in contact with a solution establishes a POTENTIALwith respect to the solution.


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would we measure the potential difference Em - Es?Em - Es cannot be measured, wecan only measure the difference between it and Em-Es for another metal:

(Em1 - Es) - (Em2- Es) = Emi - Em2

Page 5Corrosion for EngineersDr. Derek H. ListerChapter 4: Galvanic Corrosion page4-5CHANGES in potential of one electrode can be measured if the otherelectrodedoes not change, i.e., if it is a "reference electrode".There areseveral reference electrodes which are constant so long as nocurrent is drawnfrom them; potentials relative to a reference electrode aretherefore measuredwith meters (e.g., milli-voltmeters) of high impedance.kifi iMftdm*Ml/rtefer,University of New Brunswick, CanadaChulalongkorn University, Thailand

Page 6Corrosion for EngineersDr. Derek H. ListerChapter 4: Galvanic Corrosion page4-6A metal in contact with a solution of its own ions at unitactivity(thermodynamic concentration) establishes fixed potential differenceswithrespect to every other metal in the same condition OF EQUILIBRIUM(potentialsare "reversible"); THEREFORE we can set up a series of standardelectrodepotentials with respect to some reference electrode; the standardhydrogenelectrode (SHE) is chosen to have a potential of zero at25C.ifeOA.)Pttodje It Macid Mid* * *W fitfifiA fft- p'^= 0University of NewBrunswick, CanadaChulalongkorn University, Thailand

Page 7Corrosion for EngineersDr. Derek H. ListerChapter 4: GalvanicCorrosionpage4-7tNoble orCathodicActive oranodiciStandard emf series of metalsMetal-metalionequilibrium(unitactivity)Au-Au+3Pt-Pt+2Pd-Pd+2Ag-Ag+Hg-Hg2+2Cu-Cu+2Pb-Pb+2Sn-Sn+2Ni-Ni+2Co-Co+2Cd-Cd+2Fe-Fe+2Cr-Cr+3AI-AI+3Na-Na+Electrodepotentialvs. normal hydrogenelectrode at 25C,volts+1.498+1.2+0.987+0.799+0.788+0.3370.000-0.126-0.136-0.250-2.777-0.403-0.440-0.744-0.763-1.662-2.363-2.714-2.925Thishas the accepted signconvention; however, someworkers use oppositesignconvention.Source: A.J. de Bethune and N. A. S. Loud,"Standard AqueousElectrode Potentialsand Temperature Coefficient at 25C,"Clifford A. Hampel,Skokie, III., 1964. Seealso Table 9-1. These potentials are listedin accordancewith the StockholmConvention. See J. O'M. Bockris and A. K.N. Reddy, ModernElectrochemistry,Plenum Press, New York, 1970.University of New Brunswick,CanadaChulalongkom University, Thailand

Page 8Corrosion for Engineers Chapter 4: Galvanic CorrosionDr. DerekH. Lister


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pagemREFERENCE ELECTRODESAg/ solid Ag CI in 0.1 N K CI: + 0.288 V(SHE)Cu/satd. Cu S04: + 0.316 V (SHE)Hg/ solid Hg2 Cl2 in 0.1 N KCL: + 0.334 V (SHE)Bycoupling two reversible electrodes together, we get a fixedpotentialdifference:e.g.Ag/Ag+ - Cu2+/Cu0.799 V - 0.337V= 0.462 V(Discuss the

possibility of making a reversible electrode out of an alloy suchasbrass.)University of New Brunswick, Canada Chulalongkom University, Thailand

Page 9Corrosion for Engineers Chapter 4: Galvanic CorrosionDr. Derek H. Lister ^^^page _ $N.B. |f we drew current from two such electrodes (reversible Ag,Cu)THEY WOULD NO LONGER BE AT EQUILIBRIUM. THE REVERSIBILITYWOULDBE DESTROYED.silver would be deposited more than silver ions would be formed; copper ionswould be formed more than copper would be deposited;(remember, equilibrium orreversibility at an electrode means the rate ofthe "forward" reaction equals the

rate of the "back" reaction).N.B. Corroding metals are not at equilibrium.NORare they usually in contact with unit activity of their own ions.THEREFORE theEMF series is an ideal system, which may be used asan indicator for practicalsituations.University of New Brunswick, Canada Chulalongkorn University,Thailand

Page 10Ti:kTJ"t;neerS Chapter 4: Oalvanic Comson. page 4-10IN GENERAL, as a roughguide:any metal in the EMF series will displace from solution any metal aboveite.g., Fe displaces Cu from CuS04 solution,Zn displaces H2 from acidsolution.BUT "passivation" of some metals alters behavior; passivation istheformation of a very protective oxide layer that makes the metal noble.E.g.,Cr is a reactive element, but Cr metal is usually passivated andcathodic to mostcommon metals (hence Cr plating).Galvanic corrosion MAY arise when dissimilarmetals are in contact inaqueous solution.The potential difference between themwill initiate attack, the corrosion ratedepends on the surface reactions of(usually) both metals (i.e., we usuallyconsider galvanic COUPLES of just twometals).University of New Brunswick, CanadaChulalongkorn University, Thailand

Page 11Corrosion for EngineersDr. Derek H. ListerChapter 4: Galvanic Corrosion page4-11Galvanic potentials are made use of in batteries, e.g., the Daniel cell.M.IvoltsPZn1(oso*)Pot nt lo I V*" 1.1 voltaeIn the Daniel cell, the zincelectrodeis 1.1V negative with respect to thecopper electrode.Which is theanode, the cathode?Which way does the electricalcurrent flow?University of NewBrunswick, CanadaChulalongkorn University, Thailand

Page 12Corrosion for EngineersDr. Derek H. ListerChapter 4: Galvanic Corrosion page4-12AnodeZn/Zn** Interface ismite for


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de-electronation(oxidation);Zn-*Zn#%2eCathodeacu"s|Cu/Cu**Interface issite forelectronatfon(reduction):Cf+29 CuThe sign of the voltage on the Daniel cellindicates that, upon placing aload on the cell, a spontaneous de-electronationwill occur on the zincelectrode; and electronation, on the copperelectrode.University of New Brunswick, CanadaChulalongkorn University, Thailand

Page 13Corrosion for EngineersDr. Derek H. ListerChapter 4: Galvanic Corrosion page4-13A Dry Cell. Dry cells are electrochemical energy storers in which theelectrolyteis immobilized in the form of a paste. A typical dry cell is theLeclanche cell.A schematic diagram of this cell is shown below. The reactionsoccurring inthe cell during discharge are:at anode,at cathode,Zn -> Zn2+ +2e2Mn02 + 2H30+ + 2e - Mn203 + 3H20Since hydroxide ions are producedduringworking (because H30+ is consumed), thefollowing irreversible sidereactions occur:OH" +NH4+ -> H20 + NH3Zn 2+ + 2NH3 + 2Cr -> Zn(NH3)2CI2Zn2++20H- ZnO + H20ZnO + Mn203 - Mn203ZnOOwing to the above reactions, the cell is

onlypartially rechargeable and this to such asmall extent that it is never donein practice.Carbon rodZinc conPorou portillonImmobilludtltctrolyt(NH4Cl*ZnCl,*H,0)University of New Brunswick, CanadaChulalongkornUniversity, Thailand

Page 14Corrosion for Engineers Chapter 4: Galvanic CorrosionDr. Derek H. Lister _____ ^page 4. 74NOTE WELL:The analogy between aqueous corrosion processes (e.g.,galvanic couples)and cells / batteries is illuminating but limited. The cathodicreaction ingalvanic corrosion is usually oxygen reduction or hydrogen evolution,notmetal deposition.To predict galvanic corrosion of couples in seawater, usethe table of"Galvanic Series of some commercial metals and alloys in seawater"thatfollows.University of New Brunswick, Canada Chulalongkorn University,Thailand

Page 15Corrosion for EngineersDr. Derek H. Lister PlatinumGoldNoble orGraphitecathodic TitaniumSilver'Chlorimet 3 (62 Ni, 18 Cr, 18 MoM /^ LHastelloyC (62 Ni, 17 Cr, 15 Mo)Chapter 4: Galvanic Corrosionpage 4-15Activeoranodici8-8 Mo stainless steel (passive)18-8 stainless steel(passive).Chromium stainless steel 11-30% Cr (passive)Inconel (passive) (80 Ni,13 Cr, 7 Fe).Nickel (passive)Silver solder"Monel (70 Ni, 30 Cu)Cupronickels(60-90 Cu, 40-10 Ni)Bronzes (Cu-Sn)Copper.Brasses (Cu-Zn)Chlorimet 2 (66 Ni, 32Mo, 1 Fe).Hastelloy B (60 Ni, 30 Mo, 6 Fe, 1 Mn)"inconel (active).Nickel(active)TinLeadLead-tin soldersp8-8 Mo stainless steel (active) L.18-8stainless steel (active)Ni-Resist (high Ni cast iron)Chromium stainless steel,13% Cr (active)CCast ironSteel or iron2024 aluminum (4.5 Cu, 1.5 Mg, 0.6Mn)CadmiumCommercially pure aluminum (1100)ZincMagnesium and magnesiumalloys**visiteWjft ftrUniversity of New Brunswick, CanadaChuialongkorn University,


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Thailand

Page 16Coifosion for EngineersDr. Derek H. ListerChapter 4: Galvanic Corrosionpage4-16EXAMPLES (from Fontana):A yacht with a Monel hull and steel rivets became

unseaworthy because ofrapid corrosion of the rivets. Severe attack occurred onaluminum tubingconnected to brass return b^nds. Domestic hot-water tanks made ofsteel failwhere copper tubing is connected to the tank. Pump shafts and valvestemsmade of steel or more corrosion-resistant materials fail because ofcontactwith graphite packing.Other examples:Galvanic corrosion of steel pipeatbrass fitting in humid marineatmosphere.University of New Brunswick,CanadaChulalongkorn University, Thailand

Page 17Corrosion for EngineersDr. Derek H. ListerChapter 4: Galvanic Corrosion page4-17Galvanic corrosion of painted steel auto body panel in contact with

stainlesssteel wheel opening molding.University of New Brunswick,CanadaChulaiongkorn University, Thailand

Page 18

Page 19Corrosion for EngineersDr. Derek H. ListerChapter 4: Galvanic Corrosion page4-19Statue of LibertyRust staining of the Statue of Liberty torchdue to galvaniccorrosion of the ironarmature in contact with the copper skin. Courtesy ofR.Baboian, Texas Instruments, Inc.University of New Brunswick,CanadaChulalongkorn University, Thailand

Page 20Corrosion for Engineers /#,#,,#./%i ^Dr. Derek H. Lister Chapter 4:Galvanic Corrosion- page 4-20Surface oxides (e.g., "rust") are very important ingalvanic corrosion: bare metal is a better cathode than oxide-coveredmetaloxide interferes with hydrogen evolution and impedes oxygen diffusion,oxideputs an additional electrical resistance in the electrochemical circuit.Oxidefilm effects: In standard EMF series, Al is more active than Zn (-1.662 vversus -0.763 v);we might expect that in a Zn-AI couple the Al would be anodicto the Zn'NOT SO!The AI2O3 film makes the Al more noble, so that Zn is anodic toit andactually protects it when coupled to it in solution (see "Galvanic SeriesinSeawater"). The oxide film on stainless steel is electrically insulating andimpedes thecharge flow between galvanic couples. The oxide film on copper iseasily reduced. The resulting exposed metal isan efficient cathode. Oxygen isreadily reduced there. Galvanic coupleswith copper can be very corrosive.NOTE:diffusion & reduction of 02 often control galvanic corrosion, a largecathodearea relative to the anode can be disastrous; such effectscommon at joints,where structures/components may be joinedtogether with a different


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metal.University of New Brunswick, CanadaChulalongkorn University, Thailand

Page 21Corrosion for EngineersDr. Derek H. ListerChapter 4: Galvanic Corrosion page4-21Effects of area relationship on corrosion of rivets (steel-copper couple)in

seawater for 15 months.Copper rivetsin steel plateLarge anodeSmall cathodeWS?. !Steel rivetsin copper plateLarge cathodeSmall anodeDiscuss:Two differentmetals of approximately the same area are joined to form agalvanic couple in acorrosive solution; we are to reduce the corrosion bycoating (e.g., painting)one component of the couple. Do we coat the anodeor the cathode?University ofNew Brunswick, CanadaChuialongkorn University, Thailand

Page 22

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Corrosion for Engineers Chapter 4: Galvanic CorrosionDr. Derek H. Lister page4-23Generally-Galvanic corrosion is under cathodic control.... if we reduce thearea ofthe Cathode (by coating, etc.) we reduce the corrosion; if we reduce thearea ofthe Anode, corrosion will continue at the same rate but over a smallerarea, soperforation etc. will occur sooner.TO REDUCE GALVANIC CORROSION BYCOATING, THE MORECORROSION-RESISTANT (i.e., THE MORE NOBLE ORCATHODIC)COMPONENTOF THE COUPLE IS COATED.University of New Brunswick, Canada ChulalongkornUniversity, Thailand

Page 24Corrosion for EngineersDr. Derek H. ListerChapter 4: Galvanic Corrosion page4-24CATHODIC PROTECTIONTo reduce metallic corrosion, the component can madetheCATHODE of agalvanic cell(a) by impressing an electric current from an externalpower source.RectifierCurrentiTorikCurrentVK ASiI GrovelBackfillCathodicprotection of an underground tank using impressed currents.University of NewBrunswick, CanadaChulalongkorn University, Thailand

Page 25Corrosion for EngineersDr. Derek H. Listerb)WoteroutletWoterChapter 4: GalvanicCorrosion page 4-25Ground level--BackfillCathodic protection of adomestichot-water tank using a sacrificial anode.Protection of anundergroundline with a magnesium anode.University of New Brunswick,CanadaChuialongkorn University, Thailand

Page 26Corrosion for Engineers Chapter 4: Galvanic CorrosionDr. Derek H. Lister _ page4 - 26ZINC PLATING (" GALVANIZING"!Steel sheeting is coated with zinc byhot-dipping in the molten metal, byheating with zinc dust ("Sherardizing"),


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etc.The Zn coating acts as a sacrificial anode... at the inevitableimperfections,cruds, etc., zinc dissolves preferentially, deposits loose,flocculant Zn(OH)2from aqueous solution.University of New Brunswick, CanadaChulalongkorn University, Thailand

Page 27Corrosion for Engineers Chapter 4: Galvanic CorrosionDr. Derek H. Lister page4.27Protection continues as long as enough Zn is left...if large enough areasofsteel are exposed steel corrosion will occur usually at the middle oftheexposed area.If the temperature >60C, the Zn (OH)2 changes from a loose to ahard, compactform.This MAY change the polarity of the steel/Zn couple by makingthe Zn morenoble than the steel; this CAN lead to rapid failure of thesteel.NOTE:Galvanized steel should only be used in pH range 6 -12.5.. readydissolution ofZn in acids and alkalis quickly removes protection outside therange.CADMIUM PLATING ... similar action on steels to zinc plating galvanic AEless than for Zn; more protective than Zn in marine environments (chloride less

solublethan Zn Cl2-gives more protective coat); better than zinc in humidconditions indoors; used less and less because Cd is toxicUniversity ofNeWBrunswick, Canada Chulalongkorn University, Thailand

Page 28Corrosion for EngineersDr. Derek H. ListerChapter 4: Galvanic Corrosion page4-28TIN PLATINGdifferent action from Zn or Cd; Sn is CATHODIC tosteel; "pinhole"corrosion can occur at imperfections in tin plate.Tin plate commonly used onsteel cans for foodstuffs. Organic acids in foods,fruit juices etc., complexSn2+ very readily., lower potential, make tin anodic tosteel.Also, efficiency ofSn (and Fe) for H2 evolution poor in 02 - starvedenvironment inside a food can,only possible cathodic reaction is H2 evolution;if evolution rate slow,corrosion rate slow (tins don't explode very often).University of New Brunswick,CanadaChulalongkorn University, Thailand

Page 29Corrosion for Engineers Chapter 4: Galvanic CorrosionDr. Derek H. Lister . page4-29NOTE: Galvanic corrosion can occur without components of differentmetalsactually being in electrical contact;e.g., if soft water containing C02(i.e., slightly acid from carbonic acid) flowsthrough copper pipes into agalvanized tank, copper ions will deposit on thezinc as metalCu2+ + Zn -> Cu +Zn2+The Cu is an efficient cathode and will rapidly destroy Zncoating.University of New Brunswick, Canada Chulalongkom University, Thailand

Page 30Corrosion for Engineers Chapter 4: Galvanic CorrosionDr. Derek H. Lister __ page4-30MINIMIZE GALVANIC CORROSION1. Select metals as close together as possibleingalvanic series;2. Avoid small-anode/large-cathode combinations... choosefasteners of morenoble materials;3. Insulate dissimilar metals (e.g., sleeve


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bolts in flange joints, as well as useinsulating washers);4. Apply coatingscarefully, keep in good condition (esp. those on anodes);5. Add inhibitors, ifpossible, to environment;6. Avoid threaded joints where possible;7. Design foranodic member (make thicker, easily replaceable, etc.);8. Install a third metalthat is anodic to BOTH in the couple.University of New Brunswick, Canada

Sacrificial anode - Wikipedia, the free encyclopediaWikipedia is there when youneed it now it needs you.

Partially corroded sacrificial anode on the hull of a ship.A sacrificial anode,or sacrificial rod, is a metallic anode used in cathodic protection where it isintended to be dissolved to protect other metallic components. The more activemetal is more easily oxidized than the protected metal and corrodes first (hencethe term "sacrificial"); it generally must oxidize nearly completely before theless active metal will corrode, thus acting as a barrier against corrosion forthe protected metal.

1 Reaction2 Examples3 See also4 References

Electrons are stripped from the anode and conducted to the protected metal,


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which becomes the cathode. The cathode is protected from corroding, i.e.,oxidizing, because reduction rather than oxidation takes place on its surface.For example when zinc and iron are electrically connected in the presence ofoxygen and water, the zinc will lose electrons and go into solution as zinccations. Electrons released from the zinc atoms flow through metallic conduction

to the iron where, on the surface, dissolved oxygen is reduced, by gaining theelectrons released by the zinc, to hydroxide anions. Were the zinc not present,the same reduction of oxygen to hydroxide would occur on the iron surface.However in that case the electrons for reduction would be furnished by the iron,thus oxidizing the iron. Therefore, the zinc, when present, is "sacrificed" bybeing oxidized instead of the iron. The iron is "safe" until all of the zinc hascorroded. As zinc is more costly than iron, this method of protecting iron, orsteel, would not be cost effective were it not for secondary chemical reactionsthat form coatings on the iron surface, thus reducing the electrochemicalreaction to a trickle and greatly prolonging the life of the zinc anode.For this mode of corrosion protection to function there must be simultaneously

present an electron pathway between the anode and the metal to be protected(e.g., a wire or direct contact) and an ion pathway between the anode and themetal to be protected (e.g., water or moist soil) to form a closed circuit; thussimply bolting a piece of active metal such as zinc to a less active metal, suchas mild steel, in air will not furnish any protection.

A interesting recent development has been the use of sacrificial anodes to solvelimescale problems[1]. The zinc this time is sacrificed for limescaleprotection. It works by tiny amounts of zinc dissolving in the water which, areattracted to the calcium. They react to form aragonite [2] and in this form itdoesn't react with plumbing systems.

Other examples of protection by use of sacrificial anodes include protection ofvoids in the glass lining of mild steel water heater tanks via use of magnesiumor aluminum alloy anodes,[3] protection of off-shore oil rigs via special alloyanodes for use in salt water, steel pipes with zinc anodes [4] and protection oflock gates in water ways.


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Galvanic corrosion - Wikipedia, the free encyclopediaI am in. Are you?Donor:Peter ChangDate:December 10, 2009Amount:USD 35.00

Donate Now[Hide][Show]Wikipedia Forever Our shared knowledge. Our shared treasure. Helpus protect it. [Show]Wikipedia Forever Our shared knowledge. Our sharedtreasure. Help us protect it. Galvanic corrosionFrom Wikipedia, the free encyclopediaJump to: navigation, search

This article needs additional citations for verification.Please help improve this article by adding reliable references. Unsourcedmaterial may be challenged and removed. (June 2008)

Galvanic corrosion is an electrochemical process in which one metal corrodespreferentially when in electrical contact with a different type of metal andboth metals are immersed in an electrolyte. Conversely, a galvanic reaction isexploited in primary batteries to generate a voltage. A common example is thecarbon-zinc cell where the zinc corrodes preferentially to produce a current.The lemon battery is another simple example of how dissimilar metals react toproduce an electric current.Rusted corrugated steel roofWhen two or more different sorts of metal come intocontact in the presence of an electrolyte a galvanic couple is set up asdifferent metals have different electrode potentials. The electrolyte provides ameans for ion migration whereby metallic ions can move from the anode to thecathode. This leads to the anodic metal corroding more quickly than it otherwisewould; the corrosion of the cathodic metal is retarded even to the point ofstopping. The presence of electrolyte and a conducting path between the metalsmay cause corrosion where otherwise neither metal alone would have corroded.Even a single type of metal may corrode galvanically if the electrolyte variesin composition, forming a concentration cell.

Contents [hide]


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1 Examples2 Galvanic series3 Preventing galvanic corrosion4 Factors that influence galvanic corrosion5 Lasagna cell

6 Galvanic compatibility7 See also8 References9 External links

[edit] ExamplesGalvanic corrosion in Statue of LibertyRegular maintenance showed that the Statue of Liberty suffered from galvaniccorrosionA common example of galvanic corrosion is the rusting of corrugated

iron sheet, which becomes widespread when the protective zinc coating is brokenand the underlying steel is attacked. The zinc is attacked preferentiallybecause it is less noble, but when consumed, rusting will occur in earnest. Witha tin can, the opposite is true because the tin is more noble than theunderlying steel, so when the coating is broken, the steel is attackedpreferentially.A rather more spectacular example occurred in the Statue of Liberty when regularmaintenance in the 1980s showed that galvanic corrosion had taken place betweenthe outer copper skin and the wrought iron support structure. Although theproblem had been anticipated when the structure was built by Gustave Eiffel toFrdric Bartholdi's design in the 1880s, the insulation of shellac between thetwo metals failed over a period of time and resulted in rusting of the ironsupports. The renovation replaced the original insulation with PTFE. Thestructure was far from unsafe owing to the large number of unaffectedconnections, but it was regarded as a precautionary measure for what isconsidered a national US symbol.An earlier example occurred in the Royal Navy frigate HMS Alarm. The wooden hullof the vessel had been sheathed in copper to prevent attack by barnacles. It wassoon discovered that the sheathing had become detached from the hull in manyplaces because the iron nails which had been used to fasten the copper to thetimbers had been much rotted. Closer inspection revealed that some nails,which were less corroded, were insulated from the copper by brown paper whichwas trapped under the nail head. The copper had been delivered to the dockyardwrapped in the paper which was not removed before the sheets were nailed to thehull. The obvious conclusion therefore, and the one which was contained in areport to the Admiralty of 1763, was that iron should not be allowed directcontact with copper in a sea water environment if severe corrosion of the ironwas to be avoided. Later ships were designed with this in mind. Not only was seawater a very good electrolyte owing to its high salt concentration, but attackof the nails was encouraged by their very small exposed area compared with that


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of the copper-sheathed hull.[edit] Galvanic seriesMetals (including alloys) can be arranged in a galvanic series representing thepotential they develop in a given electrolyte against a standard referenceelectrode. The relative position of two metals on such a series gives a good

indication of which metal is more likely to corrode more quickly. However, otherfactors such as water aeration and flow rate can influence the process markedly.Galvanic corrosion is of major interest to the marine industry. Galvanic seriestables for seawater are commonplace due to the extensive use of metal inshipbuilding. It is possible that corrosion of silver brazing in a salt waterpipe might have caused a failure that lead to the USS Thresher sinking with allmen lost.The common technique of cleaning silver by immersion of the silver and a pieceof aluminium in a salt water bath (usually sodium bicarbonate) is an example ofgalvanic corrosion. (Care should be exercised for reasons such as this willstrip silver oxide from the silver which may be there for decoration. Use on

plated silver is inadvisable as this may introduce unwanted galvanic corrosionwith the base metal.)[edit] Preventing galvanic corrosionThere are several ways of reducing and preventing this form of corrosion.

One way is to electrically insulate the two metals from each other. Unlessthey are in electrical contact, there can be no galvanic couple set up. Thiscan be done using plastic or another insulator to separate steel water pipesfrom copper-based fittings or by using a coat of grease to separate aluminiumand steel parts. Use of absorbent washers that may retain fluid is oftencounter-productive. Piping can be isolated with a spool of pipe made ofplastic materials or made of metal material internally coated or lined. It isimportant that the spool has a minimum length of approx 500 mm to beeffective.Another way is to keep the metals dry and/or shielded from ionic compounds(salts, acids, bases), for example by painting or encasing the protected metalin plastic or epoxy, and allowing them to dry.Coating the two materials or if it is not possible to coat both, the coatingshall be applied to the more noble, the material with higher potential. Thisis necessary because if the coating is applied only on the more activematerial, in case of damage of the coating there will be a large cathode areaand a very small anode area, and for the area effect the corrosion rate willbe very high.It is also possible to choose metals that have similar potentials. The moreclosely matched the individual potentials, the lesser the potential differenceand hence the lesser the galvanic current. Using the same metal for allconstruction is the most precise way of matching potentials.

Electroplating or other plating can also help. This tends to use more noblemetals that resist corrosion better. Chrome, nickel, silver and gold can all beused.


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Aluminium anodes mounted on a steel jacket structureCathodic protection uses oneor more sacrificial anodes made of a metal which is more active than theprotected metal. Metals commonly used for sacrificial anodes include zinc,magnesium, and aluminium. This is commonplace in water heaters. Failure toregularly replace sacrificial anodes in water heaters severely diminishes the

life time of the tank. Water softeners tend to degrade these sacrificial anodesand tanks more quickly.Finally, an electrical power supply may be connected to oppose the corrosivegalvanic current. (see impressed current cathodic protection)For example, consider a system is composed of 316 SS (a 300 series stainlesssteel; it is a very noble alloy meaning it is quite resistant to corrosion andhas a high potential) and a mild steel (a very active metal with lowerpotential). The mild steel will corrode in the presence of an electrolyte suchas salt water. If a sacrificial anode is used (such as a zinc alloy, aluminiumalloy, or magnesium), these anodes will corrode, protecting the other metals.This is a common practice in the marine industry to protect ship equipment.

Boats and vessels that are in salt water use either zinc alloy or aluminiumalloy. If boats are only in fresh water, a magnesium alloy is used. Magnesiumhas one of the highest galvanic potentials of any metal. If it is used in a saltwater application on a steel or aluminium hull boat, hydrogen bubbles will formunder the paint, causing blistering and peeling.Metal boats connected to a mains shore line will normally have to have the hullconnected to earth for safety reasons. However the end of that earth connectionis likely to be a copper rod buried within the marina, resulting in asteel-copper "battery" of about 1.1V. For such cases the use of a galvanicisolator is essential - typically 2 diodes in series, preventing any currentflow while the applied voltage is less than 1.4V (i.e 0.7V per diode), butallowing a full flow in case of an earth fault. It has been noted that therewill still be a very minor leak through the diodes which may result in slightlyfaster corrosion than normal.[edit] Factors that influence galvanic corrosion

Using a protective coating between dissimilar metals will prevent the reactionof the two metals.Relative size of anode and cathode - This is known as the "Area Effect" Asit is the anode that corrodes more quickly, the larger the anode in relationto the cathode, the lesser the corrosion. Conversely, a small anode and alarge cathode will see the anode readily damaged. Painting and plating canalter the exposed areas.Aeration of seawater Poorly aerated water can affect stainless steels,moving them more towards the anodic end of a galvanic scale.Degree of electrical contact The greater the electrical contact, the easierfor a galvanic current to flow.Electrical resistivity of electrolyte Higher resistivity of the electrolytewill decrease the current, slowing corrosion.[1]Range of individual potential difference It is possible that differentmetals could overlap in their range of individual potential differences. This


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means that either of the metals could act as the anode or cathode dependingupon the other conditions that affect the individual potentials.Covering by bio-organisms Slimes that build up on metals can affect theareas exposed as well as limiting flow rate, aeration, and altering pH.Oxides Some metals may be covered by a thin layer of oxide that is less

reactive than the bare metal. Cleaning the metal can strip this oxide and thusincrease reactivity.Humidity Can affect the electrolytic resistance and transport ions.Temperature Temperature can affect the rate resistance of metals to otherchemicals. For example, higher temperatures tend to make steels less resistantto chlorides.Type of electrolyte Exposing one piece of metal to two differentelectrolytes (either different chemicals or concentrations) can cause agalvanic current to flow within the metal.

[edit] Lasagna cellThis section does not cite any references or sources.

Please help improve this article by adding citations to reliable sources.Unsourced material may be challenged and removed. (October 2008)

A "lasagna cell" or "lasagna battery" is accidentally produced when salty foodsuch as lasagna is stored in a steel baking pan and is covered with aluminiumfoil. After a few hours the foil develops small holes where it touches thelasagna, and the food surface becomes covered with small spots composed ofcorroded aluminium.This metal corrosion occurs because whenever two metal sheets composed ofdiffering metals are placed into contact with an electrolyte, the two metals actas electrodes, and an electrolytic cell or battery is formed. In this case, thetwo terminals of the battery are connected together. Because the aluminium foiltouches the steel, this battery is shorted out, a significant electric currentappears, and rapid chemical reactions take place on the surfaces of the metal incontact with the electrolyte. In a steel/salt/aluminium battery, the aluminiumis higher on the electrochemical series, so the solid aluminium turns intodissolved ions and the metal experiences galvanic corrosion.[edit] Galvanic compatibilityOften when design requires that dissimilar metals come in contact, the galvaniccompatibility is managed by finishes and plating. The finishing and platingselected facilitate the dissimilar materials being in contact and protect thebase materials from corrosion.[2]For harsh environments, such as outdoors, high humidity, and salt environmentsfall into this category. Typically there should be not more than 0.15 Vdifference in the "Anodic Index". For example; gold - silver would have adifference of 0.15V being acceptable. For normal environments, such as storagein warehouses or non-temperature and humidity controlled environments. Typicallythere should not be more than 0.25 V difference in the "Anodic Index". Forcontrolled environments, such that are temperature and humidity controlled, 0.50V can be tolerated. Caution should be maintained when deciding for this


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application as humidity and temperature do vary from regions.[2]Anodic index[2]MetalIndex (V)

Gold, solid and plated, Gold-platinum alloy0.00Rhodium plated on silver-plated copper0.05Silver, solid or plated; monel metal. High nickel-copper alloys0.15

Nickel, solid or plated, titanium an s alloys, Monel0.30Copper, solid or plated; low brasses or bronzes; silver solder; Germansilvery high copper-nickel alloys; nickel-chromium alloys0.35Brass and bronzes0.40High brasses and bronzes0.4518% chromium type corrosion-resistant steels0.50Chromium plated; tin plated; 12% chromium type corrosion-resistantsteels0.60Tin-plate; tin-lead solder0.65Lead, solid or plated; high lead alloys0.702000 series wrought aluminum0.75

Iron, wrought, gray or malleable, plain carbon and low alloy steels0.85Aluminum, wrought alloys other than 2000 series aluminum, cast alloys ofthe silicon type0.90Aluminum, cast alloys other than silicon type, cadmium, plated andchromate0.95Hot-dip-zinc plate; galvanized steel1.20Zinc, wrought; zinc-base die-casting alloys; zinc plated1.25Magnesium & magnesium-base alloys, cast or wrought1.75Beryllium1.85

[edit] See alsoElectrochemical cellGalvanic cellSacrificial anodeCorrosionLemon batteryBattery (electricity)Galvanising

[edit] References^ "Electrical Design, Cathodic Protection". United States Army Corps ofEngineers. 1985-04-22.http://www.army.mil/usapa/eng/DR_pubs/dr_a/pdf/tm5_811_7.pdf. Retrieved2008-07-02.^ a b c Handbook of Corrosion Engineering by Pierre R. Roberge

[edit] External linksGalvanic corrosion explainedCorrosion DoctorsGalvanic Corrosion Theory and documentsGalvanic seriesElectrochemistry of corrosion From the Yeager Center at CWRU.


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Bimetallic corrosionThe Straight Dope: why does ketchup dissolve aluminium?PIRA physics lecture demonstration 5e40.25Cathodic Protection 101: A basic tutorial

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