galvanic corrosion

Galvanic Corrosion:

Galvanic corrosion tends to occur when dissimilar conducting materials are connected electrically and exposed to an electrolyte. The following fundamental requirements therefore have to be met for galvanic corrosion:

Dissimilar metals (or other conductors, such a graphite).
Electrical contact between the dissimilar conducting materials
(can be direct contact or a secondary connection such as a common grounding path).
Electrolyte (the corrosive medium) in contact with the dissimilar conducting materials.

When placed in an electrolyte, different metals/alloys assume different corrosion potentials (refer to the galvanic series in seawater). It is this potential difference that is the driving force for galvanic current flow. The less noble material in the galvanic couple will become the anode and tend to undergo accelerated corrosion, while the more noble material (acting as a cathode) will tend to experience reduced corrosion effects. However, in some cases, chemical species produced at the cathode can degrade certain cathode materials.

It is not only the potential difference that controls the magnitude of galvanic current flow. The total resistance to current flow (see schematic of a simple corrosion cell, with both arrows showing conventional current direction) also needs to be considered, which can be broken down further into the following individual resistance components:

Resistance to (ionic) current flow of the electrolyte.
Resistance to current flow (by electron conduction) in the conducting materials and in the connection between them.
Resistance associated with polarization behavior of the anode.
Resistance associated with polarization behavior of the cathode (cathode efficiency).

Galvanic Corrosion Factors (Variables)

A collection of factors affecting galvanic corrosion of metals presented by Oldfield (see references below) includes the following:

Type of joint:
welded, fasteners, separated but with external connection

Total geometry:
area ratio (see further comments below), distances involved, surface shape, surface condition, number of galvanic cells

Bulk solution properties:
oxygen content, pH, conductivity, corrosivity, pollutant level

Bulk solution environment:
temperature, flow rate, volume, height above surface

Mass Transport:
migration, diffusion, convection

Protective film characteristics:
dependence on electrolyte conditions and potentials

Alloy Composition:
major and minor

Reaction kinetics:
metal dissolution, oxygen reduction overvoltage, hydrogen evolution overvoltage

Electrode potentials:
galvanic potential between metals, standard electrode potentials (the latter as a "rough" guide only)

A listing of factors (and sub-factors) by Hack was broken down into three main categories:

Material Properties:
composition, processing history, surface condition

Environment:
composition, temperature, duration, flow

Geometry:

Further variables were attributed to the above sub-factors, for example conductivity, pH, biological constituents for the environment composition sub-factor.

The following have been described as "main factors" influencing galvanic corrosion rates in Skanaluminium's on-line publication "Alubook - Lexical knowledge about aluminium".

Potential Difference between materials

Cathode Efficiency

Surface areas of connected materials (area ratio)

Electrical resistance of the connection between the materials and of the electrolyte.

Galvanic current flow can be monitored using electrochemical sensors and the zero resistance ammetry (ZRA) technique. Note that the area ratio of the anode:cathode is an important variable affecting the dissolution current density (and hence corrosion rate) pertaining to the anode. The area ratio is also important when considering the relative amount of current "available" from the cathodic reaction.

Effect of area ratio - schematic
(full coverage of the electrodes by corrosive electrolyte is assumed)
click on image to enlarge

The principle of galvanic current flow can be illustrated in a fruit powered electronic clock. By immersing copper (cathode) and zinc (anode) electrodes in the acidic electrolyte of selected fruit, a galvanic current can be generated that powers up a digital clock.

Click on image to enlarge

The above image, kindly provided by Mr. Ken Cuthbertson, shows a pear as the corrosive electrolyte; (apples, oranges, bananas and tomatoes can also do the trick).

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Link to an historical example of galvanic corrosion on the high seas.

References/Literature:

M.G. Fontana: Corrosion Engineering, McGraw-Hill, New York, 1986.

C.P. Dillon (Ed.): Forms of Corrosion - Recognition and Prevention: NACE Handbook 1, Vol. 1, NACE International, Houston, 1982.

REMR Technical Note EM-PC-1.1: Forms and Causes of Galvanic Corrosion in the Coastal Environment.

ASTM G82 Standard: Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance, ASTM International, West Conshohocken (PA).

ASTM G71 Standard: Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes, ASTM International, West Conshohocken (PA).

H.P. Hack: "Galvanic Corrosion Test Methods" (Corrosion Testing Made Easy, Volume 2), NACE International, Houston, 1993.

J.W. Oldfield: "Electrochemical Theory of Galvanic Corrosion", in "Galvanic Corrosion" - ASTM STP 979 - H.P. Hack Ed., ASTM International, West Conshohocken (PA), 1988.

Links:
Example of an unfavorable area ratio in galvanic corrosion.

Historical discoveries - galvanic current