Galvanic Corrosion,Evaluation of Galvanic Corrosion,Galvanic Corrosion,Evaluation of Galvanic Corrosion
Introduction
GALVANIC CORROSION, although listed as one of the forms of corrosion, should instead be considered as a type of
corrosion mechanism, because any of the other forms of corrosion can be accelerated by galvanic effects. Therefore, any
of the tests used for the more conventional forms of corrosion, such as uniform attack, pitting, or stress corrosion, can be
used, with modifications, to determine galvanic-corrosion effects. The modifications can be as simple as connecting a
second metal to the system or as complex as necessary to evaluate the appropriate parameters. A change in the method of
data interpretation is often all that is needed to convert conventional test methods into galvanic-corrosion tests.
This article will discuss component, model, electrochemical, and specimen tests. Additional information on galvanic
corrosion can be found in the article "General Corrosion" in this Volume.
Component Testing
Component testing is an especially useful technique for galvanic corrosion prediction. The materials in a system are often
selected primarily for reasons other than galvanic compatibility. In complex components, such as valves or pumps, many different materials can be used in a geometric configuration that is extremely difficult to model. In more complicated
cases, even the most basic prediction, such as which materials will suffer increased corrosion due to galvanic effects, may
not be possible from simple laboratory tests. Therefore, component testing becomes the best method for predicting
material behavior in complex systems.
Conducting component tests for galvanic corrosion is similar to conducting component tests for any other type of
corrosion. The same care must be taken to ensure that the materials, the operation of the component, and the environment
are similar to those in service. However, one important difference with regard to galvanic corrosion is the relationship
between the component being tested and the other elements of the system. For example, it would be a waste of effort to
expose a complicated piece of machinery in order to look for galvanic corrosion when the whole device is cathodically
protected as a result of being attached to a protected structure. Alternatively, incorrect results would be obtained for the
exposure of an isolated bronze mixed-material valve when the ultimate use was in a piping system made of a more noble
metal that could accelerate the corrosion of the entire valve galvanically. When outside interactions of this type are
possible, the interacting materials must be made part of the corrosion system by exposing the appropriate surface area of
those materials electrically connected to, and in the same electrolyte as, the component being tested.
The principal advantages of component testing are ease of interpretation of results and the lack of scaling or modeling
uncertainties. The disadvantages include high cost and the need for extremely sensitive measures of corrosion damage to
obtain results within reasonable time periods.
Modeling
Even when the galvanic behavior of panels of the materials of interest is known, the geometrical arrangement of these
materials may make galvanic corrosion prediction difficult because of the effects of solution (electrolyte) resistance on
the corrosion rates. An example of this is a heat-exchanger tube in a tubesheet. Assuming the tube to be anodic to the
tubesheet, areas of the tube near the tubesheet will have low solution resistance to the cathode and will corrode rapidly,
but areas away from the tubesheet will have a large solution resistance to the cathodic metal and will therefore corrode
more slowly. In the reverse case, in which the tubesheet is anodic to the tube, the areas of the cathodic tube near the
tubesheet will drive the galvanic corrosion of the tubesheet much more than distant areas will.
Computer Modeling. Geometrical effects can be modeled in computers by using such techniques as finite elements,
boundary elements, and finite differences. The best computer models solve a version of the Laplace equation for the
electrolyte surrounding the corroding materials and use the polarization behavior of the material in question as boundary
conditions at the metal/electrolyte interface. The analysis is similar to the heat flow analysis, with potential analogous to
temperature, current analogous to heat flux, and the polarization boundary condition analogous to a special nonlinear type
of temperature-dependent convective flux.
The only data this type of model requires are the geometry, electrolyte conductivity, and polarization characteristics of the
materials involved. The program generates potentials and current densities as a function of location, either of which can
be related back to corrosion rate. The nonlinear boundary conditions make this type of computer modeling difficult to
perform unless a large mainframe computer with sufficient computational capabilities is available. Computer modeling
provides an excellent predictive tool for geometrical effects; however, it is still seen as less satisfying than physical scale
model exposures.
Physical scale modeling must model the solution resistance effects and the relative effects of polarization resistance
and solution resistance to obtain accurate geometrical predictive capability. When solution resistance is important, the
best type of scale modeling is the scaled conductivity exposure. In this type of exposure, the model is reduced in size by
some factor from the original. To maintain proper potential and current distribution scaling, the electrolyte conductivity
must also be reduced by the same factor. Any resistive coatings, such as paints, must also have their conductivity scaled
similarly. In the case of paints, this can be accomplished by applying a thinner layer, by the same scaling factor used for
size, than the thickness used in practice.
For example, a one-tenth scale model of a heat exchanger designed to operate in seawater with a conductivity of 4
mho/cm should be placed in seawater diluted to a conductivity of 0.4 mho/cm. In this case, the observed potential and
current distributions will be the same between the model and the full-scale heat exchanger. For physical scale modeling,
measurements that can be taken include potential distribution by the use of a movable reference electrode, corrosion depth
as a function of location, and, if the model design permits, current to different parts of the structure and mass loss of
certain model components.

Although less expensive than full-scale component testing, physical scale modeling has many of the disadvantages of
component testing. In addition, a great inaccuracy in conductivity scaling stems from the fact that the polarization
resistance of the materials in the system of interest is often a function of solution conductivity. Thus, changing solution
conductivity may influence polarization resistance sufficiently to make the results of the model inaccurate. There is no
experimental way to avoid this shortcoming.
Laboratory Testing
Laboratory tests fall into two categories: electrochemical tests, in which the data are analyzed and reported in a way that
assists galvanic-corrosion predictions, and specimen exposures, which may or may not be electrochemically monitored.
Electrochemical Tests
The use of electrochemical techniques to predict galvanic corrosion is summarized in Ref 1. The details that relate to
testing techniques are discussed below.
Galvanic Series. When the only information needed is which of the materials in the system are possible candidates for
galvanically accelerated corrosion and which will be unaffected or protected, the information from a galvanic series in the
appropriate media is useful. Such a series is a list of freely corroding potentials of the materials of interest in the
environment of interest arranged in order of potential (Fig. 1). The galvanic series is easy to use and is often all that is
required to answer a simple galvanic-corrosion question. The material with the most negative, or anodic, corrosion
potential has a tendency to suffer accelerated corrosion when electrically connected to a material with a more positive, or
noble, potential. The disadvantages include:
· No information is available on the rate of corrosion
· Active-passive metals may display two, widely differing potentials
· Small changes in electrolyte can change the potentials significantly
· Potentials may be time dependent

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