Predicting Galvanic Corrosion

Predicting galvanic corrosion is a serious problem. For contacts between common metals, in particular between ordinary steel and stainless steel, experience shows that laboratory testing always leads to more severe results than what is actually observed under conditions of weathering. 

Most laboratory tests, including electrochemical tests, say nothing about the intensity of possible galvanic corrosion of aluminium in contact with other metals. 

In practice, for constructions exposed to bad weather, laboratory tests can predict a possible risk of galvanic corrosion and determine the efficiency of the planned protection. 

However, for common situations of heterogeneous contacts, the risk of galvanic corrosion can only be assessed by professional experience. Over the years, prescriptions concerning the protection of heterogeneous contacts have been progressively adapted on the basis of accumulated experience. 

Of course, this is not an issue for immersed assemblies that must be protected in any case. Several methods are available for the measurement of galvanic corrosion between aluminium and other metals or alloys. 

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Guide for development and use of a galvanic series for predicting galvanic corrosion performance... 

ASTM G82-83, Standard Guide for Development and use of a Galvanic Series for Predicting Galvanic Corrosion Performance, Philadelphia, PA, 1983. 

R. Baboian, Predicting Galvanic Corrosion Using Electrochemical Techniques, Electrochemical Techniques for Corrosion Engineering, R. Baboian, Ed., National Association of Corrosion Engineers, 1986... 

Using electrochemical tests to predict galvanic corrosion performance has the same caveats as in any other type of predictive testing. Conditions must be as close as possible to those being modeled. This includes not only solution chemistry and flow, but also the surface condition of the samples. Polished samples can behave quite differently from samples with more representative surface conditions emd corrosion products developed by pre-exposure. 

Baboian, R., Electrochemical Techniques for Predicting Galvanic Corrosion, Galvanic and Pitting Corrosion—Field and Laboratory Studies, ASTM STP 576, R. Baboian, W. France, Jr., L. Rowe, and J. Rynewicz, Eds., ASTM International, West Conshohocken, PA, 1976, pp. 5-19. 

Each environment requires a different galvanic series, for example, a galvanic series in static seawater cannot be used to predict galvanic corrosion in turbulently flowing seawater. 

Figure 12.13 illustrates the comparison of the BEM model predicted galvanic corrosion and the experimental measurement of the total corrosion for AZ91D coupled with a 10mm steel iusert. Both the BEM model and the experimental measurements indicate maximum corrosion at the Mg-steel interface, and both indicated a decrease in corrosion rate with increasing distance from the interface. 

Laboratory tests for evaluating/predicting galvanic corrosion f l into two categories electrochemical tests, in which the data are analyzed and reported in a way that assists galvanic corrosion predictions (e.g., potential measurements and polarization measurements) and specimen exposures, which can or cannot be electrochemically monitored (e.g., immersion tests... 

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