Galvanic Corrosion

Description

Accelerated corrosion which can occur when dissimilar metals are in electrical contact in the presence of an electrolyte (i.e. conductive solution). An example of this corrosion phenomenon is increased rate of corrosion of steel in seawater when in contact with copper alloys. Galvanic attack can be uniform in nature or localized at the junction between the alloys depending on service conditions. Galvanic corrosion can be particularly severe under conditions where protective corrosion films do not form or where they are removed by conditions of erosion corrosion.

Prevention or Remedial Action

• selection of alloys which are similar in electrochemical behavior and/or alloy content.
• area ratio of more actively corroding material (anode) should be large relative to the more inert material (cathode).
• use coatings to limit cathode area.
• insulate dissimilar metals.
• use of effective inhibitor.

Standard Test Methods

• ASTM G-71 - guide for conducting and evaluating galvanic corrosion tests in electrolytes.
• ASTM G-82 - guide for development and use of a galvanic series for predicting galvanic corrosion performance.
• ASTM G-104 - test method for assessing galvanic corrosion caused by the atmosphere.

Evaluation for Galvanic Corrosion

Many people utilized the standard galvanic series of materials in seawater to predict service performance relative to galvanic corrosion. In fact, this galvanic series is specific to only seawater at near ambient conditions. Other factors such as temperature and the presence of other chemical species can greatly affect the rank ordering of materials. Such differences in environmental conditions can reverse galvanic couples whereby the material expected to be the cathode may actually be the anode and experience severe corrosion.

In making galvanic corrosion measurements, it is good practice to try to separate the effects if crevices between contacting materials and actual galvanic corrosion. This is the reason that in many tests, the actual electrical coupling of the two materials is performed in a region protected from the environment or externally from the environment. The external coupling is a good idea since it allows for measurement of the mixed potential of the couple and the galvanic corrosion current. While the potential serves as a measure of the thermodynamic driving force for galvanic corrosion, it is the galvanic corrosion current that indicates the acceleration of corrosion by the influence of the galvanic couple.