Managing Galvanic Corrosion

If proper design, material selection, implementation, and maintenance steps are followed, it is relatively simple to avoid the occurrence of galvanic corrosion in a new system. MIL-STD-889 (active) is a DOD standard on dissimilar metals. The purpose of this standard is to define and classify dissimilar metals and establish requirements for protecting coupled dissimilar metals in all military equipment parts, components and assemblies. To further aid in properly avoiding this form of corrosion, Table 12 provides a brief list of guidelines to minimize galvanic corrosion. Some of these are explained in more detail in the sections to follow.

Area Effects

Taking into account the relative areas of galvanically coupled metallic systems can minimize galvanic corrosion. The size of the cathodic metal in the bimetallic system should not be significantly larger than the size of the anodic metal, since this would cause a greater degree of corrosion of the anodic member. Instead, the anodic metal should have an equal or larger area. For example, the more noble metal should be used for rivets, bolts and other fasteners, thus making the area of the anode much greater than that of the cathodic component. 

Cathodic Protection

Galvanic corrosion can be intentionally induced in order to protect a more important metallic component. This method of protection involves using a highly active metal, one that is lower on the galvanic series, to be sacrificially corroded. This sacrificial anode protects the more important, cathodic metal from corrosion. Magnesium and zinc are commonly used as sacrificial anodes. Sacrificial anodes are often replaced in-service as they are consumed through galvanic corrosion, as intended.

Insulate Dissimilar Metals

Electrically resistive, non-metallic materials can be used to insulate two dissimilar metals. This in effect, breaks the electrical connection or at least increases the electrical resistivity resulting in a reduction, if not elimination, of the potential for galvanic corrosion.

Coatings

Metallic coatings are commonly used to protect bimetallic systems against galvanic corrosion. These coatings can provide protection by acting as barriers to corrosion or by readily accepting corrosion, thereby saving the important metal component from being corroded. For example, zinc is often used as a coating for steel, and since it is not very corrosion resistant, it will corrode preferentially to protect the steel. Thus, the zinc coating acts as a sacrificial anode.

Noble metal coatings are typically used as barrier coatings, since they are relatively unreactive. These coatings can isolate the important metal from the surrounding environment; however, pores, defects, or damaged areas in these barrier coatings are areas still susceptible to being galvanically corroded. Furthermore, the areas under these discontinuities (also known as holidays) in the coating system are likely to be targeted for severe localized corrosion. In addition, if the anodic metal in the galvanically coupled system is coated with a barrier coating without coating the cathodic member as well, it can have severe negative effects due to the reduced anodic area. Moreover, if the anode is coated, while the cathode is not, the former cathode may become anodic to the former anode.

Crevices

Threaded joints with dissimilar metals that are far apart on the galvanic series should be avoided. It is recommended that crevices be sealed either by welding or brazing to protect against galvanic corrosion.