Galvanic Corrosion

Galvanic corrosion occurs when two metals having different electrical potentials (dissimilar metals) are electrically connected, either through physically touching each other or through an electrically conducting medium, such as an electrolyte. Systems meeting these requirements essentially form an electrochemical cell which will conduct electricity. The induced electrical current can then attract electrons away from one of the metals, which thus acts as the anode in the electrochemical cell. This usually results in acceleration of the rate of corrosion of the anode. The opposing metal, the cathode, will consequently receive a boost in its resistance to corrosion, since it can supply any imminent corrosion reactions with electrons from an external source. Galvanic corrosion is usually observed to be greatest near the surface where the two metals are in contact. Figure 9 shows galvanic corrosion on a metal component near a dissimilar metal fastener.

In general, corrosion is the result of an electrochemical reaction that occurs between an anode and a cathode. In the case of uniform corrosion, the metal being corroded acts as both the anode and the cathode in the reaction, where localized areas on the surface of the metal have slightly different electrical potentials. However, galvanic corrosion occurs between two dissimilar metals. The metal with a lower potential relative to the other metal acts as the anode, while the metal with a higher potential acts as the cathode. The corrosion reaction/corrosion current (flow of electrical current) is driven by an electrical potential gradient. Some typical electrical potentials for some common metallic elements are shown in Table 8. (Note – these potentials were taken in standard conditions, but actual potentials vary in metals and alloys, especially under various environmental conditions.)