The purpose of this guide is to provide general information about bimetallic corrosion. More detailed information can be obtained from British Standards Institution Published Document PD6484: 1979 (1). Further advice on specific issues can be obtained from the organisations listed at the end of this guide.
When a metal is immersed in a conducting liquid it takes up an electrode potential (also known as the corrosion potential). This is determined by the equilibrium between the anodic and cathodic reactions occurring on the surface and it is usually measured with reference to a standard electrode such as the saturated calomel electrode (SCE).
Bimetallic corrosion occurs when two metals, with different potentials, are in electrical contact while immersed in an electrically conducting corrosive liquid, Because the metals have different natural potentials in the liquid, a current will flow from the anode (more electronegative) metal to the cathode (more electropositive), which will incease the corrosion on the anode, see Figure 1
This additional corrosion is bimetallic corrosion. It is also referred to as a galvanic corrosion, dissimilar metal corrosion or contact corrosion.
In general, the reactions which occur are similar to those that would occur on single, uncoupled metal, but the rate of attack is increased, sometimes dramatically. With some metal combinations the change in the electrode potential in the couple potential can induce corrosion which would not have occured in the uncoupled state (e.g. pitting). In some environments the change in potential of the cathode in the couple can also introduce problems (e.g. hydrogen embrittlement).