Corrosion caused by two dissimilar metals in contact in the presence of an electrolyte.
Three things must all be present: two metals with different potentials, an electrical connection between them, and an electrolyte bridging the joint. Remove any one and the corrosion stops — which is why the fix is usually a washer or a gasket rather than a different material.
The anodic metal corrodes. The accepted design rule uses the difference in anodic index: under 0.25 V is generally safe, 0.25–0.50 V only in a dry environment, and above 0.50 V the couple needs insulation.
Ignoring the area ratio. A small anode coupled to a large cathode corrodes far faster than the voltage difference alone suggests — a stainless bolt in a large aluminium plate is the classic case.
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