When two electrically connected metals with different electrochemical tendencies share an electrolyte, they can form a galvanic couple. The member that more readily oxidizes under those conditions tends to become the anode and dissolve faster, while the other member supports more of the cathodic reduction.

The severity is not determined by material pairing alone. Geometry matters because total cathodic current must be supplied by anodic dissolution.

A particularly unfavorable configuration is a small anode connected to a large cathode. If the same total current is concentrated onto a small anodic area, the local corrosion current density and penetration rate can become large.

Conversely, deliberately attaching a more easily oxidized material can protect a structure. A sacrificial zinc or magnesium anode corrodes preferentially while shifting steel toward cathodic behavior.

Electrical insulation between dissimilar metals can break the galvanic circuit. Coatings can also help, but coating only the anodic member can be dangerous if small coating defects expose tiny anodic areas coupled to a large coated cathodic structure.

Galvanic design therefore requires considering electrode behavior, electrical contact, electrolyte access and anode-to-cathode area ratio together.