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Galvanic corrosion and stainless steel

When two different metals are in contact in an electrolyte, the less noble one (the anode) corrodes while the more noble one (the cathode) is protected — this is galvanic corrosion, deliberately used with sacrificial zinc anodes on boats. Steel becomes stainless with at least about 10.5% chromium, which forms a self-healing, passive oxide layer (Cr2O3).

anode (less noble):corrodescathode (more noble):protected\text{anode (less noble)}: \text{corrodes} \quad \text{cathode (more noble)}: \text{protected}galvanic corrosion

Example

A zinc anode connected to a steel hull sacrifices itself: the zinc corrodes instead of the steel.

Avoid connecting very different metals directly in damp environments, unless one is meant to be sacrificed.
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Part of Materials Technology: Heat treatment and corrosion.