Galvanic and stress corrosion

Corrosion is the deterioration of a metal by chemical or electrochemical reaction with its environment.

Galvanic corrosion

When two dissimilar metals are in electrical contact in the presence of an electrolyte (water, moisture), a galvanic cell forms. The more active metal (anode) corrodes and the more noble metal (cathode) is protected.

• Anode: $M \rightarrow M^{n+} + n e^-$
• Cathode (in neutral water): $O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$

The metals' positions in the galvanic series decide which is the anode (e.g., Mg, Zn and Al are anodic to steel; steel is anodic to copper and stainless steel).

Corrosion is more severe with a larger potential difference, a small anode area compared with the cathode area, and a highly conductive electrolyte.

Example: steel bolts used on a copper plate, or an aluminium sheet riveted with steel rivets in sea water.

Prevention: select metals close together in the galvanic series, insulate them from each other, keep the anode area large, apply coatings, or use sacrificial anodes.

Stress corrosion cracking (SCC)

SCC is brittle cracking of a normally ductile metal under the combined action of tensile stress (applied or residual) and a specific corrosive environment.

At the crack tip, the stress breaks the protective film and the exposed metal dissolves, so the crack advances (intergranularly or transgranularly) with little overall metal loss.

Example: season cracking of brass cartridge cases, and chloride SCC of 304 stainless steel pipes.

Prevention: stress-relief annealing, shot peening, changing the alloy, controlling the environment.