Corrosion that occurs when two dissimilar metals are electrically connected in the presence of an electrolyte, causing the more anodic (less noble) metal to corrode preferentially.

Principle

Electrical contact between dissimilar metals establishes a galvanic cell driven by the difference in their electrochemical potentials (the galvanic series). The metal with the more negative potential becomes the anode and dissolves; the cathodic metal is protected. The rate is strongly influenced by area ratio (large cathode/small anode accelerates anode loss), electrolyte conductivity, and environmental factors.

Demonstration

A steel bolt contacting a copper plate in seawater: steel (more anodic) corrodes preferentially while copper becomes cathodic. Deliberate use of sacrificial anodes (zinc on ship hulls) protects a structure by offering a more anodic metal that corrodes instead.

Misapplication

Assuming galvanic corrosion will occur regardless of electrical isolation or electrolyte (it requires both electrical contact and conductive electrolyte); ignoring area ratios and thinking any dissimilar metal contact is harmful; misplacing sacrificial anodes so that protected area becomes the anode if area ratios are unfavorable.

Consequence

Can cause rapid degradation of the anodic metal, leading to structural weakness or leaks. Proper design can exploit galvanic principles for protection (sacrificial anodes, insulation, selecting metals close in potential, coatings). Assessment and mitigation require considering potentials, area effects, and electrolyte properties.

Reversal

Reversal is electrical isolation or matching potentials so no galvanic couple forms: disconnected or insulated dissimilar metals in a nonconductive medium do not produce galvanic corrosion. Alternatively, deliberately connecting a more anodic sacrificial metal reverses damage onto that anode.

Boundary

Requires an electrochemical circuit: (1) galvanically dissimilar metals electrically connected, (2) an electrolyte providing ionic conduction, and (3) spatial separation of anodic and cathodic sites. It excludes purely chemical corrosion without electron flow and nonconductive environments where ionic current cannot flow.

Semantic Tension

Terminology overlaps with 'bimetallic corrosion' and sometimes with contact corrosion; tension arises in distinguishing galvanic effects from local electrochemical heterogeneities (e.g., compositional differences within a single alloy) and from mechanically assisted corrosion processes.

Synthesis

Galvanic corrosion is the preferential electrochemical dissolution of the anodic metal when dissimilar metals are electrically coupled in an electrolyte; predictable by potential differences and area effects and manageable by isolation, material choice, or sacrificial protection.