Galvanic Corrosion

Galvanic corrosion occurs when dissimilar conductive materials are electrically connected while sharing an electrolyte. The electrolyte may be seawater, rainwater, condensate, soil moisture, cooling water, process fluid, or even a thin film of contaminated humidity. The material with the lower electrochemical potential becomes anodic and dissolves faster than it would in isolation. The more noble material becomes cathodic and is partly protected.

The mechanism is the same as a battery. Electrons flow through the metallic path from the anode to the cathode, while ionic current closes the circuit through the electrolyte. At the anode, metal atoms oxidize and enter solution. At the cathode, reduction reactions consume electrons, commonly oxygen reduction in aerated water. The anodic metal loses material, which can produce local thinning, pits, leaks, fastener failure, or joint seizure.

Factors that control severity

Material pairing matters, but the galvanic series alone is not enough. The actual corrosion rate depends on the environment and geometry. A small anodic area coupled to a large cathodic area is dangerous because the anodic current is concentrated over a small surface. A stainless steel plate fastened with small carbon steel screws in seawater is a typical high-risk arrangement. The reverse area ratio is less severe because the anodic current is spread over a larger surface.

Electrolyte conductivity strongly affects the current. Galvanic corrosion is usually more severe in seawater than in pure freshwater because salts carry ionic current efficiently. Temperature, dissolved oxygen, flow velocity, deposits, crevices, and wet-dry cycling also influence the rate. Coatings can help, but a damaged coating on the anodic member can make the problem worse by exposing only a small anodic area to a large protected cathode.

Design controls

Good design starts with compatible material selection. Engineers avoid unnecessary dissimilar metal couples, select pairings close in the galvanic series for the service environment, or intentionally use a sacrificial material such as zinc coating to protect steel. Electrical isolation is another common control: insulating washers, sleeves, gaskets, and sealants can break the metallic path if they remain intact in service.

Environmental control is equally important. Drainage, ventilation, sealed joints, controlled water chemistry, corrosion inhibitors, and avoidance of stagnant crevices reduce the electrolyte exposure. In marine and buried systems, cathodic protection may be used deliberately, but it must be designed carefully because overprotection can create coating damage or hydrogen-related problems in susceptible materials.

Inspection and failure analysis

Galvanic corrosion often appears near joints, fasteners, flanges, weld attachments, inserts, and interfaces between coatings and exposed metal. Failure analysis should identify the material couple, confirm electrical continuity, characterize the electrolyte, estimate the cathode-to-anode area ratio, and separate galvanic attack from general corrosion, pitting, crevice corrosion, and stray-current corrosion.

A common mistake is to treat galvanic corrosion as a material property rather than a system condition. A metal is not simply “galvanically bad” in isolation. Risk appears when material potential, electrical connection, electrolyte, and geometry combine in a way that drives damaging current through the anodic member.