galvanic action

Galvanic action is the electrochemical corrosion that occurs when two metals of different electrode potentials are placed in electrical contact, usually in the presence of an electrolyte such as water, salt solution, or moist soil. One metal, the more active one, acts as an anode and corrodes preferentially, while the other, more noble metal acts as a cathode and is protected. The result is not merely that both metals corrode at their normal rates, but that the active metal corrodes far more rapidly than it would alone.

The driving force behind galvanic action is the difference in electrode potential between the two metals. Zinc and steel form a classic pair: zinc has a standard electrode potential of about -0.76 volts relative to the standard hydrogen electrode, while steel sits closer to -0.44 volts. In a galvanic couple immersed in seawater or brackish water, the zinc corrodes away while the steel surface remains largely protected. This is not accidental; it is the basis of cathodic protection systems used in offshore pipelines, ship hulls, and storage tanks.

The severity of galvanic corrosion depends on several factors: the magnitude of the potential difference, the relative surface areas of the two metals, the electrical resistance of the path between them, and the conductivity of the electrolyte. A small area of noble metal coupled to a large area of active metal will cause rapid, localized attack on the active metal. Conversely, a large area of active metal coupled to a small cathode produces more uniform but less intensive corrosion. In humid tropical climates or near salt water, the electrolyte conductivity is high and galvanic action proceeds quickly.

Recognition and control

Galvanic corrosion is most dangerous when it occurs in hidden locations: in lap joints, under washers, inside crevices, or at fastener holes where water can pool. Stainless steel fasteners used in carbon steel structures, copper piping soldered to steel tanks, or aluminum trim attached to steel vehicles are all prone to failure from this mechanism. The corrosion can be visually distinctive, sometimes forming white or colored salt deposits around the joint.

Prevention relies on isolation or material selection. Insulating washers, plastic sleeves, or coatings of bitumen or epoxy paint can interrupt the electrical path. When coupling dissimilar metals is unavoidable, the metals chosen should be close in the electrochemical series, or a sacrificial anode of truly active metal such as zinc or magnesium can be deliberately introduced to draw the corrosive attack away from the structural metals. In seawater systems, zinc anodes are replaced every 3 to 5 years depending on water temperature and current demand.