sacrificialanode

A sacrificial anode is a metal piece that is deliberately more anodic (less noble) than the metal it protects, used to prevent corrosion of ferrous structures by galvanic protection. It is placed in electrical contact with the protected component and immersed in an electrolyte such as seawater, freshwater, soil, or concrete pore water.

Principle and operation

When two dissimilar metals are electrically connected in an electrolyte, a galvanic cell forms. The sacrificial anode acts as the anode and corrodes in preference to the protected metal, which becomes the cathode. The driving force for corrosion is the difference in electrochemical potential between the metals. The protection efficiency depends on factors such as surface area ratios, electrolyte conductivity, temperature, and the proximity of the anode to the protected surface.

Common materials and applications

Typical sacrificial anode materials include zinc, magnesium, and aluminum alloys. Zinc anodes are widely used to protect steel structures in seawater, such as ship hulls and offshore equipment. Magnesium anodes are common in freshwater or low-conductivity soils, where a high driving voltage is advantageous. Aluminum alloys are used in a range of marine and buried applications due to favorable performance and weight.

Design and maintenance

Anodes are sized so that their consumption rate provides sufficient protection for the coated or uncoated surface. They must be electrically connected to the protected metal, and coatings or insulating gaps should be avoided between them. Sacrificial anodes are consumables and require periodic inspection and replacement when their mass is depleted or protective potentials fall outside recommended ranges. In contrast, impressed-current systems provide active, externally powered protection and are used in situations requiring greater control.