Galvanic corrosion is most rapid and severe when which relationship between anodic and cathodic surface areas exists?

• A. Anodic surface area smaller than cathodic surface area
• B. Anodic surface area larger than cathodic surface area
• C. Surface areas are equal
• D. No contact between metals

Answer: (A)

When two different metals are in electrical contact in an electrolyte, one becomes the anode and dissolves while the other acts as the cathode. The rate of the anodic dissolution depends on how much current can flow between them, which is influenced by the relative surface areas. If the cathodic area is much larger than the anodic area, the circuit can carry more current, and that current is concentrated over the small anodic surface. This high current density on a small area drives rapid and aggressive corrosion of the anodic metal. If the anodic area were larger, the current density on that area would be lower, so the rate per unit area would be less; equal areas yield intermediate effect; and if there’s no contact, galvanic corrosion doesn’t occur.