Galvanic corrosion is likely to be most rapid and severe when which condition applies?

• A. The anodic area is larger than the cathodic area
• B. The anodic area is smaller than the cathodic area
• C. The metals are identical in area
• D. No galvanic corrosion occurs

Answer: (B)

Galvanic corrosion happens when two dissimilar metals are in electrical contact in an electrolyte; one metal acts as the anode and dissolves, while the other is the cathode and is protected. The rate of this corrosion depends on the relative sizes of the anodic and cathodic areas. When the anodic metal is small and the cathodic metal is large, the same current must pass through a small area, creating a high current density on the anodic surface. This concentrates dissolution of the anodic metal, making the corrosion rapid and severe. The large cathodic area readily accepts electrons, sustaining the reaction. If the anodic area were larger than the cathodic area, the current density on the anodic surface would be lower, so the corrosion would be less aggressive per unit area. If the metals are identical in area, or if there is no electrolyte or no electrical contact, galvanic corrosion is minimized or does not occur.