Which combination is considered the worst for galvanic corrosion?

• A. A large stainless steel mass with a small carbon steel mass
• B. A large stainless steel mass with a large carbon steel mass
• C. A small stainless steel mass with a large carbon steel mass
• D. A large aluminum mass with a small steel mass

Answer: (A)

Galvanic corrosion happens when two dissimilar metals are in electrical contact in an electrolyte. The more noble metal becomes the cathode and the less noble metal the anode; the corrosion rate on the anodic metal is driven by the galvanic current, which increases when the cathode area is large relative to the anode area.

Pairing a large mass of stainless steel (noble, cathode) with a small mass of carbon steel (active, anode) creates a big cathodic area feeding a tiny anode. The small carbon steel surface has to carry a relatively large current, so the current density there is high and the carbon steel dissolves rapidly, while the stainless steel stays largely protected. That combination therefore produces the most severe galvanic attack on the anodic metal, making it the worst case.

The other scenarios dilute that effect: when both metals are large, the area ratio is more balanced and the overall galvanic current is lower; when the anodic metal is large relative to the cathode, the current is spread over a bigger area and the corrosion rate per unit area on the anodic metal is reduced; and while aluminum versus steel introduces another active-passive behavior, it does not create the same extreme cathode-to-anode area advantage as the first case.