Which metal ion is listed as contributing to severe corrosion of aluminum equipment when introduced in small amounts?

• A. Copper
• B. Lead
• C. Mercury
• D. Iron

Answer: (D)

Small amounts of certain metal ions can destabilize aluminum’s protective oxide film and set up a galvanic interaction that drives rapid, localized attack on aluminum. Aluminum naturally forms a protective oxide layer, but in the presence of a more noble metal in the same electrolyte, a galvanic couple forms: the aluminum acts as the anode and dissolves, while the noble metal acts as the cathode. This accelerates corrosion of the aluminum surface, even with only trace amounts of the contaminant.

Iron is a common contaminant in process streams and environments, and it readily forms this kind of galvanic situation with aluminum. Even tiny amounts can create localized cathodic sites or deposits that promote severe corrosion of aluminum equipment. Copper and mercury can also be aggressive toward aluminum in some contexts, but the emphasis here is on iron because of its frequent presence and strong potential to cause this galvanic attack. Lead is less implicated in this particular mechanism.