Extruded aluminum alloy is particularly susceptible to which type of corrosion?

• A. Fretting corrosion.
• B. Intergranular corrosion.
• C. Dissimilar metal corrosion.
• D. Stress corrosion.

Answer: (B)

Intergranular corrosion is the attack that travels along the boundaries between grains in a metal. In extruded aluminum alloys, the extrusion process creates elongated grains and, with heat treatment, can produce precipitates along grain boundaries or deplete solute at those boundaries. This makes the grain boundaries anodic relative to the grain interiors, so in the presence of an electrolyte, corrosion tends to initiate and propagate along those boundaries, producing intergranular attack. Other forms—fretting from mechanical wear at contact points, dissimilar-metal galvanic corrosion, or stress-assisted cracking under load—are not specifically tied to the extrusion-induced grain-boundary structure in aluminum.