What is intergranular corrosion and which alloys are at risk?

• A. Attack along grain boundaries caused by uniform corrosion
• B. Localized pits within grains
• C. Exfoliation due to surface layers
• D. Attack along grain boundaries often due to improper heat treatment or impurities; susceptible in some aluminum alloys (e.g., older 2024/7075 variants) and stainless steels

Answer: (D)

Intergranular corrosion is corrosion that travels along the grain boundaries of a metal rather than through the grain interiors. This happens when boundaries become susceptible due to processing or impurities—such as when elements are depleted at the boundary or sensitizing precipitates form there. In stainless steels, chromium carbide precipitation at high temperatures depletes chromium near boundaries, making them prone to attack. In some aluminum alloys, particularly certain heat-treatable ones, aging or impurities can create boundary-weakness that favors corrosion along the grain interfaces.

The option correctly states that the attack occurs along grain boundaries and is linked to improper heat treatment or impurities, with specific risk for some aluminum alloys (like older 2024/7075 variants) and stainless steels. Other choices describe different corrosion modes—uniform attack across the surface, pits inside grains, or exfoliation as a surface-layer phenomenon—that do not define intergranular corrosion.