Intergranular corrosion is defined as an attack on the grain boundaries of alloys under specific conditions. Which term best describes this phenomenon?

• A. Grain boundaries
• B. Grain interiors
• C. Surface oxide layer
• D. Pores

Answer: (A)

Intergranular corrosion occurs when the corrosive attack follows the interfaces between grains in a metal, not through the grain interiors. The best term to describe this phenomenon is grain boundaries, since the attack concentrates along those boundaries under the right conditions. Grain boundaries can be more electrochemically active due to factors like segregation of alloying elements, precipitate formation, or sensitization that depletes certain elements at the boundary, creating ready-made pathways for corrosion in some environments. This is different from intragranular corrosion, which would occur inside the grains; from corrosion limited to a surface oxide layer, which is about the surface film rather than the internal boundaries; or porosity-related attack, which follows voids rather than the crystal interfaces.