Intergranular corrosion tends to present with what observable by-product behavior?

• A. By-products lifting grains apart at boundaries
• B. Pitting within grains
• C. Delamination along grain boundaries
• D. Threadlike filaments under coating

Answer: (A)

Intergranular corrosion is attack that follows the grain boundaries, weakening the bonds where grains meet. As this boundary attack progresses, corrosion products and dissolved metal can accumulate between grains, creating pressure and reducing the cohesion holding neighboring grains together. The observable by-product effect is that grains appear to be lifted apart or pushed away from each other at the boundaries due to these products and the loss of boundary strength. This lifting or separation along boundaries is a direct sign of the boundary-focused attack.

Pitting within grains is more typical of corrosion that targets the grain interiors rather than the boundaries, so it doesn’t reflect intergranular behavior. Delamination along grain boundaries is a related mechanical consequence, but the by-product behavior described here—grains being lifted apart by boundary products—best captures the hallmark observable of intergranular corrosion. Filiform corrosion under coatings is a different phenomenon tied to under-coating attack rather than boundary-specific degradation.