What is the term for the lifting or flaking of metal surfaces due to delamination of grain boundaries from corrosion products?

• A. Exfoliation
• B. Pitting
• C. Spalling
• D. Erosion

Answer: (A)

Exfoliation describes a corrosion process where corrosion products build up between the grains of a metal, pushing the grain boundaries apart and causing entire layers to lift off or peel away in sheet-like flakes. This creates a flaky, laminated surface as the outer layers delaminate from the underlying metal. It's most noted in certain alloys, like some aluminum alloys, where the swelling of corrosion products between grains drives the peeling action.

Pitting is localized tiny holes from focused attack, so it doesn’t involve lifting whole layers. Spalling refers to the shedding of surface chunks due to mechanical or thermal stresses, not specifically grain-boundary delamination from corrosion products. Erosion is material loss from abrasive or fluid flow impacts rather than corrosion-driven delamination.