ATO Corrosion Practice Exam

Crevice corrosion is driven by a localized chemistry in a confined space where solution becomes stagnant. When a crevice or gap traps fluid, diffusion of fresh oxygen into that tiny volume is limited, so the oxygen inside gets depleted while metal ions released from the metal accumulate. This creates an electrolyte inside the crevice that is poorer in oxygen and richer in metal ions than the solution outside. The inside of the crevice becomes anodic and dissolves, while the surrounding metal acts as a cathode, sustaining a localized attack. The result is a focused corrosion attack right at the crevice boundaries. Other types describe different situations. Filiform corrosion happens under a coating, not in a trapped crevice. Fretting corrosion arises from repetitive mechanical wear at contact points under load. Hot corrosion occurs at high temperatures, typically with molten salts.