ATO Corrosion Practice Exam

Pitting corrosion is a localized form of attack that creates small, deep pits at discrete spots on the metal surface. It starts when the protective oxide or passive film breaks down in a localized area, often underchloride conditions, moisture, or stress. Once a pit forms, it concentrates corrosive species and water inside the pit, driving deep penetration while the rest of the surface may look relatively untouched. In thin sheet metal, these pits can probe through the thickness, leading to perforation and a hole that can compromise strength or containment. This is why perforation is the key consequence: the damage is localized yet capable of sudden failure, unlike uniform thinning from uniform corrosion. Discoloration can accompany pits, but the critical failure mode is the through-thickness hole that perforates the sheet.