Stress corrosion cracking is the failure of a normally ductile metal caused by the combined effect of a corrosive environment and tensile stress. Three conditions must be present for stress corrosion cracking to occur: a corrosive environment, a susceptible material, and tensile stress above a threshold level. Stress corrosion cracking initiates with cracks forming at defects on the material surface. Cracks then propagate as the passivation film at the crack tip is broken, exposing fresh reactive metal to corrosion. When the stress intensity at the crack tip exceeds the material's fracture toughness, an unstable brittle fracture occurs.