Stress Corrosion Cracking

Stress corrosion cracking (SCC) results from a combination of tensile stress and corrosion. Initiation of stress corrosion cracks usually begins at a small surface corrosion pit (see pitting corrosion) that is subjected to tensile stress. The tensile stress “stretches” the opposite sides of the pit apart which exposes new material at the bottom of the pit to further corrosion. As this “corrosion – tensile stress cycle” continues, the resulting separation grows into a crack which penetrates further and further into the part until complete fracture occurs.

The tensile stress that contributes to SCC is typically significantly lower than that required to produce a tensile fracture, however, the continuing corrosion process weakens the metal at the advancing crack front to the point at which it fractures under this tensile stress.

Naturally, a corrosive environment is required for stress corrosion cracking to occur. This environment may be extremely subtle and can range from mildly acidic rain to the highly concentrated chloride road salts. Different types of material are affected differently by various environments. Stress corrosion cracking occurs in carbon, alloy and stainless steels, from exposure to chlorides. Copper alloys such as brass and bronze are susceptible to SCC in chloride or ammonia environments.

Stress Corrosion Cracking is easily mistaken for other failure modes Analysis of SCC should be performed by engineers with Failure Analysis experience in this fracture mechanism.