Stress Corrosion Cracking

Stress corrosion cracking (SCC) is a delayed failure mode where a ductile metal fractures under the combined action of tensile stress and a specific corrosive environment. It initiates at the surface, often from machining or welding residual stress, and propagates without visible warning, even below yield strength. This intergranular or transgranular crack growth can lead to sudden failure in susceptible alloys like aluminum in saltwater or stainless steel in chlorides.

Stress corrosion cracking (SCC) is a delayed failure mode where a ductile metal fractures due to the combined action of tensile stress and a specific corrosive environment, even below yield strength. In CNC machining, the risk is shaped by the near-surface residual stress field from cutting, grinding, and finishing operations. Aggressive machining, tearing instead of shearing, and thermal damage leave a more tensile surface condition unfavorable for SCC resistance. In welded aluminum, high-magnesium 5XXX alloys are sensitive under saltwater conditions. For stainless steels, chlorides, elevated temperatures, and residual stress are the classic setup around weld toes and stress concentrators. On millwork floors, metal hardware and fasteners exposed to moisture, salts, or dissimilar-metal contact are at risk. Crack growth initiates at the surface, often from machining marks or grinding damage, and propagates without visible warning. The failure can be intergranular or transgranular, and it is not simple overload or rusting. Prevention involves reducing tensile stress, avoiding susceptible alloy/environment combinations, and minimizing surface damage from processing, welding, forming, or grinding.

• ⚠️Residual-stress trap: Parts pass inspection but retain high tensile residual stress from machining or welding, causing delayed SCC initiation at the surface.
• ⚠️Wrong alloy in wrong environment: A susceptible alloy used in chloride, salt spray, or other aggressive environments develops fine branching cracks and fails abruptly under service load.
• ⚠️Bad finish or stress raiser: Deep tool marks, sharp corners, or grind burns concentrate tensile stress and give SCC a preferred initiation site at the surface.