2000 NPL Stress Corrosion Cracking
Stress corrosion cracking is cracking due to a process involving conjoint corrosion and straining of a metal due to residual or applied stresses .1 Despite the introduction of polymers and composites in recent years, metals remain important in structures because of their strength, stiffness, toughness and tolerance of high temperatures. Unfortunately, metals are subject to corrosion. (The noble metals, such as gold and platinum are an exception to this, but they are rather too rare for common use). Corrosion can take many forms; the form that concerns us here is the interaction of corrosion and mechanical stress to produce a failure by cracking. This type of failure is known as stress corrosion cracking, often abbreviated to SCC.2 As will be explained below, SCC may occur by a number of mechanisms; when cracking is clearly a result of hydrogen embrittlement, this term may be used in place of SCC. However, this distinction is rather arbitrary; we are often unsure of the mechanisms of SCC, and many failures that are actually due to the effects of hydrogen would conventionally be ascribed to SCC. Similarly other specific stress corrosion cracking processes have acquired their own names; ‘season cracking’ for the cracking of brass in environments containing ammonia, ‘caustic cracking’ for the cracking of steel in strong alkalis etc. SCC is an insidious form of corrosion; it produces a marked loss of mechanical strength with little metal loss; the damage is not obvious to casual inspection and the stress corrosion cracks can trigger mechanical fast fracture and catastrophic failure of components and structures. Several major disasters have involved stress corrosion cracking, including the rupture of high-pressure gas transmission pipes, the explosion of boilers, and the destruction of power stations and oil refineries. Fortunately, the occurrence of SCC depends on the simultaneous achievement of three requirements: .. a susceptible material, . an environment that causes SCC for that material, and sufficient tensile stress to induce SCC. Consequently, SCC is relatively rare, though failures can be very costly and destructive when they do occur.