Brittle crack corrosion layer

Abrasion removes protective oxidized metal and polarized coatings to expose unoxidized metal, in addition to removing metal particles. Forms microscopic grooves and dents for concentration cell corrosion. Increases microscopic surface area exposed to corrosion. Removes strain-hardened surface layers. Cracks brittle metal constituents, forming sites for impact hydraulic splitting. Plastic deformation by high-stress metal-mineral contact causes strain hardening and susceptibility to chemical attack. 

Those alloy/environment systems that form relatively thick corrosion-product layers (e g., brass in ammonia environments) frequently exhibit preferential penetration of the corrosion products along grain boundaries. These are generally brittle products such that cracking will occur on reaching a critical depth in the presence of tensile stresses across the grain boundary. The environment again has access to the... 

Macroscopically, the crack surfaces may look brittle and diseoloured, dull or darkened by oxide layers. The initiation site may be discovered as a pit (formed by pitting or deposit corrosion) or, e.g. as a defect caused by forming or machining. On parts in service it may, however, be difficult to discover cracks, because they are narrow and filled or covered by corrosion products. As shown in the examples in Tables 7.1 and 7.2, SCC is a common and widespread corrosion form in the process industry. 

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