Corrosion and Galvanic Compatibility Knowledge
Corrosion Testing Equipment
One of the most serious metallurgical problems and one that is a major concern in most industries is stress-corrosion cracking (SCC). SCC is a type of intergranular attack corrosion that occurs at the grain boundaries under tensile stress. It tends to propagate as stress opens cracks that are subject to corrosion, which are then corroded further, weakening the metal by further cracking. The cracks can follow intergranular or transgranular paths, and there is often a tendency for crack branching.
The cracks form and propagate approximately at right angles to the direction of the tensile stresses at stress levels much lower than those required to fracture the material in the absence of the corrosive environment. As cracking penetrates further into the material, it eventually reduces the supporting cross section of the material to the point of structural failure from overload.
Stresses that cause cracking arise from residual cold work, welding, grinding, thermal treatment, or may be externally applied during service and, to be effective, must be tensile (as opposed to compressive).
SCC occurs in metals exposed to an environment where, if the stress was not present or was at much lower levels, there would be no damage. If the structure, subject to the same stresses, were in a different environment (noncorrosive for that material), there would be no failure. Examples of SCC in industry are cracks in stainless steel piping systems and stainless steel valve stems.