Stress- Corrosion Cracking:

One of the most serious metallurgical problems and one which is a mainly concern within the nuclear organization is stress-corrosion cracking (SCC). An SCC is a category of intergranular attack corrosion which occurs at the grain boundaries under tensile stress. It tends to propagate as stress opens cracks which are subject to corrosion, that are then corroded additional, weakening the metal through further cracking. The cracks could follow transgranular or intergranular paths and there is frequently a tendency for crack branching.

The cracks form and propagate around at right angles to the direction of the tensile stresses at stress levels much lower than those needed to fracture the material in the absence of the corrosive environment.  As cracking penetrates additional into the material, it eventually decreases the supporting cross section of the material to the point of structural failure from overload.

Stresses which cause cracking arise from residual cold work, grinding, welding, thermal treatment, or might be externally applied in during service and, to be effectual, have be tensile (as opposed to compressive).

SCC occurs within metals exposed to an environment whereas, if the stress was not present or was at much lower levels then there would be no damage. If the structure, subject to the similar stresses, were in a various environment (noncorrosive for which material), there would be no failure.  Instances of SCC in the nuclear organization are cracks within stainless steel piping systems and stainless steel valve stems.

The most effective means of avoiding SCC in reactor systems are: 1) designing properly; 2) reducing stress; 3) removing critical environmental species like as chlorides, hydroxides, and oxygen; 4) and preventing stagnant areas and crevices in heat exchangers where hydroxide and chloride may become concentrated. Low alloy steels are less susceptible than high alloy steels, other than they are subject to SCC in water holding chloride ions.  Nickel-based alloys, therefore, are not affected through chloride or hydroxide ions.

An instance of a nickel-based alloy which is resistant to stress-corrosion cracking is inconel. An Inconel is composed of 72 percent nickel, 14-17 percent chromium, 6-10 percent iron, and small amounts of manganese, copper and carbon.