CORROSIONDOE-HDBK-1017/1-93Properties of MetalsOne of the most serious metallurgical problems and one that is a major concern in thenuclear industry is stress-corrosion cracking(SCC). SCC is a type of intergranular attackcorrosion that occurs at the grain boundaries under tensile stress. It tends to propagateas stress opens cracks that are subject to corrosion, which are then corroded further,weakening the metal by further cracking. The cracks can follow intergranular ortransgranular paths, and there is often a tendency for crack branching.The cracks form and propagate approximately at right angles to the direction of the tensilestresses at stress levels much lower than those required to fracture the material in theabsence 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 structuralfailure from overload.Stresses that cause cracking arise from residual cold work, welding, grinding, thermaltreatment, 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 orwas at much lower levels, there would be no damage. If the structure, subject to the samestresses, were in a different environment (noncorrosive for that material), there would beno failure. Examples of SCC in the nuclear industry are cracks in stainless steel pipingsystems and stainless steel valve stems.The most effective means of preventing SCC in reactor systems are: 1) designingproperly; 2) reducing stress; 3) removing critical environmental species such ashydroxides, chlorides, and oxygen; 4) and avoiding stagnant areas and crevices in heatexchangers where chloride and hydroxide might become concentrated. Low alloy steelsare less susceptible than high alloy steels, but they are subject to SCC in water containingchloride ions. Nickel-based alloys, however, are not effected by chloride or hydroxideions.An example of a nickel-based alloy that is resistant to stress-corrosion cracking is inconel.Inconel is composed of 72% nickel, 14-17% chromium, 6-10% iron, and small amountsof manganese, carbon, and copper.One of the most important forms of stress corrosion that concerns the nuclear industry ischloride stress corrosion. Chloride stress corrosion is a type of intergranular corrosionand occurs in austenitic stainless steel under tensile stress in the presence of oxygen,chloride ions, and high temperature. MS-02Page 34Rev. 0
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