One of the most serious metallurgical problems and one that is a major concern in the nuclear industry 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 the nuclear industry are cracks in stainless steel piping systems and stainless steel valve stems.

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

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 amounts of manganese, carbon, and copper.

Chloride Stress Corrosion

One of the most important forms of stress corrosion that concerns the nuclear industry is chloride stress corrosion. Chloride stress corrosion is a type of intergranular corrosion and occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature.

It is thought to start with chromium carbide deposits along grain boundaries that leave the metal open to corrosion. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and use of low carbon steels.

Caustic Stress Corrosion

Despite the extensive qualification of inconel for specific applications, a number of corrosion problems have arisen with inconel tubing. Improved resistance to caustic stress corrosion cracking can be given to inconel by heat treating it at 620C to 705C, depending upon prior solution treating temperature. Other problems that have been observed with inconel include wastage, tube denting, pitting, and intergranular attack.

Summary

The important information in this chapter is summarized below.

Corrosion Summary

Corrosion is the natural deterioration of a metal in which metallic atoms leave the metal or form compounds in the presence of water or gases. General corrosion may be minimized by the use of corrosion-resistant materials and the addition of protective coatings and liners.

Galvanic corrosion occurs when dissimilar metals exist at different electrical potentials in the presence of an electrolyte. Galvanic corrosion may be reduced by the careful design and selection of materials regarding dissimilar metals and the use of sacrificial anodes.

Localized corrosion can be especially damaging in the presence of other destructive forces such as stress, fatigue, and other forms of chemical attack.

Stress-corrosion cracking occurs at grain boundaries under tensile stress. It propagates as stress opens cracks that are subject to corrosion, ultimately weakening the metal until failure. Effective means of reducing SCC are 1) proper design, 2) reducing stress, 3) removing corrosive agents, and 4) avoiding areas of chloride and hydroxide ion concentration.

Chloride stress corrosion occurs in austinitic stainless steels under tensile stress in the presence of oxygen, chloride ions, and high temperature. It is controlled by the removal of oxygen and chloride ions in the environment and the use of low carbon steels.

Problems occurring with the use of inconel include caustic stress corrosion cracking, wastage, tube denting, pitting and intergranular attack. Inconel's resistance to caustic stress corrosion cracking may be improved by heat treating.