Properties of Metals
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
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.
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.