- Chlorides
- Tensile residual stresses
- Temperature
- Oxygen
- Time of exposure
The amount of chlorides required depends on the stainless steel grade but for Type 304 and Type 316 it is on the order of a few parts per million. Any situation that results in splashing or alternate wet-dry situations where the chlorides may evaporate and concentrate is sufficiently aggressive for these grades. In heat transfer applications, there can be significant chloride concentration effects in the surface film adjacent to the heat-rejecting surface, even without actual evaporative concentration.
Tensile residual stresses can result from welding, from forming operations, or from operational stresses, such as those from thermal cycling. Residual tensile stresses are difficult to eliminate and difficult to detect or monitor prior to a problem.
Typically temperatures of about 150°F (65°C) or higher are associated with stress corrosion cracking of Types 304 and 316. The rate of stress corrosion cracking increases rapidly with increasing temperature. If the surface is a heat-rejecting surface, i.e., the metal is heating the liquid, for example, in a heat exchanger, stress corrosion cracking is more likely than when the metal and the solution are at the same temperature.
The amount of oxygen required is so small that in most practical situations it is impossible to have the oxygen levels low enough to prevent stress cracking.
Stress corrosion cracking does not occur instantaneously, but rather takes some time to initiate even when all of the other factors are present. Unfortunately, the time to initiation for a particular exposure may be as short as a few hours. Consequently, it is sometimes possible to operate a batch process with relatively short exposures without SCC for many years, but to rapidly observe stress corrosion cracking when precisely the same conditions are imposed in a continuous process.
Appearance
A stainless steel that has failed by chloride stress corrosion cracking will often have a web-like array of tiny surface cracks. Sometimes these cracks are made visible as a result of corrosion from the process stream leaking through the cracked area. In extreme cases, the cracking is so pervasive that the metal is so full of cracks that it can be broken off by hand. This pattern of corrosion occurs because the corrosion tends to follow the direction of highest residual tensile stress, but the actual cracking tends to locally relieve that stress.
Possible solution
Because it is usually impractical to alter the process to eliminate variables such as temperature or chlorides, replacing the component with one made from a more resistant stainless steel is often the only practical alternative. Because of its resistance to chloride stress corrosion cracking, the duplex stainless steel, 2205, is commonly used to replace 304 or 316 that has failed due to chloride stress corrosion cracking.