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Chloride Stress Corrosion Cracking - CSCC
Chloride Stress Corrosion Cracking - CSCC
Chloride Stress Corrosion Cracking is a localized corrosion mechanisms like pitting and crevice corrosion. The three conditions that must be present for chloride stress corrosion to occur are as follows.
- Chloride ions are present in the environment
- Dissolved oxygen is present in the environment
- Metal is under tensile stress
Austenitic stainless steel is a non-magnetic stainless steel grades consisting of iron, chromium, and nickel, with a low carbon content. This alloy is highly corrosion resistant and has desirable mechanical properties. One type of corrosion which can attack austenitic stainless steel is chloride stress corrosion.
Chloride stress corrosion is a type of intergranular corrosion. Chloride stress corrosion involves selective attack of the metal along grain boundaries.
In the formation of the steel, a chromium-rich carbide precipitates at the grain boundaries leaving these areas low in protective chromium, and thereby, susceptible to attack.
It has been found that this is closely associated with certain heat treatments resulting from welding. This can be minimized considerably by proper annealing processes.
This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and the use of low carbon steels. Environments containing dissolved oxygen and chloride ions can readily be created in auxiliary water systems.
Chloride ions can enter these systems via leaks in condensers or at other locations where auxiliary systems associated with the nuclear facility are cooled by unpurified cooling water. Dissolved oxygen can readily enter these systems with feed and makeup water. Thus, chloride stress corrosion cracking is of concern, and controls must be used to prevent its occurrence.
Figure 1 Intergranular Corrosion Cracking
Figure 1 illustrates intergranular stress corrosion cracking. The pressure of a tensile stress opens up intergranular cracks and accelerates further corrosion.
Chloride stress corrosion is a particularly significant problem in the operation of nuclear facilities because of the wide use of austenitic stainless steel, and the inherent presence of high tensile stresses associated with pressurization.
Chloride stress corrosion cracks have been known to propagate in austenitic stainless steel at stresses of about one-fifth yield strength with chloride concentrations of less than 50 ppm.
Tests show that the 18-8 stainless steels grade are susceptible to chloride stress attack when both the chloride ion concentration and dissolved oxygen concentration are above certain values.
The region of susceptibility for austenitic stainless steel is illustrated in Figure 2.
Figure 2 Austenitic Stainless Steel Chloride Stress Corrosion Cracking
Note that when dissolved oxygen is present at about 1 ppm, chloride stress corrosion cracking can be initiated at chloride ion concentrations near 1 ppm.
However, when the concentration of dissolved oxygen is very low, susceptibility to chloride stress corrosion cracking is reduced.
High temperature tends to decrease the time required for chloride-induced cracking to occur, but there appears to be no practical temperature limit below which cracking will not occur, given sufficient time and severe conditions.
The curve in Figure 2 is valid for temperatures in the range 470°F to 500°F.
this article is taken from: DOE FUNDAMENTALS HANDBOOK CHEMISTRY Volume 1 of 2 - DOE-HDBK-1015/1-93 JANUARY 1993
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