The most effective means for preventing stress corrosion cracking SCC are proper design, reducing stress, removing critical environmental contributors (for example, hydroxides, chlorides, and oxygen), and avoiding stagnant areas and crevices in heat exchanger where chlorides and hydroxides might become concentrated. Low alloy steel are less susceptible than high alloy steel, but they are subject to SCC in water containing chloride ions. Nickel based alloys are not affected by chloride or hydroxide ions.
Two types of SCC are of major concern to a nuclear facility.
Chloride Stress Corrosion Cracking (Stainless Steel)
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 alloy 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 stainless 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 treatment 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 lowcarbonstainless steel. Environments containing dissolved oxygen and chloride ions can readily be created in auxiliary water systems. Chloride ions can enter these systems via leaks in condenser 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.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)