Stress Corrosion Testing Methods

Stress corrosion tests are conducted for a variety of reasons, the reason frequently determining the type of test. Some of the reasons are: (1) Evaluate a metal or alloy, or various heat treatments of one alloy for susceptibility to stress corrosion in certain environments. (2) Compare stress corrosion susceptibilities of various alloys.(3) Evaluate environments which might accelerate stress corrosion cracking in various alloys. (4) Evaluate a specific service requirement with regard to the possibility of stress corrosion. (5) Evaluate the effectiveness of coatings or other protective measures for reducing stress corrosion of susceptible metals and alloys. Stress corrosion tests may be conducted under conditions of either constant deflection or of constant load. An important difference between them is that under constant load general corrosion may cause a decrease in cross-sectional area with subsequent increase in stress per unit area. This can lead to mechanical rather than stress corrosion failure. Under constant deflection, the stress per unit area tends either to stay constant or decrease. Therefore, failure by stress rupture is not as likely to occur, but complete fracture from stress corrosion also may not occur because of stress relaxation during the earlier stages of cracking. For (1) and (2), both types of tests should be conducted in order to obtain a complete picture of the susceptibility of an alloy to stress corrosion, preferably using specimens oriented in more than one direction with respect to the rolling direction. Behavior in the short-transverse direction is especially important for study. For (3) and (5), the most important considerations should be selection of a specimen type and a stressing method which provide reproducibility of stress pattern and stress level from one specimen to another, and selection of a material known to be susceptible to stress corrosion cracking. In materials that are notch sensitive, care should be taken to eliminate notches, scratches, etc., which might act as stress raisers.