Some Aspects of Stress Corrosion Cracking of Mild and Low Alloy Steels in H2O-CO-CO2 Systems

Effects of gas composition, temperature and alloying elements on stress corrosion cracking of mild and low alloy steels in H2O-CO-CO2 system were studied. Stress corrosion tests were carried out in H2O-CO-CO2 mixtures by using U-bend specimens. The test results were as follows. (1) Mild and low alloy steels suffered from transgranular stress corrosion cracking in H2O-CO-CO2 mixtures of vast ranges of CO/CO2 ratio. (2) No stress corrosion cracking of mild steel was found in pure CO or CO2 gases with water, respectively. But stress corrosion cracking occurred when pure grade CO contained small amount of air (0.03% CO2). (3) Oxygen in H2O-CO-CO2 mixture promoted the susceptibility to stress corrosion cracking of mild steel in liquid phase. H2S in water showed the inhibiting effect. (4) Stress corrosion cracking occurred at testing temperatures of 20, 40, and 70°C, but its susceptibility decreased over 100°C at which general corrosion was promoted. (5) Small amount of alloying elements in the steels, i.e., C, Mn, Si, Cr, Mo, Cu W, V, Ti, Nb and Al, had no prominent effect to stress corrosion cracking. But steels containing chromium more than 9% did not suffer from stress corrosion cracking. (6) This type of cracking was called stress corrosion cracking (anodic dissolution, not hydrogen embrittlement) because of the availability of cathodic protection. (7) Treatments to relieve residual stress in steels or application of steels containing chromium more than 9% may be used to prevent stress corrosion cracking.