Role of microstructure and environment in stress corrosion cracking susceptibility

In this study, problems in determining threshold stress intensity factor (KISCC) in corrosive environment for thin narrow sections have been identified and a novel technique circumferential notch tensile (CNT) has been successfully used to determine KISCC of thin sections. Although a narrow zone of steel weld was successfully tested by CNT technique, the KISCC data were not very reliable because of poor weld quality. Specimens with simulated microstructure of heat affected zone were used for determination of KISCC of heat affected zone and base metal, using CNT technique. Stress corrosion crack growth in caustic solution (30% NaOH) at elevated temperatures (100 ºC) has been determined using this technique. CNT technique has also been successfully used to study the effect of sensitization on KISCC and crack growth rate of thin sections of solution annealed and sensitized stainless steel in 42% MgCl2 environment. Stress corrosion crack growth was found to be faster in sensitized stainless steel than solution annealed. In this study, effect of caustic solution chemistry on stress corrosion cracking susceptibility of carbon steel has been studied. Constant load tests with fatigue pre-cracked specimens were conducted in 10, 20, 30 and 40% caustic concentration at different temperatures varying from 45-100 ºC. Caustic cracking susceptibility diagram has been verified using CNT technique. Stress corrosion crack initiation in blunt notch has also been studied in Bayer solution and plain caustic solution. Addition of impurities has significant effect on stress corrosion cracking susceptibility. Hence, constant load tests were conducted in real plant Bayer solution. Concentration of Bayer solution was altered to 10, 20, 30 and 40% free caustic concentrations by adding NaOH. Tests temperatures were decided in such a way that they lie in definite cracking, potential cracking and no cracking regions of the susceptibility diagram, 30% free caustic concentration Bayer solution was highly susceptible for caustic cracking even at lower temperatures (55 ºC). A few tests have been carried out to develop mechanistic understanding of the role of imposed electrochemical potential in caustic cracking susceptibility