Crack initiation and propagation of sensitized Type 304 stainless steel inthe mixed solutions of sulfate and borate

The stress corrosion cracking (SCC) of sensitized Type 304 stainless steel has been examined in Na2B4O7 + Na2SO4 solutions with a constant solution co nductivity by using the slow strain rate test (SSRT). The cracking behavior was observed in-situ by a dynamic observation system with a charge-coupled device (CCD) camera. It was found that an increase in the k (= [Na2B4O7]/[ Na2SO4]) value, or borate concentration, leads to a decrease in the SCC sus ceptibility by retarding crack initiation, namely, by delaying crack initia tion time and reducing crack initiation frequency. By contrast, an increase in sulfate concentration enhances SCC susceptibility by shortening crack i nitiation time and accelerating crack initiation frequency. The critical su lfate concentration to accelerate crack initiation is 10(-5) kmol/m(3) and, below this concentration, the suppressing effect of berate ions on the SCC works. In addition, it is the solution conductivity, rather than the anion type or their concentrations, which control crack propagation. (C) 1999 El sevier Science Ltd. All rights reserved.