1994 年 43 巻 490 号 p. 874-880
The susceptibility to potential-free and potential-controlled stress corrosion cracking (SCC) of SUS304 steel has been investigated in various MgCl2 solutions under two stress conditions.
The threshold concentration for SCC occurrence under the constant load condition was about 37%, above which the susceptibility to SCC had a maximum at 42%. Such a tendency, however, did not appear under the slow strain rate condition. The cracking susceptibility also decreased with decreasing temperature of MgCl2 solution, regardless of stress conditions. The critical potentials of SCC under the constant load and slow strain rate conditions were about -320mV and -360mV, respectively. Following each of SCC ranges, the uneven general corrosion occurred as the potential was shifted to more noble values. The transgranular fracture was susceptible to the larger concentration, higher temperature and noble potential conditions, and changed gradually to intergranular fracture. The corroded regions (e.g., pitting, corrosion groove and others) were found by AES and EDX analysis to be almost depleted of iron and nickel, and highly enriched in chromium and magnesium of which both phases were most likely oxides, In the stressed specimens, therefore, it is supposed that chloride ions penetrate the oxide cracks at the early stage of SCC. In addition, it is important for the potential-free SCC tests under the slow strain rate condition to choose a solution in which the peak corrosion potential has a value about equal to that under the constant load condition, i.e., the use of lower MgCl2 concentration.