Effects of Relative Humidity on Atmospheric Stress Corrosion Cracking of Stainless Steels

In order to evaluate the effect of environmental parameters on atmospheric stress corrosion cracking (SCC) of SUS 304 and 316L stainless steels, SCC tests were carried out in laboratory air controlled temperature and relative humidity (RH) environment by using U-bend specimens with the deposition of 0.5N chloride solution (NaCl, MgCl2, CaCl2, ZnCl2) and synthetic sea water droplets. SCC susceptibilities of steels strongly depend on RH and the maximum susceptibility was observed at the inherent relative humidity (RHmax) for each chloride. The values of RHmax were about 60, 30, 20 and 10% for NaCl, MgCl2, CaCl2 and ZnCl2 solutions respectively, corresponding to their vapor pressure of saturated solution (Psat), and hence, where the specimen surface is covered with a thin saturated chloride solution layer. In case of synthetic sea water, RHmax was almost same as that of MgCl2, suggesting that SCC susceptibility was affected by the salt with lowest Psat even in minor solute composition. The critical values of relative humidity (RHcrit) below which no-SCC was observed were some what lower than RHmax, where the solution dried up. Above RHmax, the SCC susceptibilities decreased with increasing RH, due to the dilution of the chloride solutions.