LOW TEMPERATURE STRESS CORROSION CRACKING OF STAINLESS STEELS IN THE ATMOSPHERE IN PRESENCE OF CHLORIDE DEPOSITS

摘要

Several cases of ceiling collapses and other failed elements have been reported in indoor swimming pool halls in the last two decades. The collapses were caused by stress corrosion cracking (SCC) of stainless steel fastening elements made of grades UNS S30400, S31600, or similar. It was shown that this phenomenon can occur under specific conditions beneath chloride deposits at temperatures as low as the room temperature. The aim of this study was to assess the application limits of different austenitic and duplex stainless steel grades subject to tensile stress and contaminated with chloride deposits in the atmosphere under non-washing conditions as a function of temperature (20-50 °C), relative humidity (15-70 % RH), and deposit composition. Austenitic stainless steel grades UNS S30400 and S31603 were susceptible to SCC in the presence of magnesium and calcium chlorides at temperatures from 30 °C and at low relative humidity. The tendency to SCC increased with increasing temperature and decreasing relative humidity. The corrosivity of salt at given exposure conditions decreased in the following order CaCl_2 > MgCl_2 > FeCl_3 > NaCl. The corrosivity of chloride deposits was governed by the equilibrium chloride concentration in the surface electrolyte formed as a result of interaction of given chloride salt and air at given relative humidity. Threshold values of the minimum chloride concentration and relative humidity intervals leading to SCC at 30 and 40 °C were established for UNS S30400 and S31603. Duplex stainless steels S32101, S32304, S32205, and S32750 corroded selectively to the maximum depth of 380 μm. Austenitic stainless steels N08904 and S31254 showed no tendency to SCC.