Stress Corrosion Cracking Behavior of Austenitic Stainless Steels in MgCl₂ Solutions

Abstract

Stress corrosion cracking behavior of SUS 304 and SUS 316 stainless steels was investigated in terms of concentration and temperature of MgCl₂ solutions with or without dissolved oxygen. The potential-MgCl₂concentration relation between SCC and pitting corrosion was also studied by constant potential method under the applied stress of 25 kg/mm².
As dissolved oxygen becomes necessary to cause SCC with decreasing the concentration or the temperature of MgCl₂ solutions, SUS 316 becomes much more resistant to SCC than SUS 304. The crack is often observed originating at the bottom of a pit.
The lower limit of MgCl₂ concentration above which cracks develop at 80°C is about 20% for SUS 304 and about 25% for SUS 316, while the critical SCC potential of SUS 316 is about 50 mV more noble than that of SUS 304. Above the lower limit of MgCl₂ concentration, the corrosion mode shifts from SCC to pitting corrosion as the applied potential is raised and the SCC potential range widens with increasing MgCl₂ concentration. Below that, only pitting corrosion occurs.