Studies Of Stress Corrosion Cracking On Austentic Stainless Steels Of 316 NG In Acid SO₄～(2-) Environment

Austenic stainless steels type-316NG and 316Tiss were studied by slow strain rate tests (SSRTs). To evaluate the susceptibility of two type alloys to stress corrosion cracking(SCC) changing pH and SO42~ concentration at 300℃. The tests of high temperature and high pressure electrochemical polarization behavior were conducted on two type alloys. Metals - phase analysis(MPA) and scanning electron microscope (SBM) were used to explore the role of micro-phase on SCC. The results show that:1. There exists a minimum SO42- concentration for initiation of stress corrosion cracking. The susceptibility to stress corrosion cracking increases with the increasing SO42- concentration from 10mg/kg to 5000mg/kg. 316NG perform excellent resistance to stress corrosion cracking in 300C , pH15. 2 and 10 mg/kg SO4 2- environment, but 316Tiss occurs stress corrosion cracking.2. Cracks expanded rate of 316NG is more quickly than that of 316Tiss in high temperature and high pressure aqueous containing SO4 2- environment.3. Increasing SO4 2- concentration will shift the critical cracking potential of 316NG and 316Tiss to more noble potentials. The critical cracking potential, at which initiation of cracks occurs, is determined by effects of SO4 2- on the repassivation rate.4. The resistance to corrosion, especially the resistance to pitting is improved when nitrogen is added in the austenitic stainless steel.5.316Tiss containing lots of micro-phase TiC performs the best resistance of stress corrosion cracking. Micro - phase TiC hinders the location in slipping, makes the slipping step difficult to rise and decreases growth rate of stress corrosion cracking.