Hydrogen stress cracking susceptibility of super duplex stainless steel 2507 in natural sea water

摘要

Super duplex stainless steels are widely use in offshore applications. These steels offer high strength, toughness and excellent corrosion resistance. They are characterized by high chromium content and a mixed microstructure of austenite and ferrite in equal amount. These steels may be sensitive to hydrogen stress cracking when exposed to cathodic protection in natural seawater. This sensitivity could induce a detrimental effect reducing the fatigue resistance of such materials under cyclic loading conditions.The aim of this study is to evaluate the high cycle fatigue resistance of the steel under cathodic protection in natural seawater by performing stress vs. number of cycles (S-N) curves. For this purpose fatigue tests under four point bending are performed in seawater on Sandvik SAF 2507 (UNS S32750) seamless tubes with a small austenite spacing (about 2 μm). These tests were performed in air, at the open circuit potential and under cathodic protection at -1100 mV/SCE (vs. saturated calomel electrode). Different load ratios, temperatures and frequencies are investigated to estimate the sensitivity of the steel to hydrogen embrittlement.At room temperature and a load ratio of 0.1, no detrimental effect of cathodic protection is noticed comparing to open circuit potential. The fatigue lifetime is even enhanced by cathodic protection in these particular conditions, and frequency has no noticeable impact. Hydrogen pre-charging tends to reduce the difference of fatigue resistance under cathodic protection and open circuit potential.