Hydrogen embrittlement (HE) of a 2205 duplex stainless steel has been studied by slow straining of tensile specimens in sulfide containing 3.5 wt% NaCl solution. A more complex sulfide-concentration-dependent loss in ductility was discovered at controlled potentials in - 1100mV (SCE). It is believed that HE was controlled by the critical concentration of S as a promoter at lower sulfide levels and recovery of ductility at higher sulfide concentrations was attributed to the role of the sulfide as a cathodic inhibitor. No stress corrosion cracking (SCC) was observed at anodic polarization in this system. The electrochemical results indicate that the corrosion potentials of 2205 duplex stainless steel in 3.5 wt% NaCl solution move to the less noble direction with increasing the sulfide concentration or with decreasing the solution acidity, This trend was not influenced by the presence of dissolved oxygen. Under severe HE environments, transgranular cleavage is the favored path for cracking.
Duplex austenitic-ferritic stainless steels are widely used in the petroleum industry and, more particularly, for submarine gas and oil lines and other off-shore applications. 1~?2Their mechanical properties are superior to those of conventional austenitic stainless steels because of their greater resistance to chloride induced stress corrosion cracking (SCC) at room temperature, and at elevated temperature.~ Another attractive feature is their general and pitting corrosion resistance. The mechanical properties of duplex stainless steels are dependent on the amounts of phases, the morphology of the ferrite and austenite; the orientation of the ferrite and austenite with respect to the applied stress, and the composition of each of the two phases. However, studies with chloride/sulfide environments have shown that the duplex stainless steels are susceptible to SCC at ambient and elevated temperatures, and they do not offer much of an improvement over austenitic 316 stainless steels.
Furthermore, there has been some concern about the hydrogen embrittlement risk if duplex stainless steels are subjected to cathodic protection, either deliberately or as a result of being coupled to a catholically protected C-Mn steel structure in sea water.2~9 In contrast to this, Valdez-Vallejo et al. 10 have reported different views about the hydrogen embrittlement susceptibility of duplex stainless steels in seawater. They concluded that duplex stainless steels with an appropriate ferrite-austenite phase ratio should not be susceptible to hydrogen embrittlement and cracking in seawater service, even under conditions of cathodic overprotection. They claimed that this is because, in service, the stress levels should not approach the tensile strength and because any microscopic cracks that may initiate at lower stress levels are prevented from propagating by the austenite phase.
For many years, considerable efforts have been expended to determine the corrosion behavior of copper-nickel alloys in sulfide-polluted seawater. 1?-16 However, little published work is reported on hydrogen embrittlement of duplex stainless steels in sulfide-polluted seawater under cathodic protection. Sulfide is usually introduced into seawater in many ways, for example, by rotting vegetation and by industrial waste discharge. Another possible source of sulfide is microbial corrosion. For instance, the activity of sulfate-reducing bacteria is associated with the presence of sulfides. 17-18 A recent investigation showed that the Arabian Gulf seawater contains high levels of sulfide pollutant, in the range of 0.005- 0.2 ppm. Although experimental results showed that the high-chromium duplex cast steel was immune to corros
