Recent studies have shown that crevice corrosion can trigger stress corrosion cracking (SCC) of stainless steels in dilute chloride or chloride-sulphate solutions, even at ambient temperatures, usually considered to be harmless. Various types of test specimens and crevice geometries have also been used to study the SSC/SCC behaviour of nickel base alloys in H 2S-CO2 service. Because of the increasing use of duplex stainless steels in the offshore oil production, a question has invoked whether the crevice corrosion impairs the sulphide stress cracking (SSC) resistance of the super duplex stainless steels. Understanding from this is required since crevices are present in well tubulars and equipment, like gaps and contact areas between collars and overlapping joints, under gaskets or seals, under deposits which have been shown to be detrimental to stress corrosion cracking resistance in chloride environments. For answering this question the SSC resistance of wrought seamless super duplex stainless steel tube UNS S39274 was evaluated in 0.02 MPa partial pressure H2S - 0.5 MPa partial pressure CO 2with 120 g/l chlorides at 90°C. The SSC performance was studied with C-ring specimens with two edge geometries, to avoid edge cracking, and with different crevice materials and geometries. The results show that the SSC resistance of the super duplex stainless steel UNS S39274 is dependent on the applied stress. It is susceptible to SSC in the threshold conditions (0.02 MPa pH2S at 90°C) given in the ISO 15156/ NACE MR0175 when loaded to yield stress or above. The results showed also that crevices enhance localised corrosion and can induce SSC in super duplex stainless steel. The present results indicate also that crevices result in slightly lower threshold stresses than without crevices i.e. oth crevice oth un-creviced. From experimental point of view, the edge rounding proved to be an effective method to reduce the stresses at the C-ring edges and thus to avoid edge cracking, which often complicates the interpretation of the test results.
|Publication status||Published - 2013|
|MoE publication type||A1 Journal article-refereed|