Recent studies have shown that crevice corrosion can trigger stress corrosion cracking (SCC) of stainless steels in dilute chloride or chloride-sulfate 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 behavior of nickel base alloys in H2S-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 sulfide stress cracking (SSC) resistance of the super duplex stainless steels.

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 CO2 with 120 g/l chlorides at 90 °C. The condition selected represent the NACE MR0175/ISO15156 environmental and materials limits for duplex stainless steels 40 < PREN < 45 used as down hole tubular components. The SSC performance has been studied with both U-bend and C-ring specimens and with different crevice materials and geometries. The results show that the crevice corrosion can initiate cracking in H2S-CO2 containing chloride solutions. So far, definite answer to the question to which extent the crevice impairs the SSC resistance has not yet been found out. One reason is that the specimen type and crevice geometry plays significant role in crack initiation. The metal-metal crevice with U-bend specimens has proved to be the most aggressive to initiate SSC. In the case of C-ring specimens the residual stress state of the C-ring specimens has to be taken account in loading and interpreting the results.

Subject
Super duplex stainless steel, Tubes, Stress cracking, Stress, Crevices, C ring stress corrosion testing, U bends, Crevice corrosion, Duplex stainless steel, Sulfide stress cracking, Ferrite, Stress corrosion cracking, Residual stress
Keywords:
Duplex stainless steel, Super duplex stainless steel, Sulphide stress cracking, Stress corrosion cracking, Sour environment, H2S, Co2, Chlorides, Crevice
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2011
Association for Materials Protection and Performance (AMPP)
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