Crevice Corrosion Induced SSC of 25Cr Tungsten Alloyed Super Duplex Stainless Steel Tubing Exposed to H_2S/CO_2Sour Environment

摘要

Recent studies have shown that crevice corrosion can trigger stress corrosion cracking (SCC) ofstainless steels in dilute chloride or chloride-sulfate solutions, even at ambient temperatures, usuallyconsidered to be harmless. Various types of test specimens and crevice geometries have also beenused to study the SSC/SCC behavior of nickel base alloys in H_2S-CO_2 service. Because of theincreasing use of duplex stainless steels in the offshore oil production, a question has invoked whetherthe crevice corrosion impairs the sulfide stress cracking (SSC) resistance of the super duplex stainlesssteels.For answering this question the SSC resistance of wrought seamless super duplex stainless steel tubeUNS S39274 was evaluated in 0.02 MPa partial pressure H_2S – 0.5 MPa partial pressure CO_2 with120 g/l chlorides at 90 ℃. The condition selected represent the NACE MR0175/ISO15156environmental and materials limits for duplex stainless steels 40 < PREN < 45 used as down holetubular components. The SSC performance has been studied with both U-bend and C-ring specimensand with different crevice materials and geometries. The results show that the crevice corrosion caninitiate cracking in H_2S-CO_2 containing chloride solutions. So far, definite answer to the question towhich extent the crevice impairs the SSC resistance has not yet been found out. One reason is that thespecimen type and crevice geometry plays significant role in crack initiation. The metal-metal crevicewith U-bend specimens has proved to be the most aggressive to initiate SSC. In the case of C-ringspecimens the residual stress state of the C-ring specimens has to be taken account in loading andinterpreting the results.