INTRODUCTION

ABSTRACT

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 ºC. 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.

Super duplex stainless steels are attractive alloys, where both high mechanical strength and excellent resistance to localized corrosion and sulfide stress cracking (SSC) are required. According to Woolin & Maligas two factors control the sensitivity of corrosion resistant alloys to SSC, (1) resistance to localized corrosion and (2) resistance to cracking in the presence of hydrogen.1 The localized corrosion risk of duplex stainless steels in sulfide solutions increases when the H2S and chloride content increase and pH decrease.2 Localized corrosion may also increase cracking risk of duplex stainless steels by exposing material to hydrogen, since the main cathodic reaction in the de-aerated sulfide solutions is hydrogen evolution.2,3 In general crevice corrosion is considered to be more detrimental than pitting corrosion since the environmental threshold values are lower for crevice corrosion than for the pitting corrosion. Crevice corrosion mechanisms studies of duplex stainless steels in sour gas environments are scarce. Azuma et al have shown that the presence of H2S raises the depassivation pH and makes active dissolution easier.4 The solution in the crevice is proposed to be acidified because of the hydrolysis of the dissolved metal ions or metal sulfide formation. Recent studies in dilute chloride or chloride-sulfate solutions have shown that crevice corrosion can trigger stress corrosion cracking (SCC) of stainless steels, even at ambient temperatures, environments usually considered to be harmless

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