Crevice Corrosion Induced SSC of 25Cr Tungsten Alloyed Super Duplex Stainless Steel Tubing Exposed to H2S/CO2 Sour Environment

Recent studies have shown that crevice corrosion can trigger 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 SCC resistance of the super duplex stainless steels.

For answering this question the SCC resistance of 25Cr tungsten alloyed wrought seamless super duplex stainless steel tube was evaluated in 0.02 MPa partial pressure H₂S – 0.5 MPa partial pressure CO₂ with 120g/l chlorides at 90ºC. The condition selected represent the NACE MR01/175/ ISO15156 environmental and materials limits for duplex stainless steels 40 < PREN < 45 used as downhole tubular components. The SCC 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 SCC in H₂S-CO₂ containing chloride solutions. So far definite answer to the question to which extent the crevice impair the performance has not been found out. One reason to this is that the specimen type and crevice geometry plays significant role in SCC initiation. The metal-metal crevice with U-bend specimens has proved to be the most aggressive to initiate SCC. In the case of C-ring specimens the residual stress state of the C- ring specimens has been taken account in loading. Results are presented and future research needs are discussed.