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Selective Dissolution Forming During Sour Testing of Duplex Stainless Steels. Is It Critical?

According to ISO 15156(1) standard, the acceptability limits of duplex stainless steels for a safe use in exploration and production environments are defined based on the PREN (Pitting Resistance Equivalent Number) of the considered alloy. For a duplex stainless steel with 30 < PREN < 40 (typical of 22%Cr steels), the alloy is acceptable up to a H2S partial pressure of 0.1 bar while this partial pressure can be increased up to 0.2 bar if 40 < PREN < 45 (typical of 25%Cr steels).

Product Number: 51323-18834-SG
Author: Christophe Mendibide, Carole Dessolin
Publication Date: 2023
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According to ISO 15156 standard, the acceptability limits of duplex stainless steels for a safe use in exploration and production environments are defined based on the PREN. For duplex stainless steels, depending on the alloying content, the maximum acceptable H2S partial pressure can be up to 0.2 bar and 5 bar for very low chloride contents. This limit is defined for any chloride contents and pH of the service environments. Field experiences, however, demonstrate that these limits seem very conservative for service conditions with low chlorides contents and/or at high pH. In this work, a cold-rolled duplex stainless steel UNS S32750 is studied in sour conditions under slightly more severe condition at 80°C with the aim to investigate the possible effect of selective dissolution (SD)
of one phase on the cracking test results. Different pH were considered to vary the level of the SD that can form during the exposures. It is demonstrated that SD competes with cracking and that under conditions leading to the formation of large area with SD, the presence of this type of corrosion can hide the susceptibility of the material to cracking. The presence of only SD after testing must therefore be considered with caution. SD initiated also without applied stress underlining that the phenomenon is correlated to a loss of passivity. The discussion is supported by Electron BackScattered Diffraction (EBSD) analysis and electrochemical monitoring.

According to ISO 15156 standard, the acceptability limits of duplex stainless steels for a safe use in exploration and production environments are defined based on the PREN. For duplex stainless steels, depending on the alloying content, the maximum acceptable H2S partial pressure can be up to 0.2 bar and 5 bar for very low chloride contents. This limit is defined for any chloride contents and pH of the service environments. Field experiences, however, demonstrate that these limits seem very conservative for service conditions with low chlorides contents and/or at high pH. In this work, a cold-rolled duplex stainless steel UNS S32750 is studied in sour conditions under slightly more severe condition at 80°C with the aim to investigate the possible effect of selective dissolution (SD)
of one phase on the cracking test results. Different pH were considered to vary the level of the SD that can form during the exposures. It is demonstrated that SD competes with cracking and that under conditions leading to the formation of large area with SD, the presence of this type of corrosion can hide the susceptibility of the material to cracking. The presence of only SD after testing must therefore be considered with caution. SD initiated also without applied stress underlining that the phenomenon is correlated to a loss of passivity. The discussion is supported by Electron BackScattered Diffraction (EBSD) analysis and electrochemical monitoring.