51313-02676-Evaluating an SCC Chloride Limit for UNS S31603 in Sour Service

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Product Number: 51313-02676-SG

ISBN: 02676 2013 CP

Author: Sytze Huizinga

Publication Date: 2013

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A laboratory testing program was carried out for an oil & gas project in the Middle East to verify ISO15156/NACE MR0175 limits for sour service cracking resistance of UNS S31603 in oil & gas production environments.

The testing program started with the validation of the ISO 15156 limit (part 3Table A.2 - 1 bar H2S 60C) for UNS S31603. Ripple Slow Strain Rate (RSRT) tests were conducted at a third party lab as per NACE Corrosion 1997 P97058. The test was performed on duplicate specimens of UNS S31603 stainless steel material to evaluate the cracking susceptibility and impact of small amounts of plastic strain when exposed to different sour environments.

The potential test matrix included nine (9) sour environments and prescribed the testing sequence logic that indicated the order in which the testing would be performed depending on test results. Following this approach testing was performed in three (3) environments to allow conclusions to be drawn.

The first environment was the ISO limit (1 bar H2S 60C) with 230000 ppm NaCl; the specimens completed the 250 load cycles without fracturing but cracks were observed on the gauge area of both specimens constituting a “Fail” result.
The second environment was the ISO limit (1 bar H2S 60C) with 132000 ppm NaCl; both specimens completed the 250 load cycles without fracturing and no cracks were observed on the gauge area constituting a “Pass” result.
The third environment was 0.37 bar H2S 85C & 230000 ppm NaCl leading to a “Fail” result both the specimens showing cracking.

It follows that the “any chloride concentration” ISO limit for UNS S31603 is too optimistic. To arrive at a less conservative limit than 132000 ppm NaCl further testing should be done in relation to expected operation conditions.