ISO15156 / NACE MR0175 1 Part 3 table A.2 allows pH2S = 1bar up to 60°C with “any” chloride concentration and pH for the use of UNS S31603. Hence, it was recommended to evaluate UNS S31603 for the flow lines, trunk lines and equipment in one of the projects in the Middle East. A detailed program of laboratory testing was designed which consisted of duplicate Cyclic Slow Strain Rate Tests (CSSRT) to verify the ISO15156 / NACE MR0175 1 limits for sour service cracking resistance of UNS S31603. It included representative environmental conditions for the application. Testing was planned to be carried out first on UNS S31603 plate produced according to ASTM A240 2 specification and later during the detailed design project phase on specimens containing a weld.

The testing program started with the validation of the ISO limit for UNS S31603 and continued with application specific conditions. Cyclic Slow Strain Rate Tests (also known as Ripple Strain Rate Tests, RSRT) were conducted at a third party laboratory as per NACE Corrosion/97 paper P97058 3. The test matrix included nine (9) sour environments and the testing sequence was determined by the test results. This resulted in only three (3) environments being tested.

It was concluded that accepting “any chloride concentration” for UNS S31603 as per ISO15156 / NACE MR0175 Part 3 table A.2 is too optimistic since testing at 1 bar H2S at 60 °C with 230,000 mg/kg NaCl gave a fail result. Lowering the concentration to 132,000 mg/kg NaCl gave a pass, showing this concentration to be acceptable. To arrive at a less conservative limit than this concentration testing at intermediate chloride levels is required.

The current testing program concluded that UNS S31603 stainless steel cannot be used under at least some of the expected operating conditions (represented by cases 1-6 in Table 2).

Subject
Environmental cracking, Test methods, Materials, Gauges, Sour environments, Sodium chloride, Structures, Corrosion cracking, Surfaces, Stress, Sodium chloride solution, Test environments, Chlorides
Keywords:
austenitic stainless steel, sour service, corrosion cracking testing, cyclic/ripple slow strain rate test, flow lines
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2013
Association for Materials Protection and Performance (AMPP)
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