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Effect of H2S on the Corrosion and Cracking Behavior of Welded API 5L X65 Steel in Supercritical CO2

Picture for Effect of H2S on the Corrosion and Cracking Behavior of Welded API 5L X65 Steel in Supercritical CO2
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Product Number: 51321-16407-SG
Author: Shiladitya Paul
Publication Date: 2021
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Welded API X65 steel specimens were prepared and these were then tested in de-aerated NaCl solution (1000 mg L-1) for 30 days purged with (i) 10 MPa CO2 and (ii) a mixture of 9.9 MPa CO2 and 0.1 MPa H2S. Tests were conducted at 40 °C with one set of specimens submerged in the solution and the other suspended above it. Two sets of specimens were prepared- (i) for corrosion test with no applied stress, and (ii) for stress corrosion cracking (SCC) test using a four-point method by applying 90% of 0.2% proof stress. For four-point testing, specimens were extracted transverse to the weld and were tested with the weld centred on the loading span with the weld root in tension. After completion of exposure, the specimens’ mass increased and microstructural characterization revealed that the specimens formed a scale. The presence of 0.1 MPa H2S reduces the calculated corrosion rate of carbon steel parent due to preferential formation of sparingly soluble mackinawite. Some fine-scale features were seen only on the tensile surface of the stressed steel specimens. These fine-scale features were deeper (length>cross-section) in specimens tested in the vapour phase. These features, however, will not be visible when seen under light optical microscope (10 x) by visual examination only.

Key words: welded pipe, sour corrosion, cracking, SSC, supercritical CO2

Welded API X65 steel specimens were prepared and these were then tested in de-aerated NaCl solution (1000 mg L-1) for 30 days purged with (i) 10 MPa CO2 and (ii) a mixture of 9.9 MPa CO2 and 0.1 MPa H2S. Tests were conducted at 40 °C with one set of specimens submerged in the solution and the other suspended above it. Two sets of specimens were prepared- (i) for corrosion test with no applied stress, and (ii) for stress corrosion cracking (SCC) test using a four-point method by applying 90% of 0.2% proof stress. For four-point testing, specimens were extracted transverse to the weld and were tested with the weld centred on the loading span with the weld root in tension. After completion of exposure, the specimens’ mass increased and microstructural characterization revealed that the specimens formed a scale. The presence of 0.1 MPa H2S reduces the calculated corrosion rate of carbon steel parent due to preferential formation of sparingly soluble mackinawite. Some fine-scale features were seen only on the tensile surface of the stressed steel specimens. These fine-scale features were deeper (length>cross-section) in specimens tested in the vapour phase. These features, however, will not be visible when seen under light optical microscope (10 x) by visual examination only.

Key words: welded pipe, sour corrosion, cracking, SSC, supercritical CO2

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