O2 contamination in SSC / HIC test environments. Impact on test results and discussion on acceptable limits for high H 2 S content
Résumé
It is a well admitted fact that oxygen contamination shall be avoided during H 2 S cracking tests of low alloy steels. In the 2016 revisions of NACE TM0177 and NACE TM0284 documents, quantitative limits of O 2 contamination were included with thresholds at 10 and 50 ppb of dissolved oxygen depending on the considered mechanical properties of the tested grade. However, the scientific basis of these values are not well established and there is still a lack of experimental data to illustrate the potential impacts of an oxygen pollution. In addition, while the revised test methods explicitly address initial contamination of the test solution before H 2 S introduction, they do not consider a continuous oxygen supply during testing. Yet, continuous contamination is extremely difficult to be completely eliminated. In order to better understand the impact of O 2 contamination on H 2 S cracking, a 3-years Joint Industrial Project was launched at the end of 2015. The objectives were to evaluate if O 2 contamination can affect H 2 S cracking test results. A range of steel grades covering different types of O&G applications for High H 2 S content were used. SSC (uniaxial tensile tests as well as 4 point-bend) and HIC tests were conducted, with well controlled and continuous O 2 contamination. Three levels of O 2 partial pressures in the gas feed corresponding to 300 ppb, 50 ppb and less than 10 ppb dissolved O 2 were used. In parallel to the standard qualification tests, hydrogen permeation and weight-loss corrosion experiments were performed with the same test matrix, covering all regions of the SSC severity diagram. This paper aims at sharing the main results of this project for high H 2 S content.
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