Environmental cracking susceptibility of low alloy steel oil country tubular goods (OCTG) (95 ksi & 110 ksi SMYS proprietary sour grades and 110 ksi SMYS non sour grade) was investigated in high H2S partial pressures using 4 point bent beam tests in autoclaves. The H2S partial pressures and testing temperatures varied from 10 bar – 100 bar to 24°C – 150°C. Sour grades – 95 ksi and 110 ksi – remained crack free in all tested conditions. Some localized corrosion occurred after one month exposure on all materials in higher H2S in the higher temperature conditions, but these corrosion initiations did not propagate after long term (3 months) immersion tests. It was concluded that the sour grades tested in this research can be used up to 100 bar H2S and up to 150°C.
The 110 ksi non sour grade (standard API 5CT P110) suffered from SSC below 66°C at 10 bar H2S. These results are in line with the temperature limit of API P110 material as defined in NACE MR0175/ISO 15156-2. The 110 ksi non sour grade also showed environmental cracking at 100 bar H2S from 107°C to 150°C. Based on these results, it is considered that the SSC temperature limits for these grades (110 ksi non sour) should be treated carefully and not applied to any H2S partial pressure conditions.
Mechanisms of cracking susceptibility were discussed from the viewpoints of corrosion reaction, hydrogen absorption and the morphologies of corrosion products on the steel surface.
