Effect of partial pressure ratio H2S/CO2 on mechanical properties of 3Cr oil tube steel

With the exploitation of high sour and deep oil well in China, the corrosion medium containing CO2, H2S and Cl⁻ threaten to the safety of oil equipment and tube steel severely. In this article, solution immersion environments containing H2S/CO2 were simulated in a high temperature and high pressure autoclave respectively. The effect of partial pressure ratio H2S/CO2 on mechanical properties of 3Cr oil tube steel was investigated and analyzed by the test technologies of tensile and impact, SEM and diffusible hydrogen. And the mechanism of partial pressure ratio H2S/CO2 on mechanical properties was discussed primarily. The results show that high concentration H2S has a significant impact on mechanical properties degradation of oil tube steel. The performance index of tensile strength, yield strength and impact toughness decreased at different degree with the addition of high concentration H2S. The performance index became more lower with the increasing of partial pressure ratio H2S/CO2. The SEM cross section micrographs of tensile specimens indicated that many pimple characteristics which represent toughness fracture exited under low concentration H2S corrosion, many river pattern and feather pattern characteristics which represent brittle fracture appeared and the pimple characteristics decreased with the addition of high concentration H2S. The results of diffusible hydrogen show that the content of diffusible hydrogen increased with the increasing of partial pressure ratio H2S/CO2, which indicated that the hydrogen generated from H2S corrosion have a significant impact on mechanical properties degradation of oil tube steel.