Effect of molybdenum on the resistance to H2S of high sulphur microalloyed steels

Abstract

The resistance to hydrogen sulphide of microalloyed steels with sulphur levels around 0.02% and containing copper, chromium, nickel and molybdenum was studied. The sulphide stress cracking (SSC), hydrogen induced blister cracking (HIBC), hydrogen embrittlement (HE) and corrosion rates of the steels were evaluated. The threshold stress during SSC testing was found to increase with an improvement in HE resistance. Molybdenum was found to improve the HE resistance which increases the SSC resistance. HIBC and corrosion rates were lowest at 0.3% molybdenum content in the steel. Additions of copper, chromium and nickel to a 0.3% molybdenum bearing steel enhance the resistance to SSC, HIBC and HE and corrosion rates. Fine precipitates 10-25 nm in size, containing molybdenum, titanium, vanadium, chromium and sulphur, uniformly distributed in the matrix were found to improve the sour gas resistance. Without molybdenum coarser precipitates 150 nm in size were observed and HIBC was found to initiate at such precipitates. Steel with low sulphur content (0.015%) but with coarser precipitates did not show any improvement to sour gas resistance when compared with a steel with high sulphur (0.028%) content, but possessing fine precipitates. © 1993.