Hydrogen embrittlement susceptibility of AISI 4140 alloy-steel under cathodic protection and in H2S and CO2 environment

Abstract

The susceptibility to hydrogen embrittlement (HE) of AISI 4140 steel when subjected to environments such as those found in offshore oil and gas fields is investigated. Hydrogen charging was carried out for 15 days using two different hydrogen sources, cathodic protection (CP) and corrosion in a saline solution containing high concentrations of H2S and CO2 (SC-sour corrosion). The steel specimens were austenitized at 850 °C, oil-quenched at ambient temperature, followed by tempering for 18 min at 350, 450, and 600 °C. The effect of tempering temperatures on hydrogen embrittlement susceptibility was also investigated. Tensile strength testing, scanning electron microscopy (SEM), and fractography analysis were used to correlate the HE with microstructural and environmental conditions. The results revealed that both hydrogen charging conditions increased the steel hardness and decreased ductility, with this effect being more pronounced at lower tempering temperatures and SC-hydrogen charging. Fracture surfaces of the steel tempered at 600 °C with hydrogen charged exhibited predominantly ductile behavior with the presence of dimples, while at 350 and 450 °C surface fractures showed cleavage facets as a consequence of brittle behavior. In general, the AISI 4140 steel hydrogen embrittlement index (%EI) was higher for hydrogen charging induced by SC compared to CP.