Study of hydrogen permeation and hydrogen embrittlement in an API 5L X80

Most of the pipelines in the oil industry are exposed to severe operational conditions and corrosive environments containing hydrogen sulphide (H2S) that might lead to hydrogen entry by diffusion in steels. An important factor that directly affects the useful life time of these pipelines is the existence of weld joints. These joints would act as preferential areas for incidence of hydrogen embrittlement, since the weld process render to residual stresses and distinctive microestrutural features compared to the base metal. This study investigates the behaviour of the API X80 steel considering base metal and a weld joint in relation to hydrogen absorption in solutions with different concentrations of H2S, at static and tensile tests conditions. It aims to compare both, weld joint and base metal, in terms of hydrogen embrittlement behaviour, using hydrogen permeation and slow strain rate tests. The work seeks out to evaluate the conditions of iron sulphide film forming and stability in different solutions of sodium tiosulphate as well as the film influence as a barrier to hydrogen permeation. Solutions with different concentrations of sodium tiosulphate were used in achieving a maximum concentration of 10ppm (massic) of aqueous H2S. It was concluded that on the base metal the most intense embrittlement was observed in 10-3M tiosulphate solutions, with and without chloride. This condition corresponds to the highest hydrogen permeation current and to the lower density iron sulphide film. Surface analysis as SEM, EDS and XRD confirmed the relationship among different H2S concentrations, sulphide film morphology and hydrogen diffusion attenuation. The most protective sulphide film was formed in 10-2M tiosulphate solution. It was also observed that the mackinawite sulphide film can effectively function as a barrier to hydrogen entrance. Pyrrhotite formation occurrence diminished the hydrogen barrier effect. It was concluded that 10-3M tiosulphate solutions with and without chloride are the most aggressive ones in relation to hydrogen permeation and embrittlement. The weld joint tested showed lower loss of ductility than the base metal, being less affected by hydrogen embrittlement in the tested conditions.