Effect of iron sulphide film formation on hydrogen embrittlement of an API 5L X80 steel in H_2S solutions

摘要

With the growing application of high strength steel, as API 5L X80, for pipelines in the oil industry, and related geothermal energy generation plants, comes the concern about the risks involving hydrogen diffusion and hence, embrittlement due to severe operational conditions and corrosive environment containing hydrogen sulphide (H_2S). Although many efforts have been made to predict and understand those phenomena, the mechanisms involving H_2S corrosion combined with applied strain are still not entirely understood. In addition, the growth of iron sulphides scales can influence on the diffusion process. This study investigates the behaviour of the API X80 steel in relation to hydrogen absorption in solutions with different concentrations of H_2S, at static and tensile test conditions. It aims to evaluate hydrogen embrittlement susceptibility of the steel by means of hydrogen permeation and slow strain rate tests. Furthermore, the present work seeks out to evaluate, particularly in this embrittlement process, the conditions of iron sulphide layer forming and stability in different solutions of sodium thiosulphate, as well as the film influence as a protective barrier to hydrogen entry. Investigations using electrochemical impedance spectroscopy and surface analysis by SEM, EDS and XRD indicated that exists a relationship between different concentrations of H_2S, scale formation and its barrier protectiveness to hydrogen generation and uptake.