Assessment of hydrogen embrittlement severity of an API 5LX80 steel in H2S environments by integrated methodologies

Abstract

High strength steel, as API 5L grade X80 and above, have been applied in pipelines for sour service in oil and gas industry. Hence, comes the concern about the risks comprising hydrogen diffusion and embrittlement due to combined effect of operational conditions and corrosive environment containing hydrogen sulphide (H2S). Many efforts have been made to predict and understand the mechanisms involving hydrogen embrittlement under applied strain, nonetheless they are still not entirely understood. Furthermore, the growth of iron sulphide scales can influence on the diffusion process. This study investigates the behaviour of the API X80 steel concerning hydrogen absorption in solutions with different concentrations of H2S, by integrating different test methodologies at static and tensile conditions. It aims to evaluate hydrogen embrittlement susceptibility of the steel by means of hydrogen permeation and slow strain rate tests and a new complementary image processing methodology. Particularly, the embrittlement phenomenon is studied alongside with iron sulphide film influence as a protective barrier to hydrogen entry in sodium thiosulphate brines containing up to 10 ppm of H2S. Investigations by means of electrochemical impedance spectroscopy and surface analysis indicated that exists a relationship between different concentrations of H2S, scale precipitation and its barrier protectiveness to hydrogen uptake. It was observed a diminution of hydrogen permeation through the steel due to formation of mackinawite, however under tensile stress the film breakage may allow not only hydrogen diffusion into the steel, but also the occurrence of hydrogen embrittlement.