Estudo da permeação de hidrogênio e fragilização pelo hidrogênio de um aço API 5L X80

Abstract

With the growing application of high strength steel, as API 5L X80, for pipelines in the oil industry, comes the concern about the risks involving hydrogen diffusion and hence, embrittlement due to severe operational conditions and corrosive environment containing hydrogen sulphide (H2S). Although many efforts have been made to predict and understand those phenomena, the mechanisms involving H2S corrosion combined with applied strain still not entirely understood. In addition, the growth of iron sulphides scales can take an important part on the diffusion process. This study investigates the behaviour of the API X80 steel in relation hydrogen absorption in solutions with different concentrations of H2S, at static and tensile tests conditions. It aims to evaluate hydrogen embrittlement behaviour by means of hydrogen permeation, DevanathanStachurski method, and slow strain rate tests. Furthermore, the present work seeks out to evaluate 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 as SEM, EDS and XRD indicated that exists a relationship between different concentrations of H2S, scale formation and its protectiveness, being the tiosulphate concentration of 10-3M the most aggressive in relation to hydrogen embrittlement to this steel due to a conjoint formation of pyrrhotite and mackinawite. The FeS film as mackinawite presented to be the most efficient as a barrier to hydrogen uptake and stress resistance, resulting in lower current densities of hydrogen permeation and lower levels of embrittlement in the 10-2M solution.