Influence of metallurgical factors on the hydrogen induced cracking of carbon steel wires in H2S-containing environments

Abstract

Flexible pipes armor wires used in offshore oil and gas fields can corrode if water and acid gases, such as H2S, build up in the annulus. Such an environment may provide the conditions for hydrogen-induced cracking (HIC) depending on the corrosive solution composition and the metallurgical characteristics of the armor wire, such as microstructure and microhardness. The critical zones of HIC incidence were investigated for tensile and pressure armor wires with carbon content of 0.7 and 0.3 wt% C, respectively. Results were obtained through immersion testing and hydrogen permeation measurements in near neutral substitute ocean water (solution A) and standard acid solution (solution B) at ambient temperature bubbled with a gas mixture (1 % H2S + 99 % N2). Scanning electron microscope (SEM) and computed tomography (CT) evidenced the microstructure and microhardness features associated to crack morphology and location. According to the results, the increase of carbon content in cold drawn steels promoted the accumulation of hydrogen mainly in regions of high microhardness, inducing a high density of cracks in a stepwise cracking (SWC) propagation. This indicates that the driving force for HIC was associated with metallurgical factors, such as microhardness, the presence of hydrogen trapping sites of high energy, dislocations and ferrite/cementite morphology and distribution, occurring even in the absence of non-metallic inclusions. Additionally, environmental conditions, such as the low pH found in solution B, increased the criticality of HIC. Both materials presented a crack sensitivity ratio (CSR) that grew over time in solution B, but no cracks were observed in the pressure armor wire tested in solution A. Hydrogen permeation curves were delayed for both tested armor wires in solution A due to its near neutral pH characteristic. Hydrogen diffusivity (Deff) of the tensile armor wire was lower than the pressure armor wire, indicating the presence of a higher density of hydrogen trapping sites in a harder and elongated pearlite.