Effect of hydrogen diffusion and trapping behavior on the hydrogen assisted cracking of linepipe steels

DoctorThe hydrogen assisted cracking (HAC) is one of the greatest concerns for the linepipe steel when the high strength low alloy (HSLA) linepipe steel is exposed to acidic crude oil containing hydrogen sulfide (H2S). Although hydrogen trapping in the steel as well as the interaction between hydrogen and lattice iron have been widely accepted as the primary cause of hydrogen related cracking and a great deal of researches have attempted to explain the relationship among hydrogen behavior in the steel, microstructure and the cracking characteristics, the mechanism of hydrogen induced cracking (HIC) and sulfide stress cracking (SSC) have not been clearly understood. This dissertation is dealing with two research objectives. One is to design a proper electrochemical hydrogen permeation technique (HPT) to assess the hydrogen diffusion and trapping behavior in the steel. Another is to investigate the effect of microstructure and constant elastic load on the hydrogen trapping efficiency of linepipe steel. Eventually, they are correlated with HIC and SSC characteristics. Hydrogen trapping efficiency of ferrite/degenerated pearlite (F/DP), ferrite/acicular ferrite (F/AF) and ferrite/bainite (F/B) was assessed using the HPT with minor modification in reference to ISO17081. It was revealed that the AF and B acted as the reversible trapping site and the hydrogen trapping efficiency was increased in the order of DP, B and AF, with AF being the most efficient. Initiation site of internal crack was proved to be not non-metallic inclusions often reported as the typical initiation sites of cracks, but the locally agglomerated M/A constituents for the steel with F/AF or F/B microstructure since M/A constituents are easily embrittled by hydrogen. As the microstructure changed from F/AF to F/B, the HIC susceptibility increased, and this can be explained by high toughness of the AF which is impeding the crack propagation. For the electrochemical permeation study on the hydrogen diffusion and trapping behavior of linepipe steel under constant load, the HPT combined with the constant load test (CLT) was especially designed. The stable oxide film anodically grown in 0.1M NaOH solution was used instead of thin palladium (Pd) coating to avoid poor reliability and reproducibility triggered by non-uniformity of Pd film during the elastic deformation of steel membrane. As a result, it was proved that the oxide film can be applicable to the designed HPT even without Pd coating if carefully controlled. Regardless of hydrogen charging condition, increase in the elastic load increased the amount of diffusible hydrogen. It is explained by the lattice expansion during elastic deformation of the steel and easy hydrogen diffusion through enlarged interstitial sites. Increased diffusible hydrogen is available to react with defects in the steel microstructure as well as to embrittle the surrounding microstructure around an inclusion, accelerating SSC propagation