Two types of cracking, namely sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC), were evaluated in linepipe steels using NACE standard solution to establish the metallurgical parameters that control HIC and SSC. Quantitative experiments indicate a threshold hydrogen concentration for HIC below which no cracking will initiate. Propagation of cracks occurs by hydrogen assisted fracture of the matrix surrounding the site where initiation occurs. The SSC and HIC susceptibility of steels containing Cr, Ni, and Mo under stress and hydrogen diffusion was investigated by cathodic polarization. As the content of alloying elements increased, the apparent hydrogen diffusion coefficient (D) and threshold hydrogen permeation rate (Jth) for hydrogen embrittlement decreased. Hydrogen concentration (Co) in the steel in NACE TM-0177 solutions increased due to decreasing hydrogen diffusion coefficient, D, with increasing Cr, Ni and Mo. Although hydrogen permeability (J) decreased, and the susceptibility to hydrogen-induced cracking increased as a consequence.
Keywords: Sulfide stress cracking, HIC, inclusions, hydrogen permeation, hydrogen diffusion, alloying elements, linepipe steels.