Effect of applied cathodic potential on susceptibility to hydrogen embrittlement in high strength low alloy steel

In order to investigate the effects of applied cathodic potential and chemical hydrogen absorption promoter (thiourea) on the susceptibility to hydrogen embrittlement of the high strength low alloy (HSLA) steel, slow strain rate tensile (SSRT) tests were conducted in various buffer aqueous solutions of pH 10.0. During SSRT tests, the specimens were cathodically polarized under potentiostatic control, Polarization levels were varied from the corrosion potential (E-corr) to hydrogen gas evolution potential (E-H2gas), including the adsorbed hydrogen potential (E-Hads) determined by cyclic voltammetry (CV). Additionally, thermal desorption spectroscopic FDS) analyses were applied to the specimens fractured by SSRT test and the dependence of hydrogen absorption characteristic on applied potential was also examined for understanding the SSRT test results. Experimental results revealed that the hydrogen embrittlement behavior was strongly dependent on the applied cathodic potential. In the E-Hads region, the susceptibility to hydrogen embrittlement increased with increasing cathodic overpotential, On the other hand, in the E-H2gas region, the susceptibility was independent of the applied potential. Moreover, by adding thiourea to the solution, the susceptibility to hydrogen embrittlement increased slightly in the E-H2gas region compared with that in the solution without thiourea. There was a relatively good correlation between the amount of hydrogen absorption per unit time (C-H/t(i)) and the time to failure (t(f)), namely, the susceptibility to hydrogen embrittlement has a tendency to increase monotonically with increasing C-H/t(f). As a consequence, the potential dependence of hydrogen embrittlement was closely associated with the variation in hydrogen absorption rate under applied potential.