Reassessing the HCl immersion method for evaluating hydrogen-induced delayed fracture in press-hardening steels: Pit-induced stress dominance over hydrogen embrittlement

Abstract

Press-hardening steels (PHS) with tensile strengths exceeding 1500 MPa are susceptible to hydrogen-induced delayed fracture (HIDF), posing challenges for automotive applications. Currently, constant load testing in hydrochloric acid (HCl) is widely used to assess HIDF risk, based on the assumption that hydrogen adsorbed via Volmer reaction diffuses into substrate and triggers fracture. However, existing studies have not demonstrated whether hydrogen generated from the reaction between steel and HCl can effectively penetrate substrate. Furthermore, industrial applications reveal ambiguous results when using HCl immersion method to evaluate HIDF risk. In this study, four-point bending tests under HCl immersion were performed on bare and Al-Si coated 2000MPa-grade PHS. Corrosion behavior and hydrogen evolution were examined using scanning electron microscopy (SEM) and thermal desorption analysis (TDA). Both specimens exhibit severe surface corrosion in HCl solution. While trace hydrogen ingress occurs initially, corrosion promotes effusion, preventing inward diffusion. Consequently, hydrogen concentration drops below 0.03 wppm within 10 h and remains stable. Despite these low levels, fractures still occur. Fractographic analysis reveals cleavage and microvoid coalescence without hydrogen embrittlement features. Notably, corrosion pits are observed in tensile region, particularly near laser-cut edges. Finite element analysis (FEA) confirms that pits induce stress concentrations exceeding a factor of 2. These findings indicate that fractures in this context are caused by pit-induced stress concentration rather than HIDF. The results challenge validity of current HCl immersion for assessing HIDF risk in PHS, as it fails to distinguish between corrosion-driven failure and hydrogen embrittlement, leading to misjudgment of actual HIDF risk.