Influence of Optimized Decarburization on Hydrogen Uptake and Aqueous Corrosion Behaviors of Ultrasong Martensitic Steel

This study examined the effects of microstructural alterations by controlling the surface carbon gradient via a thermal decarburizing process on hydrogen evolution, adsorption, and permeation along with neutral aqueous corrosion behavior of an ultra-high-strength steel with a tensile strength of 2.4 GPa. Microstructural analyses showed that an optimized decarburizing process at 1100 ◦C led to partial transformation to ferrite without precipitating Fe3C in a marked fraction. Electrochemical impedance spectroscopy along with the permeation results revealed that there was a notable decrease in hydrogen evolution and subsurface hydrogen concentration. Moreover, immersion test in a neutral aqueous condition showed slower corrosion kinetics with a comparatively uniform corroded surface, indicating improved corrosion resistance. However, the extent of improvement is significantly limited under non-optimized decarburizing conditions, specifically when the temperature is below or above 1100 ◦C, due to insufficient decarburization or the formation of coarse-spheroidized Fe3C particles, accompanied by a porous subsurface layer. In particular, a far greater adsorption tendency at bridge sites on Fe3C (001) in a pre-charged surface is highlighted. This study provides insight that the adjustment of the carbon gradient through an optimized annealing process can be an effective technical strategy to overcome the critical drawbacks of ultrastrong martensitic steels under hydrogen-rich or corrosive conditions.