Effect of Heat Treatment Conditions on Corrosion and Hydrogen Diffusion Behaviors of Ultra-Strong Steel Used for Automotive Applications

of them focused on the mechanical degradation of pre-charged steel under electrochemical cathodic polarization, and the corrosion behaviors on the steel surface were not considered. In particular, there have been a limited number of studies on the ef-fects of microstructural modifications by heat treatment on the corrosion and HE of ultra-strong automotive steel. Therefore, this study examined the corrosion-induced HE of ultra-strong steel samples prepared under different heat treatment conditions, and the controlling factors for corro-Effect The purpose of this study was to examine the influence of conditions for quenching and/or tempering on the corrosion and hydrogen diffusion behavior of ultra-strong automotive steel in terms of the localized plastic strain related to the dislocation density, and the precipitation of iron carbide. In this study, a range of analytical and experimental methods were deployed, such as field emission-scanning electron microscopy, electron back scatter diffraction, electrochemical permeation technique, slow-strain rate test (SSRT), and electrochemical polarization test. The results showed that the hydrogen diffusion parameters involving the diffusion kinetics and hydrogen solubility, obtained from the permeation experiment, could not be directly indicative of the resistance to hydrogen embrittlement (HE) occurring under the condition with low hydrogen concentration. The SSRT results showed that the partitioning process, leading to decrease in localized plastic strain and dislocation density in the sample, results in a high resistance to HE-induced by aqueous corrosion. Conversely, coarse iron carbide, precipitated during heat treatment, weakened the long-term corrosion resistance. This can also be a controlling factor for the development of ultra-strong steel with superior corrosion and HE resistance.