With the stricter international regulations on CO₂ emissions, fuel economy, and auto-safety, the application of novel materials with both higher strength and lower weight is becoming a major technical issue in automotive industries. Among the various lightweight concepts, ultra-strong GIGA STEEL with a tensile strength of more than 2 GPa is a major breakthrough in light of the remarkable weight reduction of vehicle without a decrease in auto-safety. Despite the outstanding mechanical performance, hydrogen embrittlement induced by aqueous and/or atmospheric corrosion is a serious problem that has restricted the application of steel to auto-parts. This study reports that such a critical challenge can be overcome by Ni-alloying, which leads to a lower cathodic reduction rate on the steel surface and slower H-infusion kinetics in the steel matrix. In contrast to the beneficial effects of Ni-alloying, conflicting results can be obtained when steel with a higher Ni content (≥1 wt.%) is exposed to neutral-corrosive environments, but the results have not been verified using conventional metallurgical approaches. This paper proposes a mechanism for these conflicting results, and provides a new and economic strategy for superior resistance to corrosion-induced hydrogen embrittlement, by making optimal use of Ni-alloying of ultra-strong steel.

Various experimental analyses on hydrogen evolution, absorption, and cracking behaviors were conducted to gain a fundamental understanding of the hydrogen embrittlement of ultrastrong steel sheets with galvanized (GI) and galvannealed (GA) coatings. The hydrogen evolution and absorption behaviors are controlled primarily by the potential differences between the coating and exposed steel substrate, and the corrosion-induced damage pattern of the coating. The higher absorption rate of hydrogen was more pronounced in corroded GI-coated steel caused by the larger cathodic polarization applied to the exposed substrate, and a more severe form of coating dissolution by aqueous corrosion in a 3.5% NaCl + 0.3% NH₄SN solution. In contrast, the corrosive species can only penetrate through the pre-existing cracks in the brittle Fe-Zn intermetallic phases composed of the GA coating, and the driving force for hydrogen evolution becomes smaller. These result in significant differences in hydrogen penetration and cracking behaviors between the two coated ultrastrong steels.

This study examined the effects of alloying elements (C, Mo) on hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) behaviors of A516-65 grade pressure vessel steel in sour environments. A range of experimental and analytical methods of HIC, SSC, electrochemical permeation, and immersion experiments were used. The steel with a higher C content had a larger fraction of banded pearlite, which acted as a reversible trap for hydrogen, and slower diffusion kinetics of hydrogen was obtained. In addition, a higher hardness in the mid-thickness regions of the steel, due to center segregation, resulted in easier HIC propagation. On the other hand, the steel with a higher Mo content showed more dispersed banded pearlite and a larger amount of irreversibly trapped hydrogen. Nevertheless, the addition of Mo to the steel can deteriorate the surface properties through localized pitting and the local detachment of corrosion products with uneven interfaces, increasing the vulnerability to SSC. The mechanistic reasons for the results are discussed, and a desirable alloy design for ensuring an enhanced resistance to hydrogen assisted cracking (HAC) is proposed.