Inhibiting hydrogen embrittlement in ultra-strong steels for automotive applications by Ni-alloying

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.