Effects of Ni-flash coating on hydrogen evolution, ad/absorption, and permeation behaviors of advanced high-strength steel during electro-Zn plating

The effects of a thin Ni-flash coating, tens of nanometers thick, on hydrogen evolution, ad/absorption, and permeation of advanced high-strength steel were examined for a deeper understanding of the hydrogen infusion behavior in the steel substrate during electro-Zn plating. The electrochemical permeation technique and impedance spectroscopy were used under cathodic polarization in a step-up manner. In addition to the electrochemical analyses, the hydrogen microprinting technique was employed to identify the distribution of Ag particles (locating hydrogen atoms) in the electro-Zn plated steels with and without a thin intermediate Ni-layer. The results revealed that despite the higher hydrogen evolution rate on Ni-flash coating layer than on bare steel, the intermediate Ni-layer decreased the hydrogen infusion considerably in the steel substrate during electroZn plating, due primarily to the lower hydrogen ad/absorption rate on the Ni-flash coating layer, and the predominant hydrogen trapping at the multi-interfacial areas of the Zn-layer/Ni-layer/steel substrate. These results could provide insights into the precise role of a thin (tens of nanometers) Ni-flash coating on the resistance to hydrogen embrittlement of ultra-high-strength steel alloys during electro-Zn plating.