Effect of Baking on Hydrogen Embrittlement for High Strength Steel Treated with Various Zinc Based Electroplating from a Sulfate Bath

Abstract

The hydrogen embrittlement of SK85 high-strength steel sheets was evaluated using a three-point bending test. The effects of electroplating each with zinc, zinc–SiO2, zinc–nickel, and zinc–nickel–SiO2 on hydrogen embrittlement were examined by baking the electroplated steel specimen. Each electroplating type caused hydrogen embrittlement, which was promoted by hydrogen, owing to the reduction due to hydrogen ions during electroplating. The hydrogen embrittlement of both zinc-electroplated and zinc–SiO2-electroplated SK85 steel continued after baking for 24 hours at 200°C, but that of the zinc–nickel-electroplated and zinc–nickel–SiO2-electroplated SK85 steel ceased. Furthermore, TDA revealed that the diffusible hydrogen at approximately 200°C, which was caused because of hydrogen embrittlement, was desorbed from all the electroplated specimens before the baking. However, after the baking, this diffusible hydrogen for each specimen was not desorbed. These results indicate that the hydrogen embrittlement for zinc-based electroplated high-strength steel was caused by another factor except for diffusible hydrogen. The hydrogen formed due to the electroplating was incorporated in the steel substrate, following which the hydrogen-vacancy cluster was formed in the substrate. It seems that the zinc and zinc–SiO2 film provided insufficient permeability required for the formation of the hydrogen-vacancy cluster. However, zinc–nickel and zinc–nickel–SiO2 film enabled hydrogen-vacancy cluster diffusion from the substrate.