Hydrogen embrittlement of SK85 high-strength steel sheets was evaluated using a three-point bending test. Using acid dipping before electroplating, mainly zinc and zinc-nickel alloy electroplating, we examined effects on hydrogen embrittlement in terms of hydrogen permeability of the plating films. Both zinc and zinc-nickel alloy electroplating caused hydrogen embrittlement, which was promoted by hydrogen, based on hydrogen ion reduction during electroplating. Hydrogen embrittlement of zinc-electroplated SK85 steel continued with time and film thinning, but that of the zinc-nickel alloy electroplating was reduced. These results indicate that the zinc film had insufficient permeability for hydrogen embrittlement. However, that of zinc-nickel alloy electroplating enabled hydrogen diffusion from the substrate. Results demonstrate that zinc-nickel alloy electroplating having hydrogen permeability and excellent corrosion resistance is an effective plating method to prevent hydrogen embrittlement of high-strength steel.
We fabricated Ni-P / Zn-Ni multilayered structures on a steel substrate using alternate electroplating with two electrolyte baths. We prepared two series of the multilayered structures to elucidate the effects of component layer thickness and annealing on hardness and wear resistance. In one series, the Ni-P and Zn-Ni layers had equal thicknesses of 200 nm, 500 nm, and 1000 nm, whereas the other series had constant 500 nm Ni-P layer thickness. Then, the electroplated films were annealed at 573 K for 2 hr. For all multilayered structures, the annealing increased the hardness and wear resistance because of Ni-P layer hardening. Among the multilayers having equal component thicknesses, the 500-nm layer thickness exhibited the highest hardness and wear resistance. In the annealed multilayers having constant 500 nm Ni-P thickness, the hardness and wear resistance increased concomitantly with decreasing Zn-Ni layer thickness. Our key finding is that the annealed multilayers consisting of 500-nm Ni-P and 100-nm Zn-Ni layers exhibited three times higher wear resistance than an annealed Ni-P film.