Hydrogen embrittlement of high strength steel electroplated with zinc-cobalt alloys

Slow strain rate tests were performed on quenched and tempered AISI 4340 steel to measure the extent of hydrogen embrittlement caused by electroplating with zinc-cobalt alloys. The effects of bath composition and pH were studied and compared with results for electrodeposited cadmium and zinc-10%nickel. It was found that zinc-1%cobalt alloy coatings caused serious hydrogen embrittlement (EI 0.63); almost as severe as that of cadmium (EI 0.78). Baking cadmium plated steel for 24 h at 200 °C gave full recovery of mechanical properties but specimens plated with zinc-1%cobalt and then baked still failed in 89% of the time of unplated controls. It was shown that hydrogen uptake and embrittlement could be controlled by depositing thin layers of cobalt or nickel at the steel/coating interface. For example, the least embrittlement was caused by zinc-10%nickel (EI 0.037) due to a nickel rich layer with very low hydrogen diffusion coefficient that formed during the initial stages of electroplating. Similarly, a 0.5 μm nickel layer was effective in lowering the embrittlement caused by zinc-1%cobalt to that of zinc-10%nickel. Furthermore, a 0.5 μm cobalt layer deposited before a zinc-1%cobalt coating gave virtually 100% recovery of mechanical properties after baking.