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

Abstract

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.