Hydrogen embrittlement of high strength steel electroplated with zincâ  cobalt allo

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 zincâ  cobalt alloys. The effects of bath composition and pH were studied and compared with results for electrodeposited cadmium and zincâ  10%nickel. It was found that zincâ  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 à °C gave full recovery of mechanical properties but specimens plated with zincâ  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 zincâ  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 m nickel layer was effective in lowering the embrittlement caused by zincâ  1%cobalt to that of zincâ  10%nickel. Furthermore, a 0.5 m cobalt layer deposited before a zincâ  1%cobalt coating gave virtually 100% recovery of mechanical propert