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 hours 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 zinc10%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 micron nickel layer was effective in lowering the embrittlement caused by zinc-1%cobalt to that of zinc-10%nickel. Furthermore, a 0.5 micron cobalt layer deposited before a zinc-1%cobalt coating gave virtually 100% recovery of mechanical properties after baking. INTRODUCTION Electrodeposited zinc-cobalt alloys are possible alternatives to cadmium for sacrificial corrosion protection of steel components. The coatings contain typically 1-4% Co by weight and are produced by an anomalous co-deposition mechanism (1,2) that promotes a zinc-rich alloy by suppressing the reduction of the more noble cobalt. The corrosion rate of electrodeposited zinc-4%Co is reported to be very similar to that of the more expensive zinc14%Ni . The possibility of hydrogen embrittlement is an important consideration when high strength steels are electroplated. Plating processes generally have a current efficiency of less than 100% and a proportion of the current passed during electrodeposition results in the generation of hydrogen, some of which becomes incorporated in the deposit and subsequently diffuses into the substrate. When hydrogen embrittlement is thought to be a risk a de-embrittlement treatment may be required. Following cadmium plating, for example, steels with a tensile strength over 1800 MPa are baked for 24 hours at a temperature in the range 190-230°C . The aim of the work described in this paper was to investigate the extent to which electroplating with zinc-cobalt alloys causes hydrogen embrittlement of high strength steel. Slow strain rate tests were carried out on plated tensile specimens to measure the effect of the absorbed hydrogen on the loss of the steel’s mechanical properties and the effectiveness of post-plating baking treatments in restoring these properties. For comparison, further tests were conducted on specimens plated with pure zinc, Zn-10%Ni and cadmium and also with Zn-12%Co-9%Fe to investigate ways in which hydrogen uptake could be controlled by the coating composition.