Browsing by Author "Hillier, Elizabeth M. K."

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    The effect of Zinc-Cobalt electroplating on the hydrogen embrittlement of high strength steel
    (Cranfield University, 2001-07) Hillier, Elizabeth M. K.; Robinson, M. J.
    Electroplated cadmium coatings are widely used in the aerospace industry to provide corrosion protection for components such as fasteners, which are manufactured from high strength steels. Due to environmental and safety concerns with regard to the use of cadmium, there is a requirement to investigate suitable replacements. Any alternate coating, in addition to effective corrosion protection, must not have deleterious effects on the steel substrate. Hydrogen is known to be absorbed during electroplating and there are concerns that this could cause hydrogen embrittlement of high strength steel fasteners. This study has investigated the effect of zinc-cobalt electroplating on the embrittlement of high strength steels used for aerospace applications, due to promising corrosion properties. i Permeation measurements have been used to measure the hydrogen uptake during electroplating, and slow strain rate testing performed to establish the effect of hydrogen on the embrittlement of the steel substrate. The influences of bath composition, temperature and pH have been studied, plus the addition of other iron group elements to the plating bath to minimise the risk of embrittlement. Hydrogen that has passed into the metal substrate has been located by a decorative mapping technique that also showed the high concentration present in the electroplated coatings. Corrosion tests have been executed to assess the performance of the electroplated coatings that have been tested. The factors that affect the ernbrittlement of zinc-cobalt electroplated high strength steels and the methods, including modulated multi-layer coatings and baking, that could be used for its control are described. 10
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    Hydrogen embrittlement of high strength steel electroplated with zincâ  cobalt allo
    (Elsevier Science B.V., Amsterdam., 2004-03-03T00:00:00Z) Hillier, Elizabeth M. K.; Robinson, M. J.
    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