The effect of Zinc-Cobalt electroplating on the hydrogen embrittlement of high strength steel

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