Structure, microhardness and corrosion behaviour of nanostructured CoP coatings obtained by direct current and pulse plating

Abstract A previously designed electrolyte to obtain nanocrystalline CoP (nCoP) alloys has been modified to overcome some process drawbacks, mainly bath instability and low current efficiencies. Moreover, the influence of some plating conditions, such as the anode material/configuration or the hydrodynamic regime, has been analysed to try to optimize the coatings characteristics and, at the same time, reducing production costs. Using the modified electrolyte, air agitation and a cobalt anode, nCoP alloys with low phosphorous content (1–2 wt%) and good properties can be obtained both using galvanostatic pulse plating (PP) and direct current (DC) electrodeposition. With these conditions the bath stability, the current efficiency and also the current distribution are significantly increased. Although PP and DC coatings have similar characteristic, PP alloys show smaller grain sizes and this results in an increased microhardness after annealing. This annealing has a side effect on the coatings, which is the cracking of the layer and its obvious impact on its protective performance. Despite this, these nCoP alloys are promising as alternative to traditional electroplated hard chromium coatings.