Preparation of amorphous-nanocrystalline composite structured Ni–P electrodeposits

Amorphous-nanocrystalline composite structured Ni–P deposits (with phosphorus content of 5–9 wt.%) show better corrosion resistance than that of microcrystalline structured Ni–P deposits, and better abrasion resistance than that of amorphous Ni–P deposits. In order to get amorphous-nanocrystalline composite structured Ni–P deposits on AISI 431 stainless steel, electroplating parameters that affect phosphorus content in Ni–P deposits (temperature, current density, pH, H3PO3 concentration and agitation rate) are analysed by orthogonal experimental design (OED) in this paper. The analysis indicates that pH value and phosphorous acid concentration are key variables, their interactive effect on Ni–P composition should be taken into account. The effects of pH and H3PO3 concentration of the electroplating bath on phosphorus content in Ni–P deposits are further examined by the method of steepest ascent and central composition design (CCD). On the basis of results and discussion in experimental design sections, various Ni–P deposits with approx. 5–9 wt.% phosphorus can be galvanostatically electroplated by controlling the key variables and these deposits are confirmed to be amorphous-nanocrystalline composite structure in XRD, TEM tests.