Development of Ni-P Alloy Coatings for Better Corrosion Protection Using Glycerol as Additive

A new alkaline citrate bath has been formulated for electrodeposition of Ni-P alloy coatings through conventional Hull cell method, using glycerol as an additive. Electrodeposition following induced type of codeposition was carried out on mild steel (MS) substrate at different current densities (c.d.’s), using the basic bath. The influences of bath constituents and operating parameters on the appearance, hardness, thickness and composition of the deposits have been studied. Corrosion behavior of alloy coatings, having hard-adherent mirror bright appearance developed at different c.d.’s were studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods in 5 wt.% NaCl solution. Experimental results revealed that corrosion protection efficacy of NiP coatings increased with deposition c.d., in proportion of its P content up to an optimal limit, and then decreased. The coating developed at 4.0 A dm-2 was found to exhibit the least corrosion rate (14.2×10-2 mm y-1), compared to those at other c.d.’s. The process and products of electrodeposition have been studied using different methods, such as cyclic voltammetry (CV), Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analyses. Experimental results were analyzed in the light of principles of induced codeposition and role of additives, and results are discussed.