%0 Journal Article
%T Potentiodynamic Polarization Behaviour of Electroless Ni-P-W Coatings
%A Supriyo Roy
%A Prasanta Sahoo
%J ISRN Corrosion
%D 2012
%R 10.5402/2012/914867
%X This paper deals with the synthesis of electroless Ni-P-W coatings on mild steel substrate followed by furnace-annealing process. Corrosion behaviors of the coatings after heat treatments at various annealing temperatures are evaluated by potentiodynamic polarization test using 3.5% sodium chloride solution. The electrochemical parameters, that is, corrosion potential and corrosion current density, are optimized for maximum corrosion resistance using Taguchi-based grey relational analysis, considering four coating parameters, namely, concentration of nickel, concentration of reducing agent, concentration of tungsten, and annealing temperature as main design factors. The optimum combination of these four design factors is obtained from the analysis. The analysis of variance reveals that the concentration of tungsten source and annealing temperature play the most important role on the corrosion performance of the coating. Effects of the operating parameters on microstructures, in terms of porosity formation, crystallization, phase transformation, grain growth, are investigated using SEM, EDX, and XRD techniques. 1. Introduction The performance and lifetime of engineering components can be enhanced by applying hard coating over the surface of the components, by allowing the mechanical properties of the substrate material to be maintained while protecting them against wear, friction, or corrosion. Basically the coating acts as a barrier to the substrate material by sealing it from the environment. Among the coating processes the electroless coating process is now widely accepted by the industries due to its simplicity and enhanced properties like improved friction, wear, and anticorrosive properties. The basic hypophosphite-reduced Ni-P and borohydride-reduced Ni-B coating has proved its supremacy in providing improved hardness, corrosion, and wear resistance [1¨C4]. The corrosion resistance of electroless Ni-P coating is excellent, moreover in some environments it is superior to that of pure nickel or chromium alloys due to the amorphous nature and passivity of the coating [5]. It is obvious that increasing of coating porosity decreases the corrosion resistance of the coating. The important factor that affects the coating porosity and ultimate resistance to corrosion attack is surface roughness which is influenced by mechanical preparation of the surface and electroless nickel-coating process procedure [6]. Recently, much attention is being paid towards composite electroless nickel plating. The properties of composite electroless nickel coatings are often
%U http://www.hindawi.com/journals/isrn.corrosion/2012/914867/