Evaluation of the corrosion resistance of electroless Ni-P and Ni-P composite coating by electrochemical impedance spectroscopy

Electroless Ni-P composite coatings have gained a good deal of popularity and acceptance in recent years as they provide considerable improvement of desirable qualities such as hardness, wear, abrasion resistance, etc. The disagreement among researchers on the corrosion behaviour of these coatings warrants a thorough investigation. Among the various techniques available for the determination of corrosion resistance, electrochemical impedance spectroscopy (EIS) is considered to be superior as it provides not only an assessment of the corrosion resistance of different deposits but also enables the mechanistic pathway by which the deposits become corroded to be determined. The present investigation focuses on the evaluation of the corrosion resistance of electroless Ni-P and Ni-P-Si3N4, Ni-P-CeO2 and Ni-P-TiO2 composite coatings produced using an acidic hypophosphite-reduced electroless nickel bath, using EIS. The study makes evident that the same fundamental reaction is occurring on all the coatings of the present study but over a different effective area in each case. The charge transfer resistance of electroless Ni-P and NI-P composite deposits are in the range 32,253-90,700 x3A9; cm2, whereas the capacitances of these coatings are in the range 11-17 x3BC;F/cm2. The improved corrosion resistance obtained for electroless Ni-P and Ni-P composite coatings is due to the enrichment of phosphorus on the electrode surface, which enables the preferential hydrolysis of phosphorus over that of nickel. The better corrosion resistance obtained for electroless Ni-P composite coatings can be ascribed to the decrease in the effective metallic area prone to corrosion. Among the three electroless Ni-P composite coatings, the corrosion resistance is in the following order: Ni-P-CeO2=Ni-P-Si3N4gt;Ni-P-TiO

Electroless Ni-P composite coatings

This review outlines the development of electroless Ni-P composite coatings. It highlights the method of formation, mechanism of particle incorporation, factors influencing particle incorporation, effect of particle incorporation on the structure, hardness, friction, wear and abrasion resistance, corrosion resistance, high temperature oxidation resistance of electroless Ni-P composite coatings as well as their applications. The improvement in surface properties offered by such composite coatings will have a significant impact on numerous industrial applications and in the future they will secure a more prominent place in the surface engineering of metals and alloys

Structure and phase transformation behaviour of electroless Nix2013;P composite coatings

This paper addresses the structural characteristics and phase transformation behaviour of plain electroless Nix2013;P coating and electroless Nix2013;Px2013;Si3N4, Nix2013;Px2013;CeO2 and Nix2013;Px2013;TiO2 composite coatings. The X-ray diffraction patterns of electroless Nix2013;Px2013;Si3N4, Nix2013;Px2013;CeO2 and Nix2013;Px2013;TiO2 composite coatings are very similar to that of plain electroless Nix2013;P coating, both in as plated and heattreated conditions. Selected area electron diffraction (SAED) patterns obtained on the Nix2013;P matrix of Nix2013;Px2013;Si3N4, Nix2013;Px2013;CeO2 and13; Nix2013;Px2013;TiO2 composite coatings exhibit diffuse ring patterns resembling the one obtained for plain electroless Nix2013;P coating. Phase transformation behaviour studied by differential scanning calorimetry (DSC) indicates that the variation in crystallization temperature and the energy evolved during crystallization of plain electroless Nix2013;P coating and electroless Nix2013;Px2013;Si3N4, Nix2013;Px2013;CeO2 and Nix2013;Px2013;TiO2 composite coatings is not significant. The study concludes that incorporation of Si 3N4, CeO2 and TiO2 particles in the Nix2013;P matrix does not have any influence on the structure and phase transformation behaviour of electroless Nix2013;P coatings

Deposition of zinc-zinc phosphate composite coatings on steel by cathodic electrochemical treatment

The present work aims at the development of an energy-efficient and eco-friendly approach for the deposition of zinc phosphate coatings on steel. The study describes the possibility of preparing zinc-zinc phosphate composite coatings by cathodic electrochemical treatment using dilute phosphoric acid as an electrolyte and zinc as an anode. The methodology enables the preparation of coatings with different proportions of zinc and zinc phosphate by suitably varying the applied current density, pH, and treatment time. Adhesion of the coating on mild steel and adhesion of paint film on the phosphate coating were found to be good. The surface morphology of the coatings exhibited platelet-type features and small white crystals (agglomerated at some places) which represented zinc and zinc phosphate, respectively. An increase in current density (from 20 to 50 mA/cm(2)) increased the size of the zinc crystals, and coatings prepared at 40 and 50 mA/cm(2) resembled that of electrodeposited zinc. Since the proportions of zinc and zinc phosphate could be varied with applied current density, pH, and treatment time, it would be possible to use this methodology to prepare coatings that would offer different degrees of corrosion protectio