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

Abstract

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 dierent 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-Si 3 N 4 , Ni-P-CeO 2 and Ni-P-TiO 2 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 dierent eective 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 W cm 2 , whereas the capacitances of these coatings are in the range 11±17 lF/cm 2 . 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 eective metallic area prone to corrosion. Among the three electroless Ni-P composite coatings, the corrosion resistance is in the following order: Ni-P-CeO 2 =Ni-P-Si 3 N 4 >Ni-P-TiO 2 .

Influence of the Tungsten Content on Surface Properties of Electroless Ni-W-P Coatings

“…By reacting with water, hypophosphite ions are formed according to equation (1). The adsorbed hypophosphite layer has a blocking action on the coated surface, preventing nickel hydration due to a limited water supply which is reported to be the initial stage for the formation of soluble Ni 2+ or a nickel passive layer 41,42 .…”

EVALUATION OF ELECTROLESS NI-P AND NI-P NANO-COMPOSITE COATINGS' PROPERTIES

“…[8][9][10][11][12] In general, electroless Ni-P is a barrier coating, protecting the substrate by sealing it off from the corrosive environments, rather than by sacrificial action. The high resistance is a result of the amorphous nature and passivity of the Ni-P deposits.…”

“…Amorphous alloys offer better corrosion resistance to corrosion attack than equivalent polycrystalline materials because of the freedom from grain or grain boundaries and because of the glassy film, which form on and passivate their surfaces. [8][9][10][11][12] It is evident from literature reports on Ni-P coatings that preferential dissolution of nickel occurs at open circuit potential, leading to the enrichment of phosphorus on the surface layer. The enriched phosphorus surface reacts with water to form a layer of adsorbed hypophosphite anions (H 2 PO 2 -).…”

“…This layer in turn will block the supply of water to the electrode surface, thereby preventing the hydration of nickel, which is considered to be the first step to form either soluble Ni 2+ species or a passive nickel film. [8][9][10][11][12] Fig .4. Potentiodynamic polarization curves of electroless Ni-P, Ni-P/nano-SiO2 and Ni-P/nano-SiC composite coatings in 3.5% NaCl electrolyte Fig.5 shows the Nyquist plots obtained for Ni-P, Ni-P/nano-SiO 2 and Ni-P/nano-SiC composite coatings in 3.5% sodium chloride solution at their respective open circuit potentials.…”

Effect of Heat Treatment and Bath Process Parameters on the Corrosion Behavior of Ni-P-TiO2 Composite Coatings

“…Phosphorus present on the coated surface plays a vital role in corrosion resistance. An EN composite coating with higher phosphorus content shows excellent corrosion resistance, as the phosphide particles enables preferential hydrolysis of phosphorus over nickel [20]. The composite coating deposited using sol-gel technique has excellent corrosion resistance compared to conventional composite coatings and Ni-P coatings [21].…”