Deposition of electroless Ni–P graded coatings and evaluation of their corrosion resistance

doi:10.1016/j.surfcoat.2004.10.014

Surface and Coatings Technology

Volume 200, Issue 11, 15 March 2006, Pages 3438-3445

https://doi.org/10.1016/j.surfcoat.2004.10.014 Get rights and content

Abstract

Electroless Ni–P coatings provide high hardness and excellent resistance to wear and abrasion. The present work aims to study the formation of electroless Ni–P graded coatings, with varying nickel and phosphorus contents of the individual layers and to evaluate their corrosion resistance by polarization and electrochemical impedance spectroscopic studies. The possibility of preparing electroless Ni–P graded coatings by sequential immersion in three different plating baths is discussed. The study reveals that electroless Ni–P graded coatings offer better corrosion resistance than non-graded Ni–P coatings.

Introduction

Electroless plating is a well established surface engineering process that involves deposition of a metal-metalloid alloy coating on various substrates. Although a variety of metals can be electrolessly plated, electroless Ni–P coating has received widespread acceptance as it provides high hardness and excellent resistance to wear, abrasion and corrosion [1], [2], [3], [4], [5]. Electroless Ni–P coatings are widely used for corrosion protection application in a variety of environments [1], [2], [3], [4], [5]. It is a barrier coating, protecting the substrate by sealing it off from the corrosive environments, rather than by sacrificial action. However, in this respect, only electroless Ni–high P coating is effective in offering an excellent protection whereas electroless Ni–Low P and Ni–medium P coatings are not recommended for severe environments [1], [2], [3], [4], [5].

In recent years, multilayer and graded coatings have received much attention as they could provide considerable improvement in the desired characteristics of the coatings. Electrodeposition is a proven technique to prepare multilayer and graded coatings [6], [7], [8]. Since the composition of the individual layers could be altered easily by varying the concentration of the metallic ions, current density, speed of agitation, etc., much work on multilayer and graded coatings was explored using electrodeposition technique. Pulsed current electrodeposition still makes it more amenable for the deposition of a variety of coatings [9], [10]. On the contrary, electroless plating baths are quite sensitive to variations in temperature, pH, etc., to make them amenable for preparing multilayer coatings with periodicity. However, it is possible to prepare graded coatings by sequential immersion in two or more electroless plating baths. In our earlier work, we have prepared electroless Ni–P/Ni–B duplex coatings, with both Ni–P and Ni–B as outer layers, using acidic hypophosphite-and alkaline borohydride-reduced electroless plating baths [11]. The corrosion resistance of Ni–P/Ni–B duplex coatings is better for the coating that has Ni–P as the outer layer than the one with Ni–B as the outer layer. Hence it is possible to manipulate the corrosion resistance of graded coatings depending on the type of coatings selected as the inner and outer layers. In a similar attempt, the present work aims to study the formation of electroless Ni–P graded coatings, with varying nickel and phosphorus contents of the individual layers by sequential immersion in three different hypophosphite-reduced electroless plating baths and to evaluate their corrosion resistance by polarization and electrochemical impedance spectroscopic studies.

Section snippets

Experimental details

Mild steel was used as the substrate material for the preparation of non-graded as well as graded electroless Ni–P coatings. The mild steel substrates were surface ground to a roughness of 0.45 μm and electrolytically cleaned using an alkaline solution containing sodium carbonate, sodium hydroxide and sodium lauryl sulphate. The bath composition and operating conditions used for preparing non-graded as well as graded electroless Ni–P coatings were given in Table 1. The electroless Ni–P

Results and discussion

Electroless Ni–high, medium and low P coatings obtained using baths A, B and C (Table 1), respectively, are uniform and highly adherent. Among them, the plating rate is very high for electroless Ni–high P coating. Plating rate as a function of time for coatings obtained from baths A, B and C is given in Fig. 2. Based on the amount of coating formed in a given time from each plating bath, the time of deposition for each layer of electroless Ni–P graded coatings is decided so that layers of

Conclusion

From the study it can be concluded that it is possible to prepare electroless Ni–P graded coatings by sequential immersion in three different hypophosphite-reduced electroless plating baths. One important aspect of depositing electroless Ni–P graded coatings is the application of a nickel strike between each layer, so that the deposition proceeds without any hindrance when the substrate is sequentially immersed in electroless plating baths. The graded electroless Ni–P coatings are uniform and

Acknowledgements

Financial support given to this work by the Council of Scientific and Industrial Research (CSIR), New Delhi, India was gratefully acknowledged. The authors are thankful to Prof. S.P. Mehrotra, Director, National Metallurgical Laboratory, Jamshedpur and Dr. S. Srikanth, Deputy Director, National Metallurgical Laboratory, Madras Centre, for their constant support and encouragement to carry out this research work.

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Citation Excerpt :
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Deposition of electroless Ni–P graded coatings and evaluation of their corrosion resistance

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusion

Acknowledgements

Thin Solid Films

Thin Solid Films

Electrochim. Acta

Scr. Mater.

Thin Solid Films

Electrochim. Acta

Electrochim. Acta

Surf. Coat. Technol.

Mater. Chem. Phys.

J. Alloys Compd.

J. Non-Cryst. Solids

Mater. Chem. Phys.

Mater. Sci. Eng., A

J. Res. Natl. Bur. Stand.

Chemical (Electroless) Nickel Plating