The Electroless Deposition of Nickel-Phosphorus-Tungsten Alloys
by Vijaykumar Ijeri, Snehal Bane, Komal Shah and Prerna Goradia, Grauer & Weil (India) Ltd.
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Electroless nickel-phosphorus coatings are used on metallic components to enhance corrosion resistance, wear resistance and durability. Further improvement in properties can be obtained by adding additional alloying elements or nanoparticles. Tungsten is a hard, robust metal with high melting point and good resistance to acids and alkalis. However, it is a difficult metal to deposit. This paper describes the successful electroless co-deposition of tungsten within a nickel-phosphorus matrix to yield several advantageous properties. In particular, the Ni-P-W deposits obtained by the electroless process have resultant hardness in the range of 750-850 HV which increases on annealing. Tungsten content of 2-4 wt% is found in the deposits under optimized operating bath conditions. Further, the effect of operating parameters, like pH and temperature, on the rate of deposition, alloy composition and other surface properties are presented.
Keywords: electroless alloy deposition, nickel-phosphorus tungsten alloys, corrosion resistant coatings, wear resistant coatings
The discovery of electroless plating is credited to Brenner & Riddell in the 1940s. Today electroless nickel (EN) plating has grown into a very substantial segment of the metal finishing industry. Electroless nickel coatings have been available to engineers and designers for more than five decades. This process has been called autocatalytic, chemical nickel plating or electroless nickel plating.
Electroless deposition can be done both on metals as well as nonconductors. Metals are easily coated with electroless deposits using hot baths. Nonconductors need to be activated with catalysts to initiate plating. This paper will focus only on the plating of mild steel.
There is a large variety of EN coatings, typically defined by their alloy content, the most common being Ni-P alloys. These deposits give a low coefficient of friction and are anti-galling. They have good as-plated hardness and can be further hardened by post-plating heat treatment processes. These deposits have excellent corrosion performance in many types of environments. The hardness and corrosion properties are very much dependent on phosphorus content. There have been studies on the relationships linking deposit phosphorus levels to the resulting hardness both before and after post-plating heat treatment processes.
Wear performance has also been shown to be primarily a function of the deposit hardness, and this has been related to alloy phosphorus content. A few common properties are listed in Table 1.
Nickel-phosphorus deposition with a hypophosphite reducer is usually represented by the following reactions: (1,2) dissociation of salts, (3) reduction of nickel cations and (4) oxidation of hypophosphite:
1. NiSO4 + H2O → Ni+2 + SO4-2 + H2O
2. NaH2PO2 + H2O → Na+ + H2PO2- + H2O
3. Ni+2 + H2PO2- + H2O → Ni + H2PO3- + 2H+
4. H2PO2- + H2O → H2PO3- + H2
Table 1 - Comparison of properties obtained from different Ni-P alloys.