Nickel-phosphorus (8–12% P) is deposited to give hardness of 500–600 VPN (as deposited) and 800–1000 VPN (heat treated 400⁰C for 60 min). Gives more uniform deposit than electroplating, and better corrosion resistance. Surface has good lubricating properties due to phosphorus content. Deposit thickness 75 µm.
Composition of nicrolyte
30 g l-1 nickel sulphate (NiSO4.6H2O) – source of nickel ions (5–7 g l-1). 20 g l-1 sodium hypophosphite (NaH2PO2. H2O) – reducing agent, source of energy for process. (Sulphuric acid and ammonium hydroxide added to control pH to 4.5–5.0).
- Production line consists of the following stages:
- alkaline soak or electroclean
- cold or warm water rinse
- acid pickle
- electroless plate
- Additional pre-treatment is needed to produce a catalytic surface (Pd, Rh, Ag, Au, Ni, Fe, Co, Ru, Al) for plating copper, silver and plastics.
- Process does not require sophisticated equipment, but requires good process control of the following variables: temperature (70–95°C), pH 4.5–5.0, continuous filtration and post-heat treatment.
- Barrel plating is used for small components.
- Composites can be incorporated into the layer.
- Production costs compared with hard chrome plating: electroless nickel £0.5 (0.0065 cm)-1 cm-2, hard chrome £0.2 (0.0065 cm)-1 cm–2.
- Gives more uniform deposits than electroplating.
- Operating parameters:
- nickel content of bath 57 g l-1
- solution temperature 70–95°C
- solution pH 4.5–5.0
- bath loading 61–245 cm2 l-1
- rate of deposition 20 μm h-1
- Optimum balance of bath loading can affect losses or bath solution constituents, especially sodium hypophosphate.
- Coatings can be post-heat treated to increase deposit hardness up to 1000 VPN (1 h at 400°C).
- A wide range of materials can be coated; plated directly – steels, cast iron, Al, Co, stainless steels, Ti and Mg; requiring catalytic initiation – Cu, Ag and plastics.
- Surfaces which cannot be plated directly are Zn, Cd and Pb.
- Materials which can form coatings include the following:
- electroless Ni composite coatings (25–30% by volume) with coating thicknesses of 10–25 μm
- electroless Ni-SiC (7 µm size)
- electroless NiAl2O3 (4–5 µm size)
- electroless Ni-polycrystalline diamond (6 µm size)
- electroless Ni-PTFE
- electroless nickel with the inclusion of 20–30% by volume of 6 µm polycrystalline diamond has given the best improvement in wear rate tests
|Material||Wear rate (µm h-1)|
Electroless Ni-P as-plated
Electrodes Ni-P heat treated
|Electroless Ni-polycrystalline diamond||
- Main advantage of electroless plating is that uniform deposits can be achieved, unlike electroplating which is uneven.
- Electroless coatings will penetrate into narrow through and blind holes, provided these are positioned so that there is no gas entrapment.
- No complex jigging and anode arrangements are required.
- Can be used for a wide range of engineering applications where chemical resistance, high hardness and resistance to wear and abrasion are required.
- Applications include:
- plastic injection moulds
- die casting moulds
- glass production moulds
- salvage of worn parts
- gears, bearings, crankshafts, pumps, hydraulic cylinders, ball studs and transmission thrust washers.
- Tolerances on deposit thickness range from 0.25 to 2 µm
- Competes directly with hard chrome-plating.
This article is a part of Manupedia, a collection of information about some of the processes used to convert materials into useful objects.