Medium/High Phosphorus EN
Electroless Nickel as we know it today was first discovered back in 1819. The first observation of the reduction of Nickel solution was made in 1845, but little was done with this for many decades. It wasn’t until 1944 before the process attracted any attention. The Electroless Nickel process is defined as an “autocatalytic chemical reduction of a metal from its salts”. Thus once the deposition reaction is initiated by a suitably prepared and activated surface, the metal deposit produced is itself catalytic, resulting in the process producing viable coatings up to almost any desired thickness. Unlike other electroplated deposits, Electroless Nickel produces a uniform precision thickness on most metallic substrates ranging from 5 microns to 100 microns. The Phosphorus content varies from low Phosphorus (3-5%) medium Phosphorus (5 – 10%) to high Phosphorus (10 – 12%). In general terms the higher the Phosphorus content the better the corrosion protection.
The structure of the deposit is generally described as being amorphous – a “metallic glass”
Hardness values for Electroless Nickel deposits from a Ni-P bath are in the range of 450 – 850 VPN with the high Phosphorus baths producing deposits towards the low end of the range. The hardness of the deposit can be increased by the use of heat treatment. By heating the deposit for 1 hour between 350 and 450 Centigrade the hardness values can be increased to 900 – 1100 VPN. The downside of heat treatment is that you sacrifice corrosion protection to gain hardness.
The wear resistance of Electroless Nickel deposits is greatly influenced by the amount of alloying material and wear mechanism to which it is subjected. There are two types of wear resistance, Abrasive wear and Adhesive wear.
Abrasive wear can be described as the removal of material by cutting or ploughing from the interface when a harder material impinges under load. Low phosphorus alloy deposits exhibit good abrasive wear resistance as plated and also over the range of heat treatments to bring about increases in surface hardness. It has been shown that the abrasive wear resistance decreases as the phosphorus content increases. Similar results are also found in Boron deposits.
Adhesive wear can be defined as the removal of interacting surfaces as a result of the adhesion parts. Adhesive wear is characterised by the transfer of material from one surface to another. It has been shown that the adhesive wear resistance of Electroless Nickel alloys increases as the phosphorus content increases.
Surface hardness alone should not be used as an indication of wear resistance. It is important that other properties of the deposit are taken into account, such as retention of lubricants, co-efficient of friction tendency to galling should all be taken into account.
Electroless Nickel deposits provide excellent corrosion resistant coatings, with High Phosphorus deposits providing the best. This is particularly true for marine environments, which suit the petrochemical and marine industry. It should be noted however that if the deposit is heat treated to improve deposit hardness then some corrosion resistance is sacrificed.
Nickel is a Ferro Magnetic material, so Electroless Nickel deposits should show some ferromagnetism. For Electroless Nickel deposits with phosphorus content @ 11-12% these tend to be non Ferro-magnetic. Any deposit which is subjected to heat treatment they all exhibit ferromagnetism.
Electroless Nickel deposits are ideally suited to the following industrial applications:
Petroleum and chemical industries
Medical and pharmaceutical
Moulds and dies
Salvage and repair
Key Characteristics of Electroless Nickel Plating:
No electrical power required
Even coating all over the part
Flexibility to be plated to a high and low thickness