Handed on a plate

An informed knowledge of surface engineering can provide the process engineer with the potential to optimise the efficiency of a manufacturing process or the quality of a product in the most cost-effective way. As British industry squeezes through the margins of profitability, electroless nickel can be widely used in the fight against corrosion, which costs the UK around 4 per cent of GNP per annum.

Different surface finishes surround us in everyday life from the bathroom to the boardroom, protecting structures from the inevitable corrosion of time. Across industry, there is a need for the strength of iron and steel, but without parts rusting.

The Institute of Corrosion has represented the sector of corrosion science, technology and engineering management and the economic needs of individuals and industry since 1959. Though corrosion affects £150billion worth of end products per year the surface finish of a product is often ignored.

Key to this fight is the establishment and promotion of sound corrosion management practice; the advancement of cost effective corrosion control measures; and a sustained effort to raise corrosion awareness at all stages of design, fabrication and operation - prevention is better than cure.

The right coat for the job

Knowing your process is the key to choosing the right form of protection. A design engineer, a plating engineer and the sales representative working as a team at the beginning of a project avoids a re-design - an expensive alternative when things go wrong.

The properties of electroless nickel plating have been recognised since the 19th century but it was not until 1944, when Brenner and Riddell coated the inside of gun barrels, that electroless nickel plating came into its own.

The deposited metal layer has the advantage of having an even thickness over all surfaces, regardless of shape. This cannot be achieved with electro-deposited coatings. Russell House, chairman of the Electroless Nickel Society said: `For example, on the cogs of a gear an electric current gives layers that build up on the sharpest point, but electroless nickel evenly coats all surfaces so long as the solution can reach it.' In many cases, subsequent machining will not be required.

An application in the textile industry is an example of electroless nickel providing longer component life. A carding comb, used to draw raw fibre into threads before spinning, consists of many steel teeth which pull all the threads in one direction onto the bobbin for spinning. Plated with electroless nickel and diamond of 3-5 micro m particles, the comb lasts for months rather than days.

Electroless nickel coatings also provide varying degrees of resistance to a wide range of chemicals, which makes them suitable for many applications in the chemical industry. For example, on stirrers, valves, reaction tanks and covers.

With a wide range of applications, electroless nickel plating is suitable for almost all metals, plastics and ceramics, though each substrate requires some form of surface preparation. Components made of steel, aluminium and brass can be protected from corrosion and abrasive wear. Electroless nickel prevents the dezincification of brass pumps used in soft water applications and prevents erosion to nozzles when brackish water is supplied under pressure. As House says, `as long as the item can be immersed in a tank then it can be protected by electroless nickel.'

Often in the oil industry large tanks and lifting gear are required, with uses ranging from ball and plug valves, to bearing houses on rigs and flexible joints on delivery pipelines.

The amount of corrosion protection depends on coating thickness, substrate condition and surface preparation. This determines coating porosity and ultimately resistance to corrosion. The amount of corrosion resistance is determined by the phosphorous content of the deposit. The amount of phosphorous can be accurately controlled and can be likened to the pH scale with three categories of low, medium and high phosphorous (see panel opposite).

As the physical properties of the nickel phosphorous mixture alter, depending on the phosphorus content, a deposit can be specified for a particular application. Fruit juice cartons coated inside with electroless nickel need a high phosphorous content as it offers good corrosion resistance in acidic conditions. High phosphorous content deposits are softer but can be hardened with heat treatment. A medium phosphorous content gives corrosion protection and abrasion resistance for general engineering applications with a hardness of 500-600 Vickers Pyramid Number.

At the low end of the spectrum, the deposit offers corrosion resistance in alkaline conditions plus the advantage of a hard layer. The thickness of the coating also affects porosity. Generally a thickness of 25 micro m is applied.

Turn up the heat

Electroless nickel deposits are heat treated to improve abrasion resistance, increase hardness and enhance adhesion. To obtain the maximum hardness it is necessary to treat parts at a temperature of 400 degrees C for one hour. `This may work for some metal substrates but not all - aluminium cannot be heated to 400 degrees C as it becomes mobile,' warns House. `If a nickel deposit has a low phosphorous content it won't need to be treated as the deposit already shows a good degree of hardness.' The deposit is comparable with hard chrome and in some cases can substitute it. Clearly, an application must be examined closely for the appropriate nickel deposit and any further treatment.

Electroless nickel can also be co-deposited with different particles to form composites, particularly for improved abrasion resistance. The electroless nickel deposit could contain hard silicon carbide particles, diamond or PTFE. All additives give the deposit a slippery feel and maintain hardness, but can compromise corrosion resistance.

In contrast to electroplating techniques, the electroless or chemical nickel plating operation is based on the catalytic reduction of nickel ions on the surface to be plated. The most commonly used reducing agent is sodium hypophosphite.The process avoids the complicated jig and anode arrangements associated with metal plating. Besides corrosion protection, the deposit gives accurate components within required tolerances for most engineering applications.

Electroless nickel finishing applications have widened due to numerous deposit features and effective corrosion protection across a range of applications. Prior to specifying a plating process, discussion between the plater and the end-user sets the optimum process parameters. A quality finished product is guaranteed if the monitoring of the electroless nickel process is carefully controlled. PE

* P content of 3-7 per cent for excellent high wear resistance and corrosion resistance in alkaline conditions

* P content of 9-12 per cent for corrosion protection and abrasion resistance to most applications

* P content of 10-13 per cent for the highest demands for corrosion resistance in acidic conditions and simultaneous mechanical stress