Surface cleaning Before plating, the surface of the material must be thoroughly cleaned. Unwanted solids left on the surface cause poor plating. Cleaning is usually achieved by a series of chemical baths, including
non-polar solvents to remove oils and greases, as well as
acids and
alkalis to remove oxides, insoluble organics, and other surface contaminants. After applying each bath, the surface must be thoroughly rinsed with water to remove any residue of the cleaning chemicals. Internal stresses in the substrate created by machining or welding can affect the plating.
Plating bath The main ingredients of an electroless nickel plating bath are source of nickel cations , usually
nickel sulfate and a suitable reducing agent, such as
hypophosphite or
borohydride . With hypophosphite, the main reaction that produces the nickel plating yields
orthophosphite , elemental phosphorus, protons and molecular
hydrogen : : 2 + 8 + 2 → 2 (s) + 6 + 2 + 2 (s) + 3 (g) This reaction is
catalyzed by some metals including
cobalt,
palladium,
rhodium, and nickel itself. Because of the latter, the reaction is
auto-catalytic, and proceeds spontaneously once an initial layer of nickel has formed on the surface. The plating bath also often includes: •
complexing agents, such as
carboxylic acids or
amines to increase phosphate solubility and to prevent the white-out phenomena by slowing the reaction. • stabilizers, such as
lead salts,
sulfur compounds, or various organic compounds, to slow the reduction by co-depositing with the nickel. • buffers, to maintain the acidity of the bath. Many complexing agents act as buffers. • brighteners, such as
cadmium salts or certain organic compounds, to improve the surface finish. They are mostly co-deposited with nickel (like the stabilizers). • surfactants, to keep the deposited layer hydrophilic in order to reduce pitting and staining. • accelerators, such as certain sulfur compounds, to counteract the reduction of plating rate caused by complexing agents. They are usually co-deposited and may cause discoloration.
Surface activation Because of the autocatalytic character of the reaction, the surface to be plated must be activated by making it hydrophilic, then ensuring that it consists of a metal with catalytic activity. If the substrate is not made of one of those metals, then a thin layer of one of them must be deposited first, by some other process. If the substrate is a metal that is more
electropositive than nickel, such as
iron and
aluminum, an initial nickel film will be created spontaneously by a
redox reaction with the bath, such as: : (s) + (aq) → (s) + (aq) : 2 (s) + 3 (aq) → 3 (s) + 2 (aq) For metals that are less electropositive than nickel, such as
copper, the initial nickel layer can be created by immersing a piece of a more electropositive metal, such as
zinc, electrically connected to the substrate, thus creating a
shorted Galvanic cell. On substrates that are not metallic but are electrically conductive, such as
graphite, the initial layer can be created by briefly running an electric current through it and the bath, as in electroplating. If the substrate is not conductive, such as
ABS and other plastics, one can use an activating bath containing a
noble metal salt, like
palladium chloride or
silver nitrate, and a suitable reducing agent. Activation is done with a weak acid etch, nickel strike, or a proprietary solution, if the substrate is non-metallic.
After-plating treatment After plating, an anti-
oxidation or anti-
tarnish chemical coating, such as
phosphate or
chromate, is applied, followed by rinsing with water and dried to prevent staining. Baking may be necessary to improve the hardness and adhesion of the plating, anneal any internal stresses, and expel trapped
hydrogen that may make it brittle.
Variants The processes for electroless nickel-phosphorus plating can be modified by substituting
cobalt for nickel, wholly or partially, with relatively little changes. Other nickel-phosphorus alloys can be created with suitable baths, such as nickel-
zinc-phosphorus.
Composites by codeposition Electroless nickel-phosphorus plating can produce
composite materials consisting of minute solid particles embedded in the nickel-phosphorus coat. The general procedure is to suspend the particles in the plating bath, so that the growing metal layer will surround and cover them. This procedure was initially developed by Odekerken in 1966 for electrodeposited nickel-
chromium coatings. In that study, in an intermediate layer, finely powdered particles, like
aluminum oxide and
polyvinyl chloride (PVC) resin, were distributed within a metallic matrix. By changing the baths, the procedure can create coatings with multiple layers of different composition. The first commercial application of their work was electroless nickel-
silicon carbide coatings on the
Wankel internal combustion engine. Another commercial composite in 1981 incorporated
polytetrafluoroethylene (nickel-phosphorus PTFE). However, the co-deposition of
diamond and PTFE particles was more difficult than that of aluminum oxide or silicon carbide. The feasibility to incorporate the second phase of fine particles, the size of a
nanometer to
micrometer, within a metal-alloy matrix has initiated a new generation of composite coatings. ==Characteristics==