Electroless Nickel Coatings Offered by Val-Kro
Auto-catalytic nickel plating is commonly referred to as Electroless Nickel Plating. In contrast, with electroplating, electroless nickel (EN) plating does not require electrical current. The deposition occurs in an aqueous solution containing metal ions, a reducing agent, chelates, complexing agents and stabilizers. Chemical reactions on the surface of the part being plated cause deposition of a nickel alloy.
Since all surfaces wetted by the electroless nickel solution are plated, the deposit thickness is uniform across the entire part. This unique property of electroless nickel plating makes it possible to coat internal surfaces of pipes, valves and other parts. This uniformity of deposit thickness is difficult, if not impossible, to achieve by any other method.
The discovery of electroless nickel plating is credited to Brenner & Riddell in the 1940’s. Today electroless nickel plating has grown into a very substantial segment of the metal finishing industry. In the past 25 years the usage of this coating has grown to such proportions that electroless nickel plated parts can be found underground, in outer space, and in many areas in between.
The chemical reactions that occur when using sodium hypophosphite as the reducing agent in electroless nickel plating are as follows:
An electroless nickel coating is a dense alloy of nickel and phosphorus. The amount of phosphorus generally varies from 3 to 12 percent, depending on the bath formulation, operating pH and bath age. The deposition process is auto-catalytic. This means that once the initial layer of nickel is formed on the substrate, that layer and each subsequent layer become the catalyst that causes the above reaction to continue. This means that very thick coatings can be applied, provided that the ingredients in the plating bath are replenished in an orderly manner. In general, plating thicknesses range from 0.1 mil to 5 mils.
Electroless nickel deposits are functional coatings and are rarely used for decorative purposes only. The primary criteria for using electroless nickel generally falls within the following categories:
- Corrosion resistance
- Wear resistance to protect against galling
- Solderability and bondability
- Uniformity of deposit regardless of geometries
- Nonmagnetic properties of high-phosphorus nickel alloy
Types of Electroless Nickel (EN) Plating
Most commonly used for engineering applications. Deposits from these baths are quite hard, have natural lubricity and excellent wear and corrosion resistance.
A common electroless nickel composite coating is the co depositing of electroless nickel with Teflon (r) (Reg. TM du Pont). This coating does not provide the hardness of a Nickel Phosphorus deposit, but it does provide superior lubricity. Especially for applications involving minimum loads at maximum speeds.
Properties of Electroless Nickel (EN) Plating
The properties of electroless nickel are responsible for the rapid growth of its use as a functional metallic coating in recent years. This is no other coating that has the combination of properties offered by electroless nickel.
- Corrosion resistance: One of the most common reasons for using electroless nickel coatings in functional applications is its excellent corrosion resistance.
- Heat Conductivity: The heat conductivity of an electroless nickel coating containing 8 to 9 percent phosphorus is 0.0105 to 0.0135 cal-cm/sec/degree C.
- Melting Temperature: The melting temperature of electoless nickel coatings vary widely, depending upon the amount of phosphorus in the deposit. A generally accepted melting point is approximately 1616F for coatings with approximately 7 to 9 percent phosphorus.
- Magnetism: Electroless nickel coatings containing greater than 8 percent phosphorus are considered to be essentially nonmagnetic as plated. Coating thickness measurements with devices which rely on the nonmagnetic characteristic of the coating may become inaccurate and require special calibration in the phosphorus content of the coating is below 8 percent. Heat treatment of electroless nickel plated parts will increase the magnetism of the plated part.
- Solderability/Weldability: Electroless nickel coatings are easily soldered with a highly active acid flux. Soldering without a flux or with a mildly active flux can be difficult especially if the plated part is exposed to the atmosphere for an extended period.
- Adhesion: Excellent adhesion can be achieved on a wide range of substrates, including steel, aluminum, copper and copper alloys. Heat treatment is commonly used to enhance the adhesion of electroless nickel coatings over all substrates.
- Thickness: One of the key properties of electroless nickel plating is the ability to deposit the coating to produce a wide range of coating thickness, with uniformity and minimum variation from point to point. This uniformity can be maintained while plating both large and small parts. It is a great coating for parts with complex geometries that include recessed areas.
- Brightness: The brightness and reflectivity of electroless nickel vary significantly. The reflectivity is also affected by the surface finish of the substrate. Basically the finish of an electroless nickel plated part will mirror the surface of the part plated. Thus a bright electroless nickel deposit with appear dull if the surface of the part being plated is rough.
Electroless Nickel Coatings Offered by Val-Kro
Note: Much of the content above was obtained from MFSA quality metal finishing guide Volume 1 No 4