Electroless nickel plating
From Wikipedia, the free encyclopedia
Electroless nickel plating, or "EN", is a technique used to deposit a layer of nickel- phosporous alloy on a solid workpiece, such as metals or plastic. The alloys with different percentage of phosphorus, ranging from 2-5 (low phosphorus) to up to 11-14 (high phosphorus) are possible. The metallurgical properties of alloys depend on the percentage of phosphorus.
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[edit] Overview
Electroless nickel plating is an auto-catalytic reaction used to deposit a coating of nickel on a substrate. Unlike electroplating, it is not necessary to pass an electric current through the solution to form a deposit. This plating technique is to prevent corrosion and wear. EN techniques can also be used to manufacture composite coatings by suspending powder in the bath.
Electroless nickel plating has several advantages versus electroplating. Free from flux-density and power supply issues, it provides an even deposit regardless of workpiece geometry, and with the proper pre-plate catalyst, can deposit on non-conductive surfaces.
[edit] Pretreatment of parts for EN plating
Before performing electroless nickel plating, the material to be plated must be cleaned by a series of cleaning chemicals such as bases and acids, this process is called the pre-treatment process. Failure to remove unwanted "soils" from the part's surface would result in poor plating. Each pre-treatment chemical must be followed by water rinsing (normally two to three times) to remove the chemical that adheres to the surface. Degreasing removes oils from surface; acid cleaning removes scaling. Activation is done with a weak acid etch, or nickel strike, or, in the case of non-metallic substrate, a proprietary solution. After the plating process, plated materials must be finished with an anti-oxidation or anti-tarnish chemical (trisodium phosphate, chromate etc) and pure water rinsing to prevent unwanted stains. The rinsing materials must then be completely dried off or sometimes baked off to obtain the full hardness of the plating film.
[edit] Advantages of EN
- Does not use electrical power.
- Even coating on parts surface can be achieved.
- No sophisticated jigs or racks are required.
- There is flexibility in plating volume and thickness.
- The process can plate recesses and blind holes with stable thickness.
- Chemical replenishment can be monitored automatically.
- Complex filtration method is not required
- Matt, Semi Bright or Bright finishes can be obtained.
[edit] Disadvantages of EN
- Lifespan of chemicals is limited.
- Waste treatment cost is high due to the speedy chemical renewal.
[edit] Applications
The most common form of Electroless nickel plating produces a nickel phosphorus alloy coating. It is commonly used in engineering coating applications where wear resistance, hardness and corrosion protection are required. Applications include oil field valves, rotors, drive shafts, paper handling equipment, fuel rails, optical surfaces for diamond turning, door knobs, kitchen utensils, bathroom fixtures, electrical/mechanical tools and office equipment. It is also commonly used as a coating in electronics printed circuit board manufacturing, typically with an overlay of gold to prevent corrosion. This process is known by the acronym ENIG, which stands for "Electroless Nickel, Immersion Gold".
Due to the high hardness of the coating it can be used to salvage worn parts. Coatings of 25-100 microns can be applied and machined back to final dimensions. Its uniform deposition profile mean it can be applied to complex components not readily suited to other hard wearing coatings like hard chromium.
It is also used extensively in the manufacture of hard disk drives, as a magnetically neutral base coating on aluminium platters (disks) prior to finishing with an magnetic read/write iron oxide coating. It's use in the automotive industry for wear resistance has increased significantly, however it is important to recognise that only EOLVD compliant process types (free from heavy metal stabilizers) may be used for these applications.
[edit] Specifications
AMS 2404 Plating, Electroless Nickel
MIL-DTL-32119
ASTM B733
