Electroless nickel-phosphorus plating
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Electroless nickel plating (EN) is an auto-catalytic chemical technique used to deposit a layer of nickel-phosphorus or nickel-boron alloy on a solid workpiece, such as metals or plastic. The process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite (NaPO2H2·H2O) which reacts with the metal ions to deposit metal. 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.
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.
Pretreatment
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. De greasing 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 or chromate) 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.
The pre-treatment required for the deposition of nickel and cobalt on a non-conductive surface usually consists of an initial surface preparation to render the substrate hydrophillic. Following this initial step, the surface is activated by a solution of a noble metal, e.g., palladium chloride. Silver nitrate is also successfully used for activating ABS and other plastics. Electroless bath formation varies with the activator. The substrate is now ready for nickel deposition.[citation needed]
Advantages and disadvantages
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Advantages include:
- 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
- Matte, Semi Bright or Bright finishes can be obtained.
Disadvantages include:
- Lifespan of chemicals is limited.
- Waste treatment cost is high due to the speedy chemical renewal.
Each type of electroless nickel also has particular advantages depending on the application and type of nickel alloy.[1]
Types
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Low phosphorus electroless nickel
- Hard deposits
- Very uniform thickness
- Excellent corrosion resistance in alkaline environments
Medium phosphorus electroless nickel
- Very bright and semi-bright options
- High speed deposit rate
- Very stable
- Used for Slurry Disposal Industries
- The most common type of electroless nickel applied
High phosphorus electroless nickel
- Superior corrosion protection
- Excellent acid resistance [2]
- Lower porosity
- Non-magnetic, when phosphorus content is greater than 11.2% [3]
- Less prone to staining
- Pit-free deposits
Applications
The most common form of electroless nickel plating produces a nickel phosphorus alloy coating. The phosphorus content in electroless nickel coatings can range from 2% to 13%.[1] 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 as electroless nickel immersion gold.
Due to the high hardness of the coating it can be used to salvage worn parts. Coatings of 25 to 100 micrometres 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 way of providing an atomically smooth coating to the aluminium disks, the magnetic layers are then deposited on top of this film, usually by sputtering and finishing with protective carbon and lubrication layers; these final two layers protect the underlying magnetic layer (media layer) from damage should the read / write head lose its cushion of air and contact the surface.
Its use in the automotive industry for wear resistance has increased significantly, however it is important to recognise that only EOLVD or RoHS compliant process types (free from heavy metal stabilizers) may be used for these applications.
Standards
- AMS-2404
- AMS-C-26074
- ASTM B-733
- ASTM-B-656
- MIL-DTL-32119