Electroless nickel plating
Encyclopedia
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 metal
or plastic
. The process relies on the presence of a reducing agent
, for example hydrated sodium hypophosphite
(NaPO2H2·H2O) which reacts with the metal ion
s 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.
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.
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 such as trisodium phosphate
or chromate
, followed by water rinsing to prevent staining. The rinse object must then be completely dried or baked 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 used for activating ABS and other plastics. Electroless bath formation varies with the activator. The substrate is now ready for nickel deposition.
Disadvantages include:
Each type of electroless nickel also has particular advantages depending on the application and type of nickel alloy.
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 End of Life Vehicles Directive
or RoHS compliant process types (free from heavy metal stabilizers) may be used for these applications.
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
or nickel-boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
alloy
Alloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
on a solid workpiece, such as metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
or plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...
. The process relies on the presence of a reducing agent
Reducing agent
A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...
, for example hydrated sodium hypophosphite
Sodium hypophosphite
Sodium hypophosphite is the sodium salt of hypophosphorous acid and is often encountered as the monohydrate, NaPO2H2·H2O. It is a solid at room temperature, appearing as odorless white crystals...
(NaPO2H2·H2O) which reacts with the metal ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s 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 chemicals, this is known as the pre-treatment process. Failure to remove unwanted "soils" from the part's surface result in poor plating. Each pre-treatment chemical must be followed by water rinsing (normally two to three times) to remove chemicals that may adhere to the surface. De-greasing removes oils from surfaces, whereas 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
Tarnish
Tarnish is a thin layer of corrosion that forms over copper, brass, silver, aluminum, and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always result from the sole effects of oxygen in the air. For example, silver needs hydrogen sulfide to tarnish; it...
chemical such as trisodium phosphate
Trisodium phosphate
Trisodium phosphate is a cleaning agent, food additive, stain remover and degreaser. It is a white, granular or crystalline solid, highly soluble in water producing an alkaline solution....
or chromate
Chromate
Chromate salts contain the chromate anion, CrO42−. Dichromate salts contain the dichromate anion, Cr2O72−. They are oxyanions of chromium in the oxidation state +6. They are moderately strong oxidizing agents.- Chemical properties :...
, followed by water rinsing to prevent staining. The rinse object must then be completely dried or baked 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
Silver nitrate
Silver nitrate is an inorganic compound with chemical formula . This compound is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides...
is also used for activating ABS and other plastics. Electroless bath formation varies with the activator. The substrate is now ready for nickel deposition.
Advantages and disadvantages
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.
Low phosphorus electroless nickel
Low phosphorus treatment is applied for deposits with hardness up to 60 Rockwell C. This type offers a very uniform thickness inside complex configurations as well as outside, which often eliminates grinding after plating. It is also excellent for corrosion resistance in alkaline environments.http://corrosion-doctors.org/MetalCoatings/Electroless.htmMedium phosphorus electroless nickel
Medium phosphorus treatment has a high speed deposit rate and offers bright and semi-bright options for cosmetic particularization. The processing is very stable, used often for Slurry Disposal Industries. This is the most common type of electroless nickel applied.High phosphorus electroless nickel
High Phosphorus electroless nickel offers high corrosion resistance, making it ideal for industry standards requiring protection from highly corrosive acidic environments such as oil drilling and coal mining. With microhardness ranging up to 600 VPN, this type ensures very little surface porosity where pit-free plating is required and is not prone to staining. Deposits are non-magnetic when phosporous content is greater than 11.2%.ASTM B733 - 04(2009) Standard Specification for Autocatalytic (Electroless) Nickel Phosphorus Coatings on MetalApplications
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%. 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/mechanicalMachine
A machine manages power to accomplish a task, examples include, a mechanical system, a computing system, an electronic system, and a molecular machine. In common usage, the meaning is that of a device having parts that perform or assist in performing any type of work...
tools and office equipment. It is also commonly used as a coating in electronics printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
manufacturing, typically with an overlay of gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
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 End of Life Vehicles Directive
End of Life Vehicles Directive
The End of Life Vehicles Directive is a Directive of the European Union addressing the end of life for automotive products. Every year, motor vehicles which have reached the end of their useful lives create between 8 and 9 million tonnes of waste in the European Union...
or RoHS compliant process types (free from heavy metal stabilizers) may be used for these applications.
See also
- ElectroplatingElectroplatingElectroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
- Nickel electroplatingNickel electroplatingNickel electroplating is a technique of electroplating a thin layer of nickel onto a metal object. The nickel layer can be decorative, provide corrosion resistance, wear resistance, or used to build-up worn or undersized parts for salvage purposes.-Overview:...
- NucleationNucleationNucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
- (Electroless plating solutions are unstable, but the difficulty of nucleation means they will only act on a metal surface.) - Electroless nickelElectroless nickelElectroless nickel is a broad term that incorporates a diverse mix of technology ranging from nickel boron and ternary alloys to highly functional composites...