Electroless nickel-phosphorus plating

Last updated
Machine parts with electroless nickel plating. 15.electroless.nickel.jpg
Machine parts with electroless nickel plating.

Electroless nickel-phosphorus plating, also referred to as E-nickel, is a chemical process that deposits an even layer of nickel-phosphorus alloy on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing nickel salt and a phosphorus-containing reducing agent, usually a hypophosphite salt. [1] It is the most common version of electroless nickel plating (EN plating) and is often referred by that name. A similar process uses a borohydride reducing agent, yielding a nickel-boron coating instead.

Contents

Unlike electroplating, processes in general do not require passing an electric current through the bath and the substrate; the reduction of the metal cations in solution to metallic is achieved by purely chemical means, through an autocatalytic reaction. This creates an even layer of metal regardless of the geometry of the surface – in contrast to electroplating which suffers from uneven current density due to the effect of substrate shape on the electric resistance of the bath and therefore on the current distribution within it. [2] Moreover, it can be applied to non-conductive surfaces.

It has many industrial applications, from merely decorative to the prevention of corrosion and wear. It can be used to apply composite coatings, by suspending suitable powders in the bath. [3]

Historical overview

The reduction of nickel salts to nickel metal by hypophosphite was accidentally discovered by Charles Adolphe Wurtz in 1844. [4] In 1911, François Auguste Roux of L'Aluminium Français patented the process (using both hypophosphite and orthophosphite) for general metal plating. [5]

However, Roux's invention does not seem to have received much commercial use. In 1946 the process was accidentally rediscovered by Abner Brenner and Grace E. Riddell of the National Bureau of Standards. They tried adding various reducing agents to an electroplating bath in order to prevent undesirable oxidation reactions at the anode. When they added sodium hypophosphite, they observed that the amount of nickel that was deposited at the cathode exceeded the theoretical limit of Faraday's law. [6] [7]

Brenner and Riddel presented their discovery at the 1946 Convention of the American Electroplaters' Society (AES); [8] a year later, at the same conference they proposed the term "electroless" for the process and described optimized bath formulations, [9] that resulted in a patent. [10] [11] [12]

A declassified US Army technical report in 1963 credits the discovery to Wurtz and Roux more than to Brenner and Riddell.[ citation needed ]

During 1954–1959, a team led by Gregorie Gutzeit at General American Transportation Corporation greatly developed the process, determining the optimum parameters and concentrations of the bath, and introducing many important additives to speed up the deposition rate and prevent unwanted reactions, such as spontaneous deposition. They also studied the chemistry of the process. [1] [6]

In 1969, Harold Edward Bellis from DuPont filed a patent for a general class of processes using sodium borohydride, dimethylamine borane, or sodium hypophosphite, in the presence of thallium salts, thus producing a metal-thallium-boron or metal-thallium-phosphorus; where the metal could be either nickel or cobalt. The boron or phosphorus contents was claimed to be variable from 0.1 to 12%, and that of thallium from 0.5 to 6%. The coatings were claimed to be "an intimate dispersion of hard trinickel boride (Ni3B) or nickel phosphide (Ni3P) in a soft matrix of nickel and thallium". [13]

Procedure

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. [14]

Internal stresses in the substrate created by machining or welding can affect the plating. [14]

Plating bath

Molecular model of sodium hypophosphite, the usual reducing agent in electroless nickel-phosphorus plating. Sodium-hypophosphite-3D-balls-ionic.png
Molecular model of sodium hypophosphite, the usual reducing agent in electroless nickel-phosphorus plating.

The main ingredients of an electroless nickel plating bath are source of nickel cations Ni2+
, usually nickel sulfate and a suitable reducing agent, such as hypophosphite H
2PO−
2 or borohydride BH−
4. [1] With hypophosphite, the main reaction that produces the nickel plating yields orthophosphite H
2PO−
3, elemental phosphorus, protons H+
 and molecular hydrogen H
2: [1]

2Ni2+
 + 8H
2PO−
2 + 2H
2O → 2Ni
0 (s) + 6H
2PO−
3 + 2H+
 + 2P (s) + 3H
2 (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. [1]

The plating bath also often includes:

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: [1]

Fe
0 (s) + Ni2+
 (aq) → Ni
0 (s) + Fe2+
 (aq)
2Al
0 (s) + 3Ni2+
 (aq) → 3Ni
0 (s) + 2Al3+
 (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.[ citation needed ] 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. [ citation needed ]

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. [14]

Variants

The processes for electroless nickel-phosphorus plating can be modified by substituting cobalt for nickel, wholly or partially, with relatively little changes. [10] Other nickel-phosphorus alloys can be created with suitable baths, such as nickel-zinc-phosphorus. [15]

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. [3]

Characteristics

Advantages and disadvantages

Compared to the electrolytic process, a major advantage of electroless nickel plating is that it creates an even coating of a desired thickness and volume, even in parts with complex shape, recesses, and blind holes. Because of this property, it may often be the only option. [16]

Another major advantage of EN plating is that it does not require electrical power, electrical apparatuses, or sophisticated jigs and racks. [16]

If properly formulated, EN plating may also provide a less porous coating, harder and more resistant to corrosion and hydrogen absorption. [16]

Electroless nickel plating also can produce coatings that are free of built-in mechanical stress, or even have compressive stress. [16]

A disadvantage is the higher cost of the chemicals, which are consumed in proportion to the mass of nickel deposited; whereas in electroplating the nickel ions are replenished by the metallic nickel anode. Automatic mechanisms may be needed to replenish those reagents during plating.

The specific characteristics vary depending on the type of EN plating and nickel alloy used, which are chosen to suit the application.

Types

The metallurgical properties of the alloy depend on the percentage of phosphorus. [17]

Surface finish

Electroless nickel plating can have a matte, semi-bright, or bright finish.[ citation needed ]

Structure

Electroless nickel-phosphorus coatings with less than 7% phosphorus are solid solutions with a microcrystalline structure, with each grain 2–6 nm across. Coatings with more than 10% phosphorus are amorphous. Between these two limits, the coating is a mixture of amorphous and microcrystalline materials. [16]

Physical properties

The melting point of the nickel-phosphorus alloy deposited by the EN process is significantly lower than that of pure nickel (1445 °C), and decreases as the phosphorus content increases, down to 890 °C at about 14% P. [16]

The magnetic properties of the coatings decrease with increasing phosphorus contents. Coatings with more than 11.2% P are non-magnetic. [18]

Solderability of low-phosphorus coatings is good, but decreases with increasing P contents. [16]

Porosity decreases as the phosphorus contents increases, while hardness, wear resistance, and resistance to corrosion increase.[ citation needed ]

Applications

Electroless nickel coating is often used to smooth the platters of hard disk drives. 17.electroless.nickel.jpg
Electroless nickel coating is often used to smooth the platters of hard disk drives.

Electroless nickel-phosphorus is used when wear resistance, hardness and corrosion protection are required. Applications include oilfield 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.[ citation needed ]

Due to the high hardness of the coating, it can be used to salvage worn parts. Coatings of 25 to 100 micrometers can be applied and machined back to the final dimensions. Its uniform deposition profile means it can be applied to complex components not readily suited to other hard-wearing coatings like hard chromium.[ citation needed ]

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.[ citation needed ]

Its use in the automotive industry for wear resistance has increased significantly. However, it is important to recognize that only End of Life Vehicles Directive or RoHS compliant process types (free from heavy metal stabilizers) may be used for these applications.[ citation needed ]

Printed circuit boards

Electroless nickel plating, covered by a thin layer of gold, is used in the manufacture of printed circuit boards (PCBs), to avoid oxidation and improving the solderability of copper contacts and plated through holes and vias. The gold is typically applied by quick immersion in a solution containing gold salts. This process is known in the industry as electroless nickel immersion gold (ENIG). A variant of this process adds a thin layer of electroless palladium over the nickel, a process known by the acronym ENEPIG. [19]

Standards

See also

Related Research Articles

<span class="mw-page-title-main">Metallurgy</span> Field of science that studies the physical and chemical behavior of metals

Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.

<span class="mw-page-title-main">Electroplating</span> Creation of protective or decorative metallic coating on other metal with electric current

Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be coated acts as the cathode of an electrolytic cell; the electrolyte is a solution of a salt of the metal to be coated; and the anode is usually either a block of that metal, or of some inert conductive material. The current is provided by an external power supply.

<span class="mw-page-title-main">Chrome plating</span> Technique of electroplating

Chrome plating is a technique of electroplating a thin layer of chromium onto a metal object. A chrome plated part is called chrome, or is said to have been chromed. The chromium layer can be decorative, provide corrosion resistance, facilitate cleaning, or increase surface hardness. Sometimes, a less expensive substitute for chrome such as nickel may be used for aesthetic purposes.

<span class="mw-page-title-main">Hydrogen embrittlement</span> Reduction in ductility of a metal exposed to hydrogen

Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbed, hydrogen lowers the stress required for cracks in the metal to initiate and propagate, resulting in embrittlement. Hydrogen embrittlement occurs most notably in steels, as well as in iron, nickel, titanium, cobalt, and their alloys. Copper, aluminium, and stainless steels are less susceptible to hydrogen embrittlement.

Plating is a finishing process in which a metal is deposited on a surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.

<span class="mw-page-title-main">Copper electroplating</span>

Copper electroplating is the process of electroplating a layer of copper onto the surface of a metal object. Copper is used both as a standalone coating and as an undercoat onto which other metals are subsequently plated. The copper layer can be decorative, provide corrosion resistance, increase electrical and thermal conductivity, or improve the adhesion of additional deposits to the substrate.

<span class="mw-page-title-main">Gold plating</span> Coating an object with a thin layer of gold

Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver, by chemical or electrochemical plating. This article covers plating methods used in the modern electronics industry; for more traditional methods, often used for much larger objects, see gilding.

<span class="mw-page-title-main">Metallizing</span>

Metallizing is the general name for the technique of coating metal on the surface of objects. Metallic coatings may be decorative, protective or functional.

Hypophosphorous acid (HPA), or phosphinic acid, is a phosphorus oxyacid and a powerful reducing agent with molecular formula H3PO2. It is a colorless low-melting compound, which is soluble in water, dioxane and alcohols. The formula for this acid is generally written H3PO2, but a more descriptive presentation is HOP(O)H2, which highlights its monoprotic character. Salts derived from this acid are called hypophosphites.

Phosphinates or hypophosphites are a class of phosphorus compounds conceptually based on the structure of hypophosphorous acid. IUPAC prefers the term phosphinate in all cases, however in practice hypophosphite is usually used to describe inorganic species, while phosphinate typically refers to organophosphorus species.

<span class="mw-page-title-main">Electroless deposition</span>

Electroless deposition (ED) or electroless plating is defined as the autocatalytic process through which metals and metal alloys are deposited onto conductive and nonconductive surfaces. These nonconductive surfaces include plastics, ceramics, and glass etc., which can then become decorative, anti-corrosive, and conductive depending on their final functions. Electroplating unlike electroless deposition only deposits on other conductive or semi-conductive materials when a external current is applied. Electroless deposition deposits metals onto 2D and 3D structures such as screws, nanofibers, and carbon nanotubes, unlike other plating methods such as Physical Vapor Deposition ( PVD), Chemical Vapor Deposition (CVD), and electroplating, which are limited to 2D surfaces. Commonly the surface of the substrate is characterized via pXRD, SEM-EDS, and XPS which relay set parameters based their final funtionality. These parameters are referred to a Key Performance Indicators crucial for a researcher’ or company's purpose. Electroless deposition continues to rise in importance within the microelectronic industry, oil and gas, and aerospace industry.

<span class="mw-page-title-main">Chromate conversion coating</span> Chemical treatment of metals

Chromate conversion coating or alodine coating is a type of conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys. The coating serves as a corrosion inhibitor, as a primer to improve the adherence of paints and adhesives, as a decorative finish, or to preserve electrical conductivity. It also provides some resistance to abrasion and light chemical attack on soft metals.

<span class="mw-page-title-main">Sodium hypophosphite</span> Chemical compound

Sodium hypophosphite (NaPO2H2, also known as sodium phosphinate) 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. It is soluble in water, and easily absorbs moisture from the air.

Electrogalvanizing is a process in which a layer of zinc is bonded to steel in order to protect against corrosion. The process involves electroplating, running a current of electricity through a saline/zinc solution with a zinc anode and steel conductor. Such Zinc electroplating or Zinc alloy electroplating maintains a dominant position among other electroplating process options, based upon electroplated tonnage per annum. According to the International Zinc Association, more than 5 million tons are used yearly for both hot dip galvanizing and electroplating. The plating of zinc was developed at the beginning of the 20th century. At that time, the electrolyte was cyanide based. A significant innovation occurred in the 1960s, with the introduction of the first acid chloride based electrolyte. The 1980s saw a return to alkaline electrolytes, only this time, without the use of cyanide. The most commonly used electrogalvanized cold rolled steel is SECC, acronym of "Steel, Electrogalvanized, Cold-rolled, Commercial quality". Compared to hot dip galvanizing, electroplated zinc offers these significant advantages:

Electroless nickel immersion gold (ENIG or ENi/IAu), also known as immersion gold (Au), chemical Ni/Au or soft gold, is a metal plating process used in the manufacture of printed circuit boards (PCBs), to avoid oxidation and improve the solderability of copper contacts and plated through-holes. It consists of an electroless nickel plating, covered with a thin layer of gold, which protects the nickel from oxidation. The gold is typically applied by quick immersion in a solution containing gold salts. Some of the nickel is oxidized to Ni2+ while the gold is reduced to metallic state. A variant of this process adds a thin layer of electroless palladium over the nickel, a process known by the acronym ENEPIG.

Nickel plating may refer to:

A molded interconnect device (MID) is an injection-molded thermoplastic part with integrated electronic circuit traces. The use of high temperature thermoplastics and their structured metallization opens a new dimension of circuit carrier design to the electronics industry. This technology combines plastic substrate/housing with circuitry into a single part by selective metallization.

Nickel 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.

<span class="mw-page-title-main">Electroless nickel-boron plating</span> Metal plating process

Electroless nickel-boron coating is a metal plating process that can create a layer of a nickel-boron alloy on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing nickel salt and a boron-containing reducing agent, such as an alkylamineborane or sodium borohydride. It is a type of electroless nickel plating. A similar process, that uses a hypophosphite as a reducing agent, yields a nickel-phosphorus coating instead.

Electroless copper plating is a chemical process that deposits an even layer of copper on the surface of a solid substrate, like metal or plastic. The process involves dipping the substrate in a water solution containing copper salts and a reducing agent such as formaldehyde.

References

  1. 1 2 3 4 5 6 G. O. Mallory and J. B. Hajdu, editors (1990): Electroless plating: fundamentals and applications. 539 pages. ISBN   9780936569079
  2. ↑ Thomas Publishing Company (2020): "The Electro Nickel Plating Process". Online article at the Thomasnet.com website. Accessed on 2020-07-11.
  3. 1 2 Sudagar, Jothi; Lian, Jianshe; Sha, Wei (2013). "Electroless nickel, alloy, composite and nano coatings - A critical review" (PDF). Journal of Alloys and Compounds. 571: 183–204. doi:10.1016/j.jallcom.2013.03.107.
  4. ↑ =Georgi G. Gavrilov (1979), Chemical (Electroless) Nickel-Plating . Translation by John E. Goodman. Accessed on 2018-09-08. ISBN   9780861080236
  5. ↑ François Auguste Roux (1914): "Process of producing metallic deposits". US Patent 1207218. Granted 1916-12-05, assigned to L'Aluminium Français, expired on 1933-12-05.
  6. 1 2 Charles R. Shipley Jr. (1984): "Historical highlights of electroless plating". Plating and Surface Finishing, volume 71, issue 6, pages 24-27. ISSN   0360-3164
  7. ↑ Abner Brenner and Grace E. Riddel (1946): "Nickel plating on steel by chemical reduction". Journal of Research of the National Bureau of Standards, volume 37, pages 31–34 doi : 10.6028/jres.037.019
  8. ↑ Abner Brenner and Grace E. Riddel (1946): Proc. 33rd Annual Convention of the American Electroplaters' Society page 23.
  9. ↑ Abner Brenner and Grace E. Riddel(1947): Proc. 34th Annual Convention of the American Electroplaters' Society, page 156.
  10. 1 2 Abner Brenner and Grace E. Riddel (1950): "Nickel plating by chemical reduction". US Patent 2532283. Granted on 1950-12-05, expired on 1967-12-05.
  11. ↑ Abner Brenner (1954): Metal Finishing, volume 52, issue 11, page 68.
  12. ↑ Abner Brenner (1954): Metal Finishing, volume 52, issue 12, page 61.
  13. ↑ Harold Edward Bellis (1969): "Nickel or cobalt wear-resistant compositions and coatings". US Patent 3674447. Granted on 1972-07-04, assigned to DuPont, expired 1989-07-04
  14. 1 2 3 Thomas Publishing Company (2020): "Pretreatment of Parts for Electroless Nickle Plating". Online article at the Thomasnet.com website. Accessed on 2020-07-11.
  15. ↑ M. Bouanani, F. Cherkaoui, R. Fratesi, G. Roventi, and G. Barucca (1999): "Microstructural characterization and corrosion resistance of Ni–Zn–P alloys electrolessly deposited from a sulphate bath". Journal of Applied Electrochemistry, volume 29, pages 637–645. doi : 10.1023/A:1026441403282
  16. 1 2 3 4 5 6 7 Thomas Publishing Company (2020): "How Electroless Nickel Plating Works". Online article at the Thomasnet.com website. Accessed on 2020-07-11.
  17. ↑ "Electroless Nickel Plating". Erie Plating Co. Retrieved 8 September 2018.
  18. 1 2 ASTM (2009): "ASTM B733 - 04(2009) Standard Specification for Autocatalytic (Electroless) Nickel-Phosphorus Coatings on Metal".
  19. ↑ "Surface Finishes in a Lead Free World". Uyemura International Corporation. Retrieved 6 March 2019.
  20. ↑ ASTM (): "ASTM B733-15 Standard Guide for Autocatalytic (Electroless) Nickel-Phosphorus Deposition on Metals for Engineering Use (Withdrawn 2000)".
  21. ↑ "Electroless Nickel Specifications". Electro-Coatings. Retrieved 14 July 2020.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.