Electroless Nickel Plating
Electroless nickel (EN or E/Ni) plating is an industrial process that uses an auto-catalytic chemical reaction to deposit a layer of nickel alloy (typically nickel-phosphorus or nickel-boron) onto a solid substrate such as a metal or plastic workpiece. A reducing agent, such as hydrated sodium hypophosphite, is crucial to the electroless nickel plating process because it reacts with the metal ions and causes the nickel to be deposited. Because it depends on this chemical process, electroless nickel plating does not require the use of electricity. A comparative metal plating process is electroplating, which does use an electrical current in order to achieve the same process of metal deposition onto a substrate.
EN plating is known by a variety of other terms, including nickel coating, autocataytic coating, autocatalytic plating, autocatalytic nickel plating, chemical nickel plating, Although nickel is the most widely used metal in this type of process, it is not the only metal that can utilized in this way. Thus, the term electroless plating is used to refer to the broader family of processes characterized by the same auto-catalytic reaction.
Quick Links to Electroless Nickel Plating Information
History of Electroless Nickel Plating
Around the middle of the nineteenth century, French chemist Adolphe Wurtz accidentally discovered that the deposition of nickel-phosphorus ions occurred in a chemical reduction bath, such as sodium hypophosphite. Scientific interest in this type of chemical process grew over the following decades. In 1911, another chemist named F.G. Roux patented the "spontaneous and complete" electroless nickel coating process.
The electroless nickel process was further exploited in 1946, when Abner Brenner and Grace Riddell expanded on previous information and developed a method of plating the inner walls of tubes with nickel-tungsten alloy. Consciously giving credit to their predecessors, Brenner and Riddell successfully obtained a 1950 patent for their process, which was granted on the basis of their process being highly controlled and only coating specified catalytic surfaces. Their discoveries eventually led to related processes such as diamond and PTFE deposition, thus changing the world forever.
Advantages of Electroless Nickel Plating
Electroplating, the electrical counterpart to EN plating, is primarily used for the deposition of a layer of metal in order to provide the material being plated with a desirable property such as abrasion and wear resistance, corrosion protection, lubricity or aesthetic qualities that it would not otherwise have. Electroplating is also used to build up the thickness of undersized parts.
Electroless nickel plates are used to achieve some of the same goals as electroplating. However, EN plating offers several advantages over its electrolytic nickel. The following examples are representative of some of the most highly valued ones.
- Uniformity of Layer Deposition
- The main advantage of EN plating over electroplating is the uniformity of the layer that is commonly achieved. Without an electric current to catalyze the plating process, EN plating does not suffer from the aberrations or excessive deposits that characterizes buildups caused by electroplating.
- This natural uniformity of deposition proves highly advantageous when it comes to plating complex objects. EN plating covers recesses, holes, and back sides – i.e. normally inaccessible areas – of objects being plated. For example, the inner diameters of tubular pieces (an otherwise inaccessible area) are easier to plate via the EN process.
- Better Corrosion Resistance
- The presence of phosphorus in the nickel alloy contributes significantly to improved corrosion resistance when compared to electroplated layers. Phosphorus contributes to the unique, amorphous nature of electroless nickel, i.e. its lack of a clearly-defined crystalline structure on the microscopic level. This unique property actually contributes to a solid chemical barrier and enhances a substrate’s ability to withstand corrosion.
- Improved/High Hardness
- The unique structure of electroless plated nickel also contributes to better hardness when compared with electroplated material. Other factors, such as reduced hydrogen absorption and embrittlement (due to post-plating treatments), also contribute to this advantage.
- Better Wear and Abrasion Resistance
- Electroless plated nickel naturally possesses a low coefficient of friction. Combined with its natural hardness, this makes it offer better resistance to mechanical wear, etc. than its counterpart.
- Low Magnetic Properties
- As alluded to earlier, electroless nickel does not possess high magnetic properties. This makes it particularly well suited for specific products (especially electronic ones).
- Reasonable Cost Effectiveness
- Because it requires no electricity, the process tends to be inexpensive. The low cost of nickel also plays a role in lowering costs. Sometimes, costs associated with the bath solution (e.g. chemical waste management or adding polymers to create custom solutions and enhanced product qualities) may be high. Generally speaking, though, EN plating is affordable and not exorbitantly higher than electroplating.
Process of Electroless Nickel Plating
As stated previously, the electroless plating process revolves around a chemical reaction. The adjective "auto-catalytic" alludes to the fact that the nickel being deposited onto the substrate is the agent responsible for catalyzing the reaction. Nickel can be deposited through EN plating on a wide range of substrates, including steel (stainless, hardened, or mild), aluminum, titanium, copper, brass and zinc. Certain custom alloys are also compatible with EN plating. Steel and aluminum are the most popular substrates used in EN plating (by a wide margin).
- Pre-Treatment Methods
- Although most industrial processes are heavily dependent on process control, the electroless nickel process is particularly dependent. One of the most crucial steps in EN plating occurs at the very beginning of the process: cleaning, or otherwise pre-treating, the substrate. Uniform and firm coating adhesion (some of the main advantages of EN plating) are easily undermined by surface contaminants on the substrate. Cleaning a substrate’s surface usually takes place via various chemical treatments and thorough rinsing; some variations of this include degreasing (which targets oils) and acid cleaning (which targets scaling). Other pre-treatment methods include shot peening (plastically deforming surfaces through "shot" or round particles) as well as tumble finishing and vibratory deburring (preparing small pieces through polishing and vibration, respectively). Other pre-treatment goals include improving the surface finish of the substrate. Any pre-treatment of a substrate must also include the catalytic activation of the substrate (e.g. through special paints), since the electroless plating process depends on a chemical process linked to a conductive substrate surface.
- Bath Solution Preparation
- Preparing the bath solution that the substrate will be immersed in is another crucial first step in the process of electroless plating. This aqueous combination contains four main components: nickel ions, a reducing agent, and other chemicals that are either "stabilizing" or "complexing" agents.
- The nickel ions in the bath usually belong to a nickel alloy. The most common alloy is nickel-phosphorous, due to the corrosion protection qualities of the latter substance. Under normal circumstances, phosphorus composes 1-14% of the entire alloy. The finished, nickel-coated product will have different properties based on the percentage of phosphorus present in the alloy. Hypophosphorous acid (Ni[H2PO2]2) is a preferred nickel alloy.
- The reducing agent is responsible for depositing the nickel due to a redox (reduction-oxidation) reaction. Sodium hypophosphite is the preferred reducing agent, followed by sodium borohydride, dimethylamine borane, and hydrazine.
- "Stabilizing" agents or inhibitors provide a measure of control over the electroless plating process; they are responsible for slowing the rate of the deposition during the course of the autocatalytic reaction.
- "Complexing" agents provide another measure of control over EN plating through various means, including controlling pH levels and minimizing the concentration of "free" nickel ions.
- Properly balancing the ratio of the bath solutions’ various components is critical in ensuring a successful plating process. The size of the actual bath is almost as important as the actual composition of the bath solution. Bath loading (when the volume of the solution drastically exceeds the surface area of the object being plated) can have adverse chemical effects on the quality of the deposited metal layer.
- Submersion Process
- Once the substrate has been thoroughly prepared, through cleaning, activation, and other methods, it is submerged in the bath solution. Within the bath solution, the metal being deposited reacts due to the presence of the reducing agent into the bath solution. The reducing agent releases hydrogen and consequently reacts with the nickel alloy ions suspended in the solution, specifically, by reducing them (granting them electrons or greater control over shared electrons) during the course of a reduction-oxidation (redox) reaction. The presence of the submerged, activated substrate aids this redox reaction and ultimately results in the deposition of the nickel onto the substrate. Through the redox reaction, the dissolved nickel obtains a negative charge and adheres to the activated substrate through covalent bonding caused by the chemical reduction and resulting negative charge. The adjective "autocatalytic" that is often used to describe electroless nickel plating refers to the reality that the nickel, once it has been initially deposited, accelerates, or catalyzes, the overall redox reaction and deposition process.
- Heat Treating
- Several steps should be taken after a product has been plated in a bath solution to properly finish EN plating. Anti-oxidation agents like trisodium phosphate are applied to the product (which is then rinsed) to stabilize the surface finish. Afterward, the product usually receives some kind of heat treatment (e.g. baking in an oven) to offset the absorption of hydrogen. Fortunately, this baking process also serves to enhance and solidify the hardness of the finished electroless nickel coating.
- In contrast to electroless plating, the electroplating process uses an electrical current in order to reduce cations of a desired material from a solution instead of a chemical solution. When the reduction of cations occurs, a conductive object is coated with a thin layer of the (typically metallic) material.
Electroless Nickel Plating Types
- Cadmium Plating
- The process of depositing a thin, uniform layer of cadmium onto another substrate, such as a metal or plastic workpiece. Electroless cadmium plating is not an incredibly common process because it is under some scrutiny because of some side-effects of the process with the metal that could potentially be hazardous to the environment. However, electroless cadmium plating remains widely used in the aerospace and military industries.
- Chrome Plating
- A finishing treatment that can be either bright chrome or hard chrome. Electroless chromium (EC) plating, or chrome plating, actually refers to an alloy of chromium rather than pure chromium, which can be very expensive and requires an electrical current in order to be plated.
- Copper Plating
- The process of depositing a layer of copper onto a solid metal or plastic workpiece. Also used in the electronics industry, copper plating does not perform quite as well as either gold or silver, but is a much less-expensive option than either. In addition, copper has a higher conductivity than comparable other metals such as aluminum.
- Composite Coating
- Use hard particulate matter mixed with electroless nickel plating chemicals. Silicon carbides and synthetic diamonds are common types of composite materials.
- Electroless Plating
- Also known as chemical or auto-catalytic plating, is a type of plating that does not use electricity.
- Electroplating
- An alternate type of the coating/plating process. Using a low voltage current, charged nickel compounds are attracted to a substrate’s oppositely-charged surface; in this fashion, nickel deposits are transferred through a solution and onto the substrate.
- Gold Plating
- Involves the deposition of a thin layer of gold onto another substrate, typically a variety of metal parts. In gold plating, a thin layer of gold is deposited on the surface of another metal. It most often occurs in electronics in order to provide other metals with a corrosion-resistant and electrically conductive layer.
- High Phosphorus Plating
- Has the best corrosion resistance of any electroless nickel plating process. It is used in harsh environments, such as oil drilling and coal mining.
- Low Phosphorus Plating
- Yields very good resistance to alkaline corrosive environments. It also provides uniform thickness, so that grinding after the procedure is unnecessary.
- Medium Phosphorus Plating
- A popular form of nickel plating that has been used over the years. It generates a nice uniform coating and will not build up on the edges of the substrate.
- Metal Finishers
- Improve a product’s corrosion and wear resistance.
- Metal Plating
- The process of depositing a metal or metal alloy onto a surface.
- Nickel-Boron Coatings
- Admired for their as-plated hardness, which is greater than that of nickel-phosphorus platings. The melting point for N-B alloys is higher than that of N-P, but chemical costs for nickel-boron baths can be up to 10 times that of the nickel-phosphorus chemicals.
- Nickel Coating
- The process of coating an item with a nickel alloy to prevent oxidation.
- Nickel Plating
- The process of depositing a thin layer of nickel onto another substrate, such as metal or plastic.
- Poly Alloy Coatings
- Consist of nickel and boron or phosphorus. Other materials, such as iron, cobalt and tungsten, are also included in poly alloys. Polly alloy coatings allow maximum corrosion and high-temperature resistance, hardness and magnetic or nonmagnetic qualities.
- Rhodium Plating
- Refers to the process of depositing a thin layer of the chemical element rhodium (Rh) onto a conductive metal surface. There is electroless rhodium plating, which is typically used on precious metals such as gold and silver for commercial applications such as jewelry.
- Silver Plating
- The process of coating another substrate with a thin layer of silver. Silver plating is often utilized in the electronics industry as a less-expensive alternative to gold plating. However, silver-plated parts will not perform well in humid environments because it does oxidize.
- Tin Plating
- The deposition of tin onto another material's surface, both ferrous and non-ferrous, in order to provide increased protection from harsh conditions. Electroless tin plating is also commonly utilized in the electronics industry, specifically for PCBs. Tin is typically alloyed with other metals such as lead or copper before it is used for electroless plating.
- Zinc Plating
- An industrial process that utilizes either solely a chemical reaction of a combination of a chemical reaction and an electrical current in order to deposit a thin coating of zinc onto a metal part. Electroless zinc plating prevents oxidation of the plated metal. In addition, zinc is typically used in electroless barrel plating processes, which means that small parts are electroplated in large groups.
Applications of Electroless Nickel Plating
Electroless nickel plating is a common industrial process and is used in a wide range of applications in a wide range of industries, including:
- Petroleum
- Which it is used on essential components such as oil field valves and fuel rails.
- Chemical
- Where it is used for things such as hydraulic components, pressure vessels, and turbine blades.
- Food
- Where it is used for canning machines, molds, grills, etc.
- Automotive
- Where it is used to plate power transmission and other parts such as drive shafts, rotors, brake pistons, and mufflers.
- Manufacturing
- Where it is used for various electric and mechanical tools, fasteners, pipes, gears, etc.
- Residential
- Where it is used in coating kitchen utensils, door knobs, bathroom fixtures and more.
- Jewelry
- Where it is applied to optical surfaces used for diamond turning.
- Aeronautics
- Where it is used for engine mounts, landing gear components, propellers, etc.
- Military
- Where it is used for mirrors, firearms, fuse assemblies, etc.
- Electronics
- In order to plate electric components such as hard drive disks, printed circuit boards (PCBs), connectors, wire terminals, etc.
- Electroless Nickel
- Prized among many applications to its hardness and corrosion resistance. The low magnetic properties of electroless nickel enhance its value for the electronics industry, which uses these characteristics for products such as electromagnetic shielding.
Variations of Electroless Nickel Plating
Several variations on the electroless nickel plating process exist. One such variation is composite electroless nickel plating, which deposits silicon carbide along with a nickel alloy for even stronger resistance properties on the finished product. Duplex electroless nickel plating utilizes a nickel-phosphorus alloy just like regular electroless nickel plating; however, it is characterized by two distinct layers of nickel-phosphorous with differing phosphorus percentages (14% on the lower layer and 5% on the upper layer).
Although nickel is the most common material that is utilized in electroless plating processes, it is not the only type of material that can be electroless plated. Additional materials that can be used in electroless plating processes include: gold, silver, tin, zinc, copper, chrome, cadmium, palladium and rhodium. Of these material types, and obviously after nickel, the most commonly used materials in electroless plating processes are gold, silver, copper and palladium.
Although palladium is not a common metal, in fact is a rather rare metal with a lustrous silvery-white color only discovered in 1803, one of its more common uses is that of electroless plating. Palladium works so well for electroless plating because it provides such excellent bath stability as well as high corrosion protection.
In addition to the more common metals used in electroless nickel plating, there are those that are widely used in electroplating, but not so widely used in electroless plating including tin, zinc, chrome, cadmium and rhodium.
Choosing an Electroless Plating Manufacturer
When choosing manufacturer to meet your electroless plating needs, several factors should be taken into account:
- Manufacturer Certification
- Remember that electroless nickel plating is heavily dependent on precision and a firm grasp of process controls methods. You should consider a manufacturer’s level of certification and/or approvals according to industry standards (e.g. ASTM B733 standards, which regulate nickel-phosphorus coatings) Additionally, take note of a manufacturer’s length of experience and overall reputation within the industrial community.
- Control Processes
- You should inquire about certain aspects of a manufacturer’s control processes, such as surface preparation, operating parameters, avoidance of bath loading, post-treatment operations, etc. Investigate their level of investment in high-quality EN plating equipment (e.g. commercial baths, baking ovens).
- Plating Process Specialization
- Electroless nickel plating can come out with varying degrees of its characteristic qualities depending on variables in the manufacturing process. Chief among these variables is the presence of phosphorus, the main component in the nickel alloy. Different percentages of phosphorus in the bath solution have varying effects on the finished product:
- Low phosphorus (2-5%) products possess the best hardness and uniformity. They resist corrosion well in alkaline environments.
- Medium phosphorus products (6-9%) are the most common. They are noted for a particularly bright finish as well as a high (comparatively speaking) rate of formation.
- High phosphorus products (10-13%) offer the best corrosion resistance and resists corrosion in acidic environments well.
- Higher percentages of phosphorus also result in lower melting points and less magnetic qualities.
- Some EN manufacturers specialize in specific types of EN products with certain levels of phosphorus. Consider which type of EN product you desire, and then look for a manufacturer with strong capabilities to produce that type of product.
- Customer Satisfaction
- The best company is the one that invests time to understand its customers’ needs and modify their operations to meet those needs. Look for a manufacturer not only qualified to meet your specific plating needs, but willing to learn about your situation and customize their service accordingly.
Electroless Nickel Plating Terms
- Abrasion
- The deformation or wearing away of a surface material due to frictional forces and/or impact engendered by a nearby body or element.
- Activation
- The loss of passivity on the surface of a solid.
- Adhesion
- The sticking together or attractive force between two materials in contact. The adhesion that electroless nickel provides to most metals is excellent.
- Alloy
- A solid compound consisting of two or more metals fused together.
- Anode
- A positively-charged conductor that attracts nearby free electrons. Anodes are a uniformity factor for the electroplating process, but not electroless plating.
- Base Metal
- Metal that easily oxidizes or dissolves, forming ions.
- Bright Dip
- A process that is used to create an extremely bright surface on a metal.
- Catalysis
- The quickened rate of a chemical reaction due to a catalytic agent. Catalysts are often applied to substrates to speed up the finishing procedure.
- Coating Thickness
- The distance from the top layer of the coating material to its substrate’s outermost surface. Common thicknesses for nickel deposits range from .0005 to .001 inches.
- Compound
- A substance formed by the chemical union of two or more elements.
- Conductance
- A metal’s capacity to transmit electric current.
- Corrosion
- The deterioration of a metal due to reaction with atmospheric elements. Nickel plating is admired for its anti-corrosive qualities.
- Deburring
- The removal of burrs and sharp edges on a metal by chemical, electrochemical and mechanical processes.
- Density
- The ratio of a material’s mass to its volume. Nickel compounds used for coating purposes typically have densities in the range of 7.7 gm/cm3 to 8.5 gm/cm3, depending on the concentration of phosphorus.
- Ductility
- The ability of a metal to withstand deformation before finally fracturing.
- Electrical Resistivity
- The ability of a material to resist the flow of electrical current.
- Eutectic Alloy
- An alloyed material that has a melting point lower than that of each individual element alone.
- Hardness
- The resistance of a material to deformations by indentation. For electroless nickel plating, common hardness values range from 44 HRC to 59 HRC.
- Immersion
- The act of submerging a product. Substrates are immersed into baths containing electroless nickel plating chemicals.
- Ion
- A charged atom or molecule.
- Oxidation
- A reaction in which electrons are removed from a reactant, usually because of the addition of oxygen.
- Passivity
- A decrease in the corrosion rate of metal, which results from the application of a protective film such as electroless nickel plating.
- Substrate
- The material that is being coated or plated.
- Tensile Strength
- The maximum amount of tensile force that can be applied to a material before it is broken apart. Electroless nickel plating has comparable tensile strength to many hardened steels.
- Topography
- The surface features of a material. Substrate topography affects coating appearances for many metal products.