Introduction
For manufacturers and other heavy industries, electroplating is not only a costly procedure because of the huge electrical power consumption; it is also dangerous for workers on account of the high voltages used in the process.
Electroless nickel plating can be an alternative method.
In this post, you will learn about the advantages of electroless nickel plating, the bath constituents used in the process, and the different industries that use this method.
Table of Contents
- What is Electroless Nickel Plating?
- Advantages of Electroless Nickel Plating
- Bath Constituents in Electroless Nickel Plating
- Step-by-step Electroless Nickel Plating Process
- Industries That Extensively Use Electroless Nickel Plating
What is Electroless Plating?
Electroless plating involves the production of coatings from solutions of metal ions without the use of an external source of electrical energy. It is the most extensively used electroless coating within the manufacturing industry and the most prevalent for engineering purposes.
Advantages of Electroless Nickel Plating
- Excellent corrosion resistance
- Excellent wear and abrasion resistance
- Good ductility, lubricity and electrical properties
- High hardness, especially when heat-treated
- Good solderability
- Even and uniform thickness even down deep bores and recesses, and at corners and edges
- The coating can be applied as the final production operation and can meet stringent dimensional tolerances
- Can be used on both metallic and non-metallic substrates, provided they have been suitably pre-treated
Bath Constituents in Electroless Nickel Plating
Electroless plating relies on a reaction proceeding at a specific temperature, typically around 90°C, when a suitably activated substrate is immersed in the solution. Since most solutions used in industry are proprietary, the full formulation is never known, thus requiring the solution to be carefully controlled for optimum results. Chemical analysis of the plating solution must be performed regularly during longer production runs.
The bath constituents are detailed as follows:
Source of Metal
Most acid solutions use nickel sulphate, whilst nickel chloride is used in alkaline solutions. Deposition rate increases with increase in nickel concentration; and conversely, solution stability decreases
Reducing Agent
Sodium hypophosphite is widely used due to its low cost and availability. As with nickel content, we see the same effects with concentration on deposition rate and stability. Reducing agent is replenished at the same rate as the nickel ions to maintain the deposition rate. Most commercial chemical manufacturers supply the plating process in multiple pack systems to supply the nickel and reducing agent. Both nickel and sodium hypophosphite concentrations can be readily determined by volumetric analysis, with some large scale commercial set-ups making use of automated analysis and chemical dosing systems.
Hypophosphite consumption can be higher than expected with low surface area substrates compared to the total solution volume, especially when air agitation is utilized. Approximately 30% of hypophosphite is used to produce nickel phosphorous, whilst the remainder produces hydrogen.
Complexants
The type of complexants used depend on the nickel concentration and the chemical structure of the complexing agents themselves. Some formulations may use a single complexant, whilst others use combinations to maintain a low concentration of free nickel ions. Commonly used complexants include glycolic or lactic acid for acid-based solutions, and ammonium hydroxide for alkaline solutions. These are contained in the nickel replenisher solution to make bath management simpler.
Buffers
Hydrogen ions produced during plating causes the pH of the solution to decrease. Since pH is a major factor in controlling deposition rate and the phosphorus content of the deposit, it must be stabilized using buffers. Common buffers include acetic or propionic acids and their salts. These acids also increase the deposition rate. Even when using buffer agents, there is a slow fall in pH as plating occurs, which may be corrected by specific chemical additions or by replacement of the bath components through additions of the component chemistries.
Stabilizers
Stabilizers are used to prevent spontaneous decomposition of the plating solutions. Traditionally, these included heavy metals such as lead or cadmium at very low concentrations (<1ppm). To comply with new RoHS regulations, the heavy metals in most commercial formulations have been substituted with compounds such as molybdate or iodates.
Electroless Nickel Plating Process
The step-by-step process for electroless nickel plating is as follows:
- The metal is submerged in a series of pre-treatment baths. Each of these baths contains specific chemicals that remove oil, grease, dirt and other pollutants on the metal surface. This improves the adhesion of the deposits onto the substrate surface. The cleansing chemicals used depend upon the surface material.
- After cleaning, certain metal substrates require further treatment in an aqueous zincate solution. This tends to be a proprietary solution supplied by the electroless nickel chemistry manufacturer.
- Once immersed in the plating solution, nickel and phosphorus ions are deposited onto the metal substrate surface.
- Depending upon how thick the surface needs to be, the deposition process can be carried out from between 5 microns to 25 microns per hour.
- Once the desired plating thickness is achieved, the substrate is removed from the plating solution and inspected.
Industries that Extensively Use Electroless Nickel Plating
- Automotive: Cylinders, gears, shock absorbers, brake pistons, heat sinks, etc.
- Aviation and Aerospace: Components for rockets and satellites, valves, rams pistons, etc.
- Chemical Processing: Mixing blades, filter units, heat exchangers, pump housings, impellers, etc.
- Petroleum and Gas: Plugs, gates, balls and other valve components, pipe fittings, barrels, pumps, packers, etc.
- Plastic Manufacturing: Injection molds and dies, low and blow molding for plastics components, rollers, extruders, etc.
- Textiles: Machine parts, threaded guides, spinnerets, printing cylinders, etc.
- Food and Pharmaceuticals: Food molds, food processing machinery components, capsule machinery dies, etc.
