Process description: Electroplating nickel in metal industry
- Nickel electroplating
Nickel electroplating and electroless plating processes are used in a wide range of industrial and consumer applications. Although the prime function of these processes is to improve the resistance of substrates to corrosion, wear and abrasion, nickel provides a smooth, highly reflective and corrosion-resistant coating below a range of other coatings for decorative finishes.
Nickel plating processes – including both electrolytic and electroless (autocatalytic) systems – can conveniently be considered in separate categories.
- Nickel/chromium electroplating
The most important application of nickel is in nickel/chromium electroplated coatings, commonly called “chrome plating”. They consist of a very thin chromium topcoat (1%) over an undercoat of nickel (99%). Nickel provides a very smooth, brilliant corrosion-resistant finish.
- Nickel electroplating with other topcoats
Brass, gold and silver topcoat systems are used as alternatives to chromium.
- Nickel electroplating
Nickel can be used on its own without any topcoat. Generally, this is only for engineering purposes, such as refurbishment of worn components.
- Nickel composite electroplating systems
Almost uniquely, nickel matrixes can be formed in which inert, non-metallic particles, such as silicon carbide, diamond or PTFE are incorporated by co-deposition to improve engineering properties such as hardness, abrasion resistance and coefficient of friction.
- Nickel alloy electroplating
Electrodeposited nickel alloys of commercial significance include zinc-nickel, nickel-cobalt, and nickel-iron.
- Nickel electroforming
Nickel electroforming is a unique process that allows articles to be produced by the electrodeposition of relatively thick nickel layers – it is a vital part of the process of manufacturing compact discs, DVDs, holograms and screen printing cylinders.
- Electroless nickel plating
This is a chemical process giving hard uniform coatings. They can also be deposited on materials that cannot be electroplated, such as plastic and some alloys.
- Substrate considerations
Nickel is regularly deposited onto a wide range of metallic substrate materials commonly used in manufacturing processes such as steel, copper, brass, zinc alloys, aluminium, and magnesium as well as onto a range of plastic substrates.
It can successfully be plated directly, achieving good adhesion, onto some of these substrates (steel, copper and lead-free brass) if the correct cleaning and other pretreatment processes are properly carried out.
Zinc alloys are, however, susceptible to corrosive attack in acidic nickel plating solutions and consequently require a layer of copper deposited from a cyanide solution before nickel can be successfully deposited.
With aluminium and its alloys, because of their very high surface reactivity, it is necessary to deposit a layer of zinc (produced by non-electrolytic chemical treatments known as the “zincate” or “double zincate” processes) before a layer of copper can be applied, again from a cyanide-based solution. Magnesium alloys require similar treatment.
Watts-type nickel solutions
Watts-type nickel solutions account for the majority of solutions used in the nickel plating industry, including those used for nickel-chromium plating, nickel with other topcoat systems and composite nickel plating. Nickel sulphate (240 – 357 g/l) is used with nickel chloride (35 – 60 g/l) and boric acid (30 – 45 g/l). Operating temperatures can range from 25 to 70 °C although the more restricted range of 50 to 60°C is more common. The pH is normally 3.5 – 4.5. Formulations with nickel chloride content at the higher end of the range may be used to achieve increased deposition rates.
Watts-type solutions can be used without any additions to produce dull nickel deposits, although wetting agents are almost always added to reduce gas bubble retention on the nickel surface which would result in “pitting”. However, watts-type solutions are most frequently employed with the addition of organic compounds. These modify the metallurgical structure of the nickel to produce either a lustrous and fully bright appearance or alternatively semi-bright or satin nickel deposits. As well as altering the visual appearance of the nickel, these additives also inevitably bring about changes in deposit ductility, hardness and internal stress. Typical additions depend on the required function of the nickel deposit and varies from a small amount of organic semi-brightener (<1 ml/l) and wetting agent (<1 ml/l) for a semi-bright finish to primary and secondary brighteners at 10 – 20 ml/l and <10 ml/l wetting agent for a bright finish.
There are many types of organic compounds used to modify the properties of deposits produced from Watts-type solutions. In general, they are added as proprietary mixtures.
If regularly analysed, used and maintained with care, Watts-type solutions can have an almost indefinite working life. Nickel metal deposited at the cathode is rather more than fully replenished by that dissolved at the anode since the cathode efficiently is normally only between 96 – 98%, compared to an anode efficiency of 100%. This small difference in efficiencies is normally compensated for by removal (“drag-out”) of solution from the process tanks by work being carried forward into the rinsing system. In systems where “drag-out” is low, the solution concentration may actually increase; this may require treatment to keep the concentration within operating limits.
The problem most likely to shorten the working life of the nickel solution is the introduction of contamination which can be either inorganic or organic in nature.
Inorganic contaminants can be introduced by an impure water supply, solution carried forward into the nickel solution from those preceding it in the process chain (e.g. cleaning solutions) or metallic components accidentally dropped into the nickel solution, such as work falling from jigs, subsequently dissolving into the process solution. Some inorganic contaminants (Fe) can be removed by measures such as high pH precipitation and others (Cu and Zn) by low current density electrolysis known as “plating out”.
A considerable number of organic contaminants can be removed simply by filtration over activated carbon or suing adsorber polymers. Others may require additional treatment either with hydrogen peroxide or with potassium permanganate in order to break down the compound into simpler ones that can then be removed by active carbon treatment.
Nickel sulphate-based solutions
These solutions are widely used, and most frequently in electroforming applications where the low internal stress of the deposits they produce is absolutely vital. In these cases chloride-free solutions can be used (but only if a form of sulphur-activated nickel anode material is employed) to reduce deposit stress to a minimum. They are also used in barrel plating operations and reel-to-reel, since their higher electrical conductivity allows faster deposition rated to be used, and for thicker layers (>2000 μm).
Generally, sulphamate-based solutions are not used in situations where Watts-type solutions will prove effective due their higher cost.
The solutions are based on nickel sulphamate (rather than sulphate) in concentrations ranging from 350 to 600 g/l of the tetrahydrate salt, allowing a higher current. These solutions always contain boric acid (35 – 45 g/l) and frequently nickel chloride (1-15 g/l).
These solutions normally operate in similar temperature and pH ranges as those used for Watts-type solutions although high concentration sulphamate solutions that are used to achieve high metal deposition rates, using current densities up to 35 A/dm2, are often operated at around 70°C.
Sulphate-based solutions are frequently used without any additions, other than wetting agents to reduce “pitting”. However, selected organic compounds, such as saccharin and naphthalene tri-sulphonic acid, can be added to the solution to increase deposit hardness or to control deposit internal stress.
Like Watts-type solutions, those based on nickel sulphamate can have an almost indefinite life if analysed regularly and carefully maintained. There is, however an additional complication to be considered in relation to the chemical and electrochemical stability of the sulphamate anion. At higher temperatures and lower pH values this will hydrolyse to produce sulphate ions plus ammonium ions in solution. The ammonium ion increases the deposit stress and hardness to unacceptable levels and, furthermore, cannot be removed from the solution. In addition, if the anodes in the process solution become passive, the sulphamate anion will undergo electrochemical oxidation to produce an unspecified mixture of by-products that radically and detrimentally affect deposit properties.
The avoidance of inorganic and organic contamination and treatment are as for Watts-type solutions.
Nickel chloride-based solutions
Solutions based on nickel chloride have very limited uses due to the very high internal stresses of the deposits they produce. One exception is the Woods nickel strike solution which normally consists of 240 g/l of nickel chloride hexahydrate plus 125 ml/l hydrochloric acid and is operated at 20-30°C. It is used for one specific purpose only: to provide an initial adherent nickel layer on the surface of materials, such as stainless steel and nickel-chromium alloys, where it is difficult to achieve adhesion due to the naturally forming passive oxide film.
Other nickel plating solutions
Solutions based on nickel fluoroborate are mentioned in literature but currently find little, if any, commercial application.
Nickel alloy plating solutions
Nickel-cobalt alloys are used in electroforming because they are harder than pure nickel and nickel-iron alloys find applications in the electronics industry, generally related to their magnetic properties. Solutions used for depositing both types of alloy are normally based on the standard Watts-type or nickel sulphamate formulations, with the same issues of operation and maintenance.
Nickel-iron processes, however, require special additives to stabilise the ferrous ions in solution and prevent spontaneous oxidation to the ferric state.
Nickel-zinc alloy plating processes that produce alloys containing 10 – 14 % nickel have been developed recently and are becoming increasingly important since they can provide almost 10 times the level of corrosion protection that can be achieved with pure zinc.
Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.