Zinc electroplating is widely used because of its ability to protect steel from corrosion. The sacrificial nature of the protection and tolerance of scratches makes its performance superior to paints alone. However, use on base metals other than steel or stainless steel is rare. Zinc plating can be prepared to have an attractive appearance, and as the cost is relatively low it is a very popular coating both for small parts like bolts, nuts, rivets, washers, nails, hinges, and hooks, and for automotive parts, interior components, and so on. It also works also as an effective undercoat for paints.

Typical minimum thickness ranges are specified between 5 and 25 μm (8). The corrosion resistance of a coating depends on the layer thickness and applied post-treatment.

Three main processes are used for zinc plating: cyanide, alkaline noncyanide, and weakly acidic chloride solutions. The choice of the solution depends on the effluent treatment abilities of a plant and the type and material of components to be plated.

The cyanide process has a good macrothrowing power, and it thus gives quite even coating thickness regardless of the geometrical shape of the component. The bath is easy to maintain, and it tolerates rather large variations in concentrations. As the bath is alkaline, it does not corrode steel parts, and it also has surface cleaning properties to tolerate small amounts of organic impurities. However, due to the cyanide content, the bath is extremely toxic, which causes high rinse water and waste costs. At high current densities, the current efficiency is low, and the risk of hydrogen embrittlement for heat-treated and carbonitrided parts is high. The other downsides of cyanide baths are poor microthrowing power and difficulties in the plating of cast iron. This is due to the graphite on the surface of cast iron and its lower hydrogen overvoltage in cyanide solution compared to zinc, which will lead to hydrogen evolution on graphite spots instead of zinc precipitation (9).

Zinc exists in cyanide baths as a tetracyano complex Zn(CN)42−, which dissociates at the same speed as the Zn2+ ions are precipitated at the cathode (10). The association with the cyanide complex, which as anionic ion causes a large concentration polarization, enhances the macrothrowing power of the bath, but makes the current efficiency low. The microthrowing power (leverage) of a cyanide bath is poor. Cyanide complex makes the crystal structure of the precipitated metal fine, which gives a good basis for brighteners to make bright coatings, although the deposit brightness from a cyanide bath alone is not as good as that from other bath types. In alkaline cyanide baths, the zinc ions are also associated with hydroxyl ions as Zn(OH)42−, and there is a balance between cyanide and hydroxyl complexes, which depends on the amount of sodium hydroxide added to the bath.

Due to the adverse effects of toxic cyanide, its amount in the bath has varied from high-cyanide to middle- and low-cyanide baths. The total sodium cyanide content of the high-cyanide bath is 75–115 g l−1, while in middle-cyanide baths it is 35–55 g l−1 and in low cyanide 6–15 g l−1 (10–12). The drag-out and waste disposal costs are lower in low-cyanide baths, but the requirements for pretreatment and operating parameter variation are higher. The sodium hydroxide content should be high enough, 80–100 g l−1, to have good conductivity and anode dissolution and to produce good brightness.

The brighteners added to the cyanide bath are usually organic, since the concentration of metal brighteners is too critical. Usually, primary and secondary brighteners exist. The most common primary brightener is polyvinyl alcohol (PVA), while the typical secondary brighteners are smaller molecules that contain unsaturated bonds and polar groups, for example, aromatic aldehydes and pyridines.

The cyanide baths also contain sodium carbonate, which is formed when cyanide molecules are oxidized by the oxygen in air. Some carbonate is required to form a dense coating layer, but an excessive amount, over 70–80 g l−1, must be removed from the bath by lowering the bath temperature below the solubility limit of sodium carbonate.

Important operating parameters are relations of NaCN and Zn concentrations and cathodic current density. The higher the amount of Zn, the lower is the factor NaCN/Zn, which enhances the current efficiency. However, the macrothrowing power will be poorer, and operation of the plating process is more demanding. The selection of a cathodic current density depends on concentrations, brighteners used, temperature, and agitation. Typical values are 2–5 Adm−2. Higher current densities will lower the cathodic current efficiency. The cyanide baths are operated at room temperatures.

The purity of an anode material is important for cyanide baths, since it affects the brightness of the coating layer. Usually, the anode material is over 99.99% pure zinc, either in ball or bar form. When the bath is unused, the zinc will dissolve to the bath; to avoid excessive zinc buildup, the anodes should be removed.

Alkaline noncyanide zinc baths may be prepared by dissolving zinc oxide ZnO to sodium hydroxide and adding brighteners. The bath will contain zinc 8–10 g l−1 and NaOH 90–120 g l−1, and the zinc will be in the solution as Zn(OH)42−-ions (11,12). Most properties of the bath are determined by the brightener systems, which are usually patented and may include, for example, PVA and imines. The good properties of the baths are low metal content and usually inexpensive effluent treatment, good brightness, good macrothrowing power, and moderate microthrowing power. The danger of hydrogen embrittlement for high heat-treated steels is lower compared to cyanide baths. The bath requires better operation control than cyanide baths; in particular, the metal content must be controlled during idle periods, since too high metal concentrations will deteriorate the brightness. However, too low a metal content will lower the current efficiency. The bath is operated at the room temperature.

Weakly acidic chloride solutions have gained more market share as the properties of the solutions have been developed. The bath contains zinc 15–30 g l−1 and either ammonium or potassium chloride, so that the chloride content of the bath is 110–150 g l−1 (10,11). Organic brighteners must be included in the bath. The optimal pH range is 4.5–5.5. Zinc will be dissolved into the solution as Zn2+-ions. The precipitate will be columnar and rough unless brighteners are used. Very bright deposits can be achieved by the use of brighteners, and weakly acidic solutions are generally considered to give the best brightness among the zinc plating processes. The current efficiency is at the same time very high, around 95–98% at a large current density area, which is beneficial for the plating of carbonitrided steel and cast iron. The microthrowing power is very good, but as the current efficiency is high, the macrothrowing power is poor. This, along with the bath chemistry, requires that the parts be cleaned carefully before plating, and much attention must be paid to the plating fixtures. The adverse effect of the acidic chloride solution is the potential corrosion of the substrate materials and staining of the coating unless it is carefully rinsed off. Therefore, spot welded or other parts with lamellar structures are not suitable for the solution. The effluent treatment is usually easy, although ammonium may disturb the operation of an effluent plant by forming complexes with heavy metals. The bath is operated at room temperature, and current densities up to 6 Adm−2 can be used without significant drop in the current efficiency.