Process description: Plating zinc and zinc alloy
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- Zinc and zinc alloy plating
Zinc and zinc alloy coatings are the most widely used electrolytic surface treatment, providing corrosion resistance and/or cheap decorative coating to a very wide variety of iron and steel items for the automotive, construction and other industries. For example, they are used for steel sheet or wire, screws, washers, nuts, bolts, shopping trolleys, construction frames (chassis) and casings for domestic appliances (such as washing machines) and many other kinds of applications.
Zinc layers require post-treatment: in conjunction with various coatings, zinc layers of only 2.5 – 18 μm thick will be sufficient to protect parts for their whole working lifetime.
Zinc plating has been in existence for a hundred years and because of its wide application, many different electrolyte systems are in use. The most common ones are described below.
- Alkaline cyanide zinc
This is used mainly for technical (non-decorative) corrosion-resistant layers. The process electrolytes are easy to operate, and have compositions of zinc oxide (10 – 30 g/l), sodium hydroxide (80 – 120 g/l) and sodium cyanide (5 – 100 g/l). They operate at pH values close to 14, using both soluble and insoluble anodes.
Alkali zinc gives coating with good post-forming properties. Some sources suggest this is due to a fine structure of the deposit, others that is due to the cleaning action of the cyanide solution giving good adhesion.
The cyanide-based electrolyte has good throwing power into holes and blind spaces.
The plating solution has low conductivity, thus a higher voltage (6 – 8 V for jig plating, 10 – 15 V for barrel plating) and a higher energy demand is needed. Carbonate build-up from cyanide decomposition can be dealt with by crystallising out, which also removes metallic impurities in the electrolyte.
The current efficiency is from 30 to 75% on a well-run process at an average current density of 2 A/m2, but decreases with increasing current density.
Fume extraction of the process tanks to remove aerosols may be used.
- Alkaline cyanide-free zinc
These are mainly applied for technical corrosion-resistant layers (non-decorative). The process solutions contain zinc oxide (5 – 15 g zinc/l) and sodium hydroxide or potassium hydroxide (100 – 150 g/l). This process may require better pre-cleaning treatment than for cyanide solutions. The process can give better metal distribution than cyanide electrolytes.
The plating solution has a low conductivity, thus a higher voltage (6 – 8 V for jig plating, 10 – 15 V for barrel plating) and higher energy demand is needed.
Current efficiency is 65 – 70 %, decreasing with increasing current density. 70 – 85% us achievable at 2 A/m2 for well-managed processes.
- Acid zinc
Acid electrolytes give bright decorative layers, and used, for example, on furniture frames, shopping trolleys and baskets. In conjunction with post-treatments, they provide corrosion resistance comparable with finishes from alkaline-type electrolytes. Metal distribution is poor to acceptable, but this improves with warm electrolytes.
Electrolytes contain zinc chloride (30 – 55 g zinc/l), potassium and/or sodium chloride (130 – 180 g/l), boric acid (10 – 40 g/l) and wetting agent. Only soluble anodes are used. The solutions have good conductivity and high cathode efficiency, typically 93 – 96%. It has a lower energy demand than alkali processes.
Plating tanks may be equipped with fume extraction hoods to remove chloride-containing mists, thus preventing the corrosion of equipment.
- Zinc alloy plating
Zinc alloy coatings provide extended corrosion resistance and the main use is for automotive applications.
The main zinc alloys deposited are:
- zinc/iron (<1% Fe), from alkaline cyanide-free electrolytes
- zinc-cobalt (<3% Co), from acid or alkaline cyanide-free electrolytes
- zinc-nickel (<15% Ni), from acid (ammonium chloride-based) or alkaline cyanide-free
electrolytes. 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 than can be achieved with pure zinc. They can be deposited either from acidic chloride-based solutions or from alkaline solutions where the relatively low amounts of amines. The alkaline solutions are becoming the preferred formulations since they give a more consistent alloy composition on complex work geometry.
Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.
