Zinc, one of the most widely used metals in the surface finishing industry, is bluish-white and lustrous. Zinc electroplating creates a soft, ductile, decorative and corrosion resistant finish.
Unlike most commonly plated metals, zinc protects the substrate by sacrificing itself. The zinc corrodes before the base metal, thereby protecting the base metal. For additional corrosion resistance, chromates are applied over the zinc. Such films are typically very thin and add no measurable thickness to the overall coating. The zinc plating process is the most economical for applications exposed to atmospheric corrosion.
Cyanide Zinc, Alkaline Non-Cyanide Zinc and Acid Chloride Zinc are the typical plating processes employed. Temperature ranges for the process are between 60oF to 130oF. Plating thicknesses range from .1 mil to 2.0 mils (.0001” to .002”). Common uses for zinc plating are for appearance and corrosion protection of steel fasteners. In this instance, the plating thickness is usually .3 mils (.0003”) due to thread gauging requirements.
Cyanide zinc baths are used for steel and leaded steel substrates. The plating bath consists of zinc metal (1 to 4.5 ounces per gallon), sodium hydroxide (10 to 12 oz/gal) and sodium cyanide (1.5 to 14 oz/gal) with operating temperatures ranging from 60oF to 110oF. The pH for such baths is 12 to
Plating tanks are fabricated from steel, fiberglass, polyvinyl chloride (PVC) or polypropylene. Heating and/or cooling coils, slab anode hooks or anode baskets are fabricated from steel. Anode bags for plating baskets are not typically used. Zinc anodes for this application are 99.9% pure. Filtration and agitation are also not usually required.
Cyanide zinc is highly poisonous and corrosive to living tissue. As a result, concerns over environmental and occupational safety hazards have forced the plating industry to develop and utilize cyanide-free zinc plating systems.
Alkaline Non-Cyanide Zinc
As with cyanide based baths, alkaline non-cyanide zinc baths are used for steel and leaded steel substrates. These baths consist of zinc metal (.8 to 3 oz/gal) and sodium hydroxide (10 to 20 oz/gal) with operating temperatures ranging from 60oF to 110oF. The pH is 12 to 13. Rectification is typically 6 to 20 volts and 3 to 10 amps per square foot of part surface area for barrel applications. Rack applications are 3 to 9 volts and 10 to 40 amps per square foot of part surface area.
The quality of the water used in this process plays a major role in performance. Hard water causes dull zinc deposits that can lead to increased consumption of proprietaries and purifiers. If the quality of the water is poor, deionized water should be used as part of the bath makeup. Otherwise, using a water conditioner/purifier is recommended.
Filtration and air agitation are essential to remove metallic and/or organic contaminants. Such contaminants can be caused by human error or drag-in from the cleaner or acid tanks during the plating cycle. Contamination affects the deposit in many ways, including poor distribution, discoloration and burning. Common metallic contaminants include copper, cadmium, lead, tin, chromium and iron. The plating bath is typically turned over 1 to 2 times per hour to remove potential contaminants.
There are several proprietaries used in the alkaline non-cyanide zinc plating process. Refining agents are used to provide semi-bright uniform deposits. Brightening agents make the zinc deposit uniformly brighter by adding luster to it. Purifiers treat heavy metals and impurities introduced from zinc anodes and caustics. Water conditioners treat water hardness. Finally, wetting agents put a thin foam blanket over the surface of the plating bath to suppress fumes or spray created during the operation.
Plating tanks are fabricated from steel, PVC or fiberglass lined with PVC or polypropylene. Cooling is essential for this process while heating may be needed in colder climates. Heating and/or cooling coils, slab anode hooks or anode baskets are fabricated from steel. Anode bags used for baskets are fabricated from polypropylene. Zinc anodes used for this application are 99.9% pure.
While alkaline non-cyanide zinc plating baths result in no toxicity to humans and the formulations are noncorrosive to equipment, exposure to the chemicals of these baths can be corrosive to living tissue.
Acid Chloride Zinc
Acid chloride zinc baths are used for steel and leaded steel. They are more flexible in comparison with other substrates, such as high-carbon, heat-treated and carbonized pieces (including castings without any special treatment). Three types of bath compositions are typically used with operating temperatures ranging between 60oF and 130oF.
Because of its wide operating parameters, the ammonium chloride zinc bath is the most forgiving of the three types of acid zinc baths used. The bath consists of zinc metal (2 to 4 oz/gal) and ammonium chloride (16 to 20 oz/gal) with a bath pH of 5 to 6. The major drawback of these baths is the high level of ammonia which can cause problems in wastewater treatment. Ammonia acts as a chelator (coordination compound in which a central metal ion is attached by coordinate links to two or more non-metal atoms in the same molecule). If the rinse waters are not segregated from other waste streams, removal of metals to acceptable levels using standard water treatment practices can be difficult and expensive. Ammonia is also regulated in many communities.
Potassium chloride zinc baths are attractive because they contain no ammonia. Such baths consist of zinc metal (3 to 5 oz/gal), potassium chloride (25 to 30 oz/gal) and boric acid (3 to 5 oz/gal) with a bath pH
of 5 to 5.5. The disadvantage of these baths is a greater tendency to burn on extreme edges and their higher operating costs. These baths require the use of relatively expensive boric acid to buffer the solution and prevent burning in the high current density areas, functions performed by the ammonium chloride in the other bath types.
Mixed ammonium chloride/potassium chloride zinc baths combine the best of the ammonia and ammonia-free processes. Because potassium chloride is less expensive than ammonium chloride, the maintenance costs of mixed baths are lower than the ammonia bath and do not require boric acid. The ammonia levels in the rinse waters are low enough that they do not significantlty interfere with wastewater treatment. Such baths consist of zinc metal (2 to 4 oz/ gal), ammonium chloride (4 to 6 oz/gal) and potassium chloride (16 to 20 oz/gal) with a bath pH of 5 to 6.
Acid zinc plating baths have several advantages over cyanide and alkaline non-cyanide baths. There is less waste treatment since no cyanide is involved, eliminating disposal expenses. The resultant zinc deposits have an outstanding brightness that rivals nickel chromium in its luster. The baths provide high cathode efficiencies (90% to 95% compared to 70% to 75% for cyanide and 70% to 95% for alkaline non cyanide baths) at normal operating current densities. They provide excellent leveling and produce less hydrogen embrittlement. Finally, the wetting agents typically used in the acid process are relatively rinsefree when compared to alkaline systems.
Disadvantages of these baths are that they are extremely corrosive to both equipment and living tissue. Surface preparation is also very important. Improper cleaning and/or pickling can lead to problems in the deposits including blistering, lack of adhesion and haziness. Ductility can be lost with thick deposits. There is also a need for continuous filtration (1 to 2 turnovers per hour) and air agitation to remove iron from the solutions. This iron is derived from substrates and/or anodes.
Plating tanks are fabricated from fiberglass, polypropylene or steel lined with fiberglass, PVC, polypropylene or rubber. Tanks are leached with acid (typically 5% to 10% hydrochloric acid) prior to use. Heating and/or cooling coils are fabricated from titanium or plastic. Slab anode hooks are fabricated from either titanium or monel and are kept above solution level. If baskets are used, they are fabricated from titanium with anode bags fabricated from either Dynel® or polypropylene. Zinc anodes used for this application are 99.9% pure.
Typical Zinc Plating Sequence
1. Soak Clean
4. Acid Pickle *
6. Zinc Plate
8. Nitric Dip
*Acid Pickle tanks are used to activate and clean parts prior to plating. They consist of either 20% to 30% hydrochloric acid at room temperature or 5% to 15% sulfuric acid at 105oF to 120oF.