Aluminium forms an oxide the moment it is exposed to the air. This presents a problem when plating it, as the oxide prevents the plate from sticking. By using the Zincate process as a pre-plate dip, you chemically remove the oxide layer and at the same time, apply a layer of zinc. The zinc protects the aluminium from further oxidising until it is ready to be plated. As the part is lowered into the plating tank, the zinc is etched away by the solution and plating proceeds onto a clinically clean surface.
Procedure for Zincating Bead Blast or clean in an etch solution to remove all heavy oxides and contaminants. Buff and polish to the required shine. The part should then be thoroughly degreased in either CASWELL SD-175 Degreaser or a suitable solvent such as Acetone or Lacquer Thinners.
Make up a solution of Zincate by adding 25% by volume of the concentrate to a plating tank (Pyrex, plastic etc.) and fill with distilled water. i.e.:- Add 250 ml of Zincate, then top up to 1 litre mark with distilled water.
Heat the solution to 24°c. An aquarium heater will do this quite easily.
Immerse the part in the solution for 15 sec. to 2 minutes. Immersion time will depend on the alloy, zincate condition and temperature. If the alloy reacts violently with the solution, then shorten the time immersed. If no reaction is visible, then immersion times can be extended.
A uniform grey appearance is usually a sign of a properly zincated surface, however there are cases where a dark, slightly streaked surface is acceptable. As the Zincate solution is depleted, there will be a noticeable increase in the gassing. You can replenish the solution by adding about 10% of the Zincate concentrate, or simply make up a fresh batch.
Rinse the part and prepare for Nickel Plating in our Regular Nickel (with anodes & power supply), Electroless Nickel kit, Copy Chrome or Flash Copper kit. This process is not suitable for Brush Plating.
Zincating Aluminium prior to Electro/ Electroless Plating
The Caswell zincate treatment of aluminium is an essential step of pre-treatment before electro/ electroless plating other metals or metal alloys. The product is a concentrate which is mixed to a working solution of 1 part Zincate to 3 parts De-ionised water.
Aluminium is plated:
- to improve the appearance of the surface. (plating decorative nickel and chromium, gold, zinc with black passivation)
- to increase the corrosion resistance. (copper or brass as intermediate layer, cadmium, electroless nickel)
- to reach a higher abrasion resistance and (hard chromium, electroless nickel)
- to increase the soldering and welding properties. (tin, but also copper, zinc, silver and nickel)
- to increase the electrical contact ability. (silver, gold, electroless nickel)
Basics and Mechanism of the Zincate Process
The most important process during the pretreatment of aluminium before electroplating is the zincate treatment. The natural oxide layer is removed and the surface of the aluminium is activated creating a clinically clean work surface. A thin conductive intermediate layer is deposited in an electroless manner, which prevents the re-oxidation of the surface until the part is inserted into a following plating bath. It is primarily responsible for the good adherence of subsequent plating layers.
A usual process sequence includes the following steps, suited for aluminium alloys containing up to 1-2 % silicon:
Mildly alkaline aqueous immersing cleaning, pH 7-10, 50-60 °C, 3-15 min.; to remove polishing pastes (SD-Klene416), the usage of ultra sonic is recommended. (Caswell Ultra-Sonic6 cleaner)
At room temperature, 0.5-5 minuets, possibly slight air or bath agitation. For more difficult aluminium alloys and for higher quality demands, the zincate treatment can be applied twice. To do so, the first zincate layer is removed in nitric acid (preferable in a separate tank which is only used to remove the first zincate layer), then the aluminium surface is immersed again, after rinsing, into the zincate solution. The second zincate layer has a better adherence, finer crystals and is more dense.
Before the zincate treatment, the aluminium part has to be rinsed very thoroughly to prevent dragging in acids from any de smutting. This would impair the quality of deposition. The rinse after the zincate treatment should have a pH of 7 to 11. Higher or lower pH values may attack the zincate layer. To prevent precipitation of calcium hydroxide on the surface, the rinsing is preferably done in deionised water.
The process consists only of one component. Bath ingredients are consumed equally and they are replenished in the same ratio. The zincate treatment is easy to handle and it may be controlled and adjusted with high process reliability.