Alkaline Zinc Plating Revisited

Alkaline non-cyanide zinc (ANCZ) plating has been prevalent in the plating industry for about 50 years. It has become a preferred process for finishers who require a fairly bright zinc-plated finish on parts that have challenging geometrical shapes. ANCZ is typically the chosen zinc process for complicated geometrical shapes due to its ability to plate zinc at a preferred low ratio of zinc thickness from the high current density areas to the low current density areas. Thickness ratios can range anywhere from 1:1 to 2:1 from the high current density area to the low current density area.

Bath Components

ANCZ baths normally contain the following components:

  • Sodium Hydroxide
  • Zinc Oxide
  • Sodium Carbonate
  • Brightener
  • Polymer
  • Purifier
  • Water Conditioner
  • LCD Booster
  • Zinc Anodes – Hull Cell

Sodium hydroxide is the main electrolyte and provides conductivity to the bath, as well as contributing to the dissolution of the zinc anodes. In larger operations, a generator tank is required to dissolve the anodes in steel baskets. The size of the generator tank is recommended to be about 15-20% of the volume of the plating tank. The plating solution is pumped or gravity fed from the plating tank to the generator tank, then filtered upon return to the plating bath.

Zinc oxide is used only to make up a new tank or to raise the zinc metal in the plating bath whenever the zinc metal drops to an unacceptable concentration and cannot be brought back to proper levels by the generator tank. Ideally, the zinc metal is kept at working concentrations by the dissolving of the anodes in the generator tank. If the zinc metal gets too concentrated in the plating bath, then controlling the flow from the generator tank is important to keeping the zinc metal within target values.

The brightener provides grain refinement and luster to the zinc deposit. The brightener is consumed by electrolysis and should be added to the bath on a regular basis. A brightener feeder based on ampere hours works very well to maintain the proper concentration in the bath. Hull cell analysis is an accepted method of determining if brightener additions are required. A couple of accepted methods are 1 amp, 15 minutes and 5 amp, 5 minutes. Contact your Asterion representative for more information on how to interpret the results from hull cell testing.

The polymer is consumed by dragout and provides grain refinement, deposit ductility and low current density brightness. In some operations, the polymer is used sparingly and is only added if the zinc thickness ratio gets too high. The polymer will slow down zinc deposit build up in the high current density areas and allow more zinc to plate in the low current density areas, thus keeping the ratio lower from high current density areas to low current density areas.

The purifier is only added if needed to address concerns with impurities in the bath, specifically iron. Parts that are tubular or have blind holes or deep recesses will likely have areas that do not get plated and will have a tendency to leave behind iron particles in the bath. Iron impurities will manifest by the appearance of dark deposits, particularly in the low current density area. A good rule of thumb is to add purifier at 10% of the addition rate of brightener.

Water conditioner is a sparingly used component. Water sources that are high in calcium and other minerals (hard water) can be problematic in an ANCZ bath. Hard water will manifest by a cloudy deposit that is uniform from high current density to low current density. A hull cell test can confirm if water conditioner will resolve the issue. Always try adding brightener first (by hull cell) to determine if a cloudy issue can be resolved.

LCD Booster is a supplementary low current density brightener that can be used if the bath does not respond to normal additions of brightener and polymer. It is highly concentrated and should be used sparingly. Again, a hull cell test will confirm if LCD Booster resolves the issue.

Hull Cell Analysis: Today’s ANCZ baths are more efficient than those from the past, and the deposits are quite bright when the baths are operated within specification and maintained routinely. Of course, as in all plating processes, the pre-treatment of the parts is very important to the success of the plating. The Hull Cell is a valuable tool for monitoring ANCZ baths. The plater should become familiar with the Hull Cell methods for controlling the ANCZ bath. Keeping rinses clean and free from impurities being dragged into the plating bath is very important. Post plate rinses and chromates are also important to maintaining a consistent and high quality finish.

Alkaline non-cyanide zinc plating processes are very popular today and offer many benefits to the plater that will afford them multiple opportunities to expand their business and maximize revenues.