Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
Doís and Doníts of Alkaline Non-Cyanide Zinc Plating
Formulas for cyanide free alkaline zinc plating solutions have been improved significantly since the early 60ís when cyanide came into disfavor for use in zinc plating solutions. Plating suppliers have produced formulas that avoid delayed blistering, fast build up of carbonates, poor throwing and covering power that were early problems. Low concentration and higher concentration cyanide free plating solutions are available, each with special characteristics both are better than the early formulas. Today I believe that these new formulations are better acting than the old cyanide containing formulas. The present formulas are available in low concentration and higher formulations. Both have certain advantages. Sodium hydroxide based formulas are the most common, but there are also potassium hydroxide formulas. Each has characteristics differing slightly from each other.
The constituents are: Low concentration High concentration
Sodium hydroxide 10-16 oz/gal 16-24 oz/gal
Zinc 0.7-1.2 " 0.8-3.0 "
A potassium formula, not widely used, has14-23 oz/gal potassium hydroxide with 0.7 to 1.2 oz/gallon zinc and a high concentration solution using 18-32 oz/gal potassium hydroxide with 0.8 to 2.2 oz/gal. zinc.
Proprietary additions agents are added for smoothing and brightening the deposits. Purifiers and fume suppressants are also commonly used. All of these work without causing delayed blistering. It is believed that delayed blistering was the result of stresses in the deposits due to impurities, and/or the type of brighteners used early on. Of course, proper pre-cleaning methods and preparation must be used to clean adequately to play a major role in prevention of blistering.
Water used to make up new solutions and water used prior to the plating tank must be DI, or water treated with water conditioning agents to remove hard water impurities. The rinse before the Zinc plating tank must use conditioned water. Use soft water or DI water. Water hardness causes dull zinc deposits. Adding more brightener or other addition agents leads to higher cost, and brittle deposits.
The raw materials used to make up and maintain must be as pure as possible. The zinc solution, if made by dissolving Zinc must use 99.99% pure zinc for the solution. The sodium hydroxide (caustic soda) must be "Rayon grade." Zinc oxide, if used to make up the plating solution must be as pure as possible, 80.3% zinc with less than 0.005 Cadmium, (Cd) and 0.002% , lead (Pb) .
The solutions are chellate-free and do not build carbonates as fast as earlier cyanide zinc plating solutions. Deposits are bright, and not as brittle as the older versions. There is no cyanide to waste treat. The plating solutions provide good plate distribution (throwing power and covering). Very good brightness can be achieved with good ductility. The columnar structure allows better paint, or powder coating adhesion then other zinc deposits. Rack marks are a problem for other zinc solutions. Alkaline non-cyanide solutions are less prone to rack marks. Should rack marks appear, test and raise the brighteners and/or increase the current density. And alter the anode area closer to the rack mark areas if possible.
Use proprietary addition agents
And follow the instructions including
The use of pure chemicals for make up an
Perform regular analysis and Hull tests.
Use high carbon mild steel for anode baskets.
Use polypropylene anode bags.
Use automatic feeders for addition agents.
Filter the solution 2+ turns/hour, 10-15 Micron.
Use unconditioned water for bath make up
or for the rinse before the plating bath.
Add chelates to the solutions.
Let the chemistry get out of balance.
Do not use titanium baskets for anode holders.
Use cotton bags. They dissolve in alkaline baths.
Let impurities accumulate in the zinc bath.
(Maximum impurities are: chrome 2 ppm;
Copper 20 ppm; Cadmium 1 ppm; Lead 1ppm;
and Tin 10 ppm.
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