Zinc Plater Finds New Market

In early 1992, Elk Grove Plating Company, Elk Grove, Illinois, installed a trivalent chromium plating line to replace its hexavalent chromium line. Even though both lines served the same function, plating decorative finishes on office supplies, the switch was made because trivalent chromium was more environmentally friendly.

However, after several years of running this line, the Brosio family, owners of Elk Grove, noticed that the market for trivalent chromium was diminishing. The office supply parts that it was plating were manufactured in Southeast Asia and shipped to the U.S. to be plated. But, the office supply manufacturers soon found out that it was cheaper for them to plate the parts themselves. Because trivalent chromium was no longer economically viable, Elk Grove decided it was time to look for a new market.

After some research and discussion, the company found a new market for parts that needed a functional zinc coating with a chromate conversion coating. The parts came from a variety of industries, including the automotive, computer, hand tool and telecommunications. Elk Grove already had a chloride zinc line that provided clear decorative finishes. However, neither this line nor the trivalent chromium line could cost effectively produce the functional coatings required in the new market. "In order for us to achieve the desired plating thickness, it was necessary to decrease the rack population and slow down the chloride zinc line," said Michael Brosio, principal partner. "Also, the parts we were plating could not pass the minimum salt spray requirements."

Because the trivalent chromium line produced a single layer bright nickel and chromium finish, the line was not capable of meeting the neutral salt spray requirements. To meet the requirements for the new market, Elk Grove would have had to add new plating solutions. It also would have needed to rebuild to accommodate the new solutions. Therefore, Elk Grove changed its trivalent chromium line to an alkaline, non-cyanide zinc line.

In its initial search, Elk Grove looked at five baths. Because it felt comfortable working with OEMs who could provide local support, Elk Grove was able to narrow it down to two baths. Out of these two, it chose the Enthobrite^® NCZ Dimension^™ alkaline, non-cyanide zinc bath from Enthone-OMI, New Haven, Connecticut. Mr. Brosio stated, "We chose Enthone because of its integrity, reputation and the key people we know in the organization. We have had a long-standing relationship with them."

This zinc plating process displays properties that result in increased benefits to the user. Metal distribution is better than traditional alkaline non-cyanide zinc. A wide operating range for zinc, temperature and brightener additions make the bath a reliable solution. The deposits are bright and ductile over a broad current density range. The higher temperature range will reduce cooling costs. The process is designed to replace cyanide zinc plating without sacrificing the quality of the zinc deposit. Waste treatment is greatly simplified and requires only a simple pH adjustment to precipitate zinc metal.

Having chosen the new bath, Elk Grove had to convert its trivalent chromium line to the alkaline non-cyanide zinc line. The first step was to decontaminate the plating line. All of the pipes, pumps and tanks had to be completely flushed of trivalent chromium. Then the plating tank had to be rebuilt and enlarged. "The tank size was increased to maintain our rack production and achieve the desired plating thickness without slowing down the line," explained Mr. Brosio. The tank now has a capacity of 7,050 gal. After the tank was rebuilt, Elk Grove added an evacuation system to handle misting of sodium hydrate, which can irritate the skin and make breathing difficult, and a low-foam blanket in the bath. Although the misting did not cause any environmental problems, it did affect employee health and safety.

The entire process took six to eight months to complete. The only chemistry that needed to be changed was the zinc bath and the chromate bath, which was completed in one weekend. Elk Grove was able to keep the pretreatment system from the trivalent line. The bath is kept clean through continuous filtration and agitation.

The line has been up and running since November 1996. Currently, the line plates 6,750 sq ft per shift, with one shift per day. With only one shift, the line is operating at just 25 pct capacity. However, the Brosios have been looking for more work to increase that percentage. The chromate conversion coatings can be done in four colors: blue bright, yellow, black and olive drab. The bath plates at 3.5 to five mils on all parts.

The alkaline non-cyanide zinc line has several advantages over the chloride zinc line. Elk Grove Plating found that it could deposit 3.5 to five mils of zinc in a much shorter time on the new line. The new line also provides increased throughput because of the shorter plating times and increased rack-loading capabilities.

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Table I - Alkaline Non-Cyanide Zinc Plating Process
STEP	TEMPERATURE	TIME	CONCENTRATION
1. Alkaline Soak	185F	4 min	14-16 oz/gal free caustic
2. Rinse	slightly above ambient
3. Rinse	slightly above ambient
4. Reverse Electroclean	115F	2 min	10-12 oz/gal free caustic
5. Rinse	slightly above ambient
6. Reverse Electroclean	115F	2 min	10-12 oz/gal free caustic
7. Rinse	slightly above ambient
8. Acid Drip	ambient	3 min	17 oz sulfate-based acid salt
9. Rinse	slightly above ambient
10. Alkaline Zinc Plate	85F (14 pH)		2.3 oz/gal zinc metal 23 oz/gal sodium hydrate
11. Rinse	slightly above ambient
12. Chromate	90F
13. DI Rinse

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Table II - Potassium Cyanide Zinc Plating Process
Step	Temperature	Time	Concentration
1. Alkaline Soak	185F	4 min	14-16 oz/gal free caustic
2. Rinse	slightly above ambient
3. Rinse	slightly above ambient
4. Reverse Electroclean	115F	2 min	10-12 oz/gal free caustic
5. Rinse	slightly above ambient
6. Reverse Electroclean	115F	2 min	10-12 oz/gal free caustic
7. Rinse	slightly above ambient
8. Acid Drip	ambient	3 min	17 oz sulfate-based acid salt
9. Rinse	slightly above ambient
10. Potassium Chloride Zinc	80F (4.9 pH)		5.0 oz/gal zinc metal 21 oz/gal potassium chloride 3.5 oz/gal boric acid
11. Rinse	slightly above ambient
12. Chromate	90F
13. DI Rinse