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Zinc Anodizing

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Q. What is zinc anodizing? Is it similar to aluminum anodizing? Is the process widely used? –F.S.

A. If you were a purist you would say no, but if you are a pragmatist, the answer is yes. A purist will tell you that a true anodized coating consists of essentially 100 percent oxides of the metals. Aluminum is by far the most common example of this. The so-called zinc anodized surface really consists of a mixture of oxides, phosphates and chromates. The zinc anodizing process uses alternating current as opposed to direct current, and again, most but not all aluminum anodizing processes use direct current. I will go with the pragmatists and call it zinc anodizing. A review of the history of the zinc anodizing process supports this position.

Here are a few details of the zinc anodizing process: The process was introduced commercially approximately 50 years ago under the trade name Iridize. It has not been a screaming success in the marketplace. I was aware of one company in the U.S. ten years ago. I do not know for sure if the company is still around. The process gives coatings that have excellent corrosion resistance, but because of the cost involved and the required equipment, it is not widely used.

Four different colors are available: green, gray, charcoal and brown. The commercial process requires a three-phase power supply with a variable output of 50 - 250 volts AC. The baths are run at a temperature of 150 – 180°F and at a current density of 35 – 45 ASF.

A military specification was issued for zinc anodizing, MIL-A-81801, “Anodic Coatings for Zinc and Zinc Alloys,” but it was cancelled in the mid-‘90s.

I should also point out that, besides aluminum, magnesium, titanium and zinc, other metals have been “anodized.” These include beryllium, tin, copper, germanium and cadmium. In most cases, the methods used are strictly experimental/ laboratory methods.

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