Vacuum Deposition of Selenium

Question:

We vacuum deposit selenium on 3003 and 6063 aluminum alloy drums for copy machines. The drums must be exceedingly clean. We have tried a number of cleaning methods to promote good coating adhesion. These include a non-silicated alkaline cleaner, various etch solutions, including potassium hydroxide, sodium hydroxide,hydrofluoric acid, followed by nitric acid, phosphoric acid and other desmutters. We have even used a sulfuric-chromic mixture to clean the parts and desmut in one step. This worked great and so does 50% nitric acid, but we have some safety concerns with these. What can you recommend that is both safe and will provide the “tooth” necessary for good adhesion of the selenium? J.M.

Answer:

A gentleman I used to work with would say on a regular basis, “anodizing (metal finishing) is not a trouble-free business.” Many times there are no easy answers in metal finishing. Experience teaches that most processes are a compromise between cost, safety and the environment on the one hand and effectiveness of the process on the other. It becomes a matter of learning what you can live with within these bounds. All of the chemicals you mentioned can be pretty nasty, and they all must be handled carefully to prevent injury to personnel.

I have been a fan of the sulfuric acid-chromic acid mixture for many years. The problem is the chrome. Treatment requires reducing the hexavalent chrome to trivalent and precipitating it out of solution. However, this is a standard procedure and details can be found in just about any metal finishing waste treatment reference. There is no question that the sulfuric-chromic solution provides superior cleaning along with very mild etching of the aluminum surface. As you found, “it works great!” You might want to consider setting up to handle the waste from this bath and use it so you can sleep at night knowing you are getting a superior product. By the way, a good mixture is 1 gallon of concentrated H2SO4 and 45 oz of chromic acid in 9 gallons of water. Use a temperature from 140 to 180F and determine the time, up to about 5 minutes, that works best for you.