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Zincating of aluminum: single or double and why?
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On the subject of - Zincating aluminum . As a process engineer for a relatively new plating company, I am always looking to prove why certain methods are incorporated into certain processing of base metals. Aluminum is one of which requires Zincating of the surface, and while that's not hard to get platers to understand , the "why" of zincating, what is most difficult is to get them to understand the why of "double" zincating and the benefits. Yet while I stand at the aluminum pre-plate line , I see single zincates more than I do doubles, I yearn to get some one to back me on the reasons for this double zincate- VS- single.
I am responsible for corrective actions on rejects and thus I answer them as to blistering/poor adhesion off aluminum, with " improper surface treatment" We plate EN direct on aluminum, and also live entry cyanide copper strike. So here is my request. HELP me explain better the benefits of double -VS- Single Zincate on aluminum. And any other input you can give on the processing of cast alum.
Thank you much.
todd Huehn- Blaine, Minnesota
First of two simultaneous responses--
-Hi todd.
When you zincate aluminium, well, aluminium and zincate change each other. The first etching in zincate is to build up etch structure in the aluminium area only for pure aluminium not cast aluminium. You remove this zincate layer in hydrofluoric acid or nitric acid and etch in the zincate for a second time about 20 sec; this zincate has a hexagonal compounding with the aluminium substrate. If you do this for example on cast aluminium, the aluminium area will then get a high contamination in the area because the area is too much to etch. This contamination can be hard to remove and can also destroy the EN bath. Most Lab reports will show you this. The zincate compound is harder when you double zincate. I hope this will help you. I have really tried to translate this into English.
Regards,
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Anders Sundman surface finishing engineer Sweden |
Second of two simultaneous responses--
-Todd, the shortest answer I can give is this: The difference between the single vs. double zincate is that the first film is rather thick and spongy. You can see this by the dark color on the parts. As you strip the first coating off and re-apply it you see a much different appearance. This is because the second coating is thinner and more dense than the first. The zincate strip doesn't take the first coating off entirely, thereby providing a good base for the second coat. The differences between parts plated with one zincate dip vs. two are usually better adhesion due to the E.N. or copper strike bonding much better to the part. A single zincate film will tend to dissolve more quickly in a plating bath before the initial coating is formed. Sometimes this leads to blistering and peeling of the coating when stressed or heated.
Jim Conner
- Salisbury, Maryland