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Blackening aluminum without paint

Q. I need to make a bunch of aluminum, self-locking washers look black. It doesn't need to be a pristine finish, just look dark from ten to fifteen feet away. It does need to be a permanent and weather resistant finish as the washers will be installed on rooftops, exposed to sun, rain, snow, etc. A ten year life to the finish is the expectation.

I am not considering paint for several reasons. It isn't all that durable, may chip or fade, and takes a lot of time and space to prep (the washers need to be "right side up" and I'll be treating lots of 5000 or more). I'm hoping there is a chemical bath that I can do with five gallon buckets.

This is an adaptive use of an HVAC product for residential solar applications. I am currently using Plastikot rubberized spray finish. I also recently found Rustoleum Leak Seal which seems similar. I'm hoping that these finishes will last longer than paint, but don't know what to expect. Again, just like spray paint, they are impractical for small parts in large quantities.

If the product is popular, I will source stock material that is anodized from the start, before the stamping takes place. For now, I need to be able to modify what's readily available.

Thom Westergren
- Denver, Colorado, USA
January 27, 2012

Personally, I would never stamp anodized aluminum where you have a concern about looks and lifespan because you will have chipping of the anodize as well as the non-anodized edges.

James Watts
- Navarre, Florida
January 31, 2012

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If it does not have to be jewelry, then chromate and dye the chromate. It will take some lab experimenting to get the right pH and temperature, but, black from 15 feet away is easy.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

February 1, 2012