Summary for low resistance black coating Discussion
Low resistance inexpensive black coating on aluminum was the goal, but the thread suggests chromate conversion is only a partial fit. Michael D. Miles, PE asked for a black, electrically conductive aluminum finish that would be cheaper than black nickel, zinc, or chrome over copper/nickel, and wanted process parameters, cosmetic expectations, and whether local sheet metal shops could handle it. The reply points to MIL-C-5541 for chromate conversion coatings on aluminum and says to specify Class 3 for low electrical resistance. However, the answer warns that Class 3 gets its lower resistance by using a thinner coating, which reduces corrosion resistance and likely limits how dark the finish can be. The responder says a proprietary dark tint might exist, but would be surprised if it matched the saturation of black electroplate or black anodize. No other specific process or source is endorsed in the excerpt.
Q. I am a consulting design engineer (Mechanical) in the electronics packaging industry. In a recent project I encountered a need for a black coating on aluminum that was electrically conductive. Alternatives such as Black Nickel, Zinc, and Chrome over a copper and/or nickel flash have been discussed however they are cost prohibitive. Recently a black (or very dark) chromate tint was proposed. Can anyone suggest parameters or specifications for the process and sources for the tint? What cosmetic quality can be expected? Assuming it is (just) a bath change for the tint, would it be a major hurdle for local sheet metal fabricators to supply or is it best done by another source? Are there other suggestions of materials and processes to pursue? Thank you for your consideration.
Michael D. Miles, PE
A. Hi, Michael. MIL-C-5541covers chromate conversion coatings on Aluminum. For low electrical resistance, specify Class 3. This class achieves its lower rating via a thinner coating–which naturally sacrifices some corrosion resistance, and probably some of the saturation required for a dark coating.
There may be a proprietary that offers a shade dark enough to meet your needs, but I’d be surprised to find anything even approaching the saturation of a black electroplate or black anodize coating. Good luck.
Ted Mooney, P.E.
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