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Black coating on steel insert of aluminum part that was Iridited / Chem filmed / Chromated




Q. Recently a local sheet metal shop told me that they had to use stainless steel inserts (instead of zinc plated steel) in my aluminum part because they could not press the inserts in after bending and since the part was going through a clear Iridite bath an insert with a zinc finish would get damaged. I don't want to use stainless inserts, because it is necessary that I use stainless screws (I don't want any galling to occur). Another vendor sampled the original part as per the original design and indeed the zinc plated inserts came back with a black coating on them and thread looks like it is rusted. What is happening to the zinc plated steel inserts when they are run through a clear Iridite bath (designed for aluminum finishing)?

Jesse Moon
designer - Ottawa, Canada
July 28, 2009


A. Let's see. The first plater said that the Iridite process would damage the inserts and you did not believe him, so you sent them to a plater that would try. They came back damaged, just like plater 1 said. If you do not believe what you are told, why will you believe me or any other plater, when I say that the Iridite solution is a moderately strong acid solution, and zinc reacts extremely fast with acids. Also, the zinc plating is very thin, so it is stripped quite rapidly leaving bare steel that has been activated and thus rusts easily.
You do not want it to gall, so use an "anti gall" material on the bolts. Any heavy lubricant or wax or formal product will work. Or, use aluminum bolts.

James Watts
- Navarre, Florida
July 30, 2009




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