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Ask the Expert Question-and-Answer Archive by Ron Joseph February, 2005 Galvanic Corrosion - Zinc and Aluminum Q. I have a group of sign cabinets which are constructed of sheet aluminum and within the structure, there are operable doors which are likewise constructed of sheet aluminum product. The doors are formed from this aluminum material with the product formed as a tube type section. It is then formed into a rectangular frame which is used to hold lexan windows in place. The manufacturer used galvanized steel "L" brackets inside of the corners of the frames so as to increase the rigidity of the door. My question relates to the brackets of galvanized steel coming in contact with the aluminum casing. Is the galvanizing within the industry standard for corrosion protection between these dissimilar metals? These signs units are being utilized in a very corrosive atmosphere, which was only supposed to be a high humidity (95% condensing) area. The corrosive element was a bit of an extra added attraction. Thanks for any assistance you can offer. A. Thanks for a great question. You are obviously well aware of the galvanic corrosion that can take place at the interface of the galvanized bracket and the aluminum frame. If I were in your position I would prevent the inter-metallic contact and not take any chances. After all, once your customers take delivery of the cabinets you don't want them to come back in a few months complaining of corrosion. On the other hand, you might not want to spend the money or impede your production rate by inserting an insulating layer between the contact surfaces. A simple way to determine the risk is to affix a few brackets to a corresponding number of aluminum samples and send the assemblies to a laboratory for exposure in a salt spray cabinet, and/or a humidity chamber. You should see the results within 96 hours of exposure (4 days). If the interfaces look good after that period you might decide to dispense with insulating the mating surfaces. Again, if I were in your position I would not want to take the risk, and in any case I would like to have some lab backup in case of corrosion problems down the road. You should be able to locate a suitable lab in your area, but if you would like me to assist you with the testing, please get back to me. Best wishes, Ron Joseph |