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November 11 - November 15, 2013

I have galvanized steel in direct contact with powder-coated aluminum in an outdoor exposure. The galvanized steel is corroding. What is the best way to remedy the situation?

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Selected Answers

From Robert Cameron of Belzona Houston on November 22, 2013:
There are two-part epoxies available that are tolerant to surface contaminants. I would remove loose contaminants with hand tools and even refer to SSPC-SP 11, Power Tool Cleaning to Bare Metal, if possible on the galvanized section (no need to knock off the galvanized layer if it is tightly adhered). Then mask the powder-coated area, and apply two coats of this two-part, surface- tolerant epoxy.

From Karen Fischer of Amstar of Western New York, Inc. on November 19, 2013:
Though there isn't much information here as to what kind of structure this might be and how the two materials are connected (fixed or movable), my first inclination would be to isolate the two dissimilar metals with a neutral, non-reactive material of some kind to eliminate the direct contact between these two metals. Clearly, the galvanized metal is reacting with the powder-coated material. I don't know what kind of structure this is, but some type of rubber or plastic spacer may be enough to stop this reaction. Painting of the galvanized metal may not provide sufficient isolation, especially if the connection is movable. Repair and re-galvanizing the corroded area can then be accomplished.

From Carl Thompson of Hill Brothers on November 11, 2013:
I would tape the powder-coated aluminum where it meets the steel. Aluminum is amphoteric and can be attacked by either acid or alkali. Treat the galvanized steel with a 20% solution of phosphoric acid. Wait 15 minutes, then wipe the rust and corrosion away. If some corrosion remains, repeat the process. When cleaned, neutralize the galvanized steel with a very dilute solution of ammonia. You should see a white residue on the surface . This is an oxide of phosphoric and should help keep further corrosion away. Any holes developed from corrosion should be filled with a metallic, 2-part epoxy putty. If painting is desired, brush away any loose white powder with a stiff copper brush and proceed. To prevent any galvanic reaction between the aluminum and the stainless steel, painting the galvanized metal would be preferred.

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Tagged categories: Aluminum; Corrosion; Galvanized steel

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