“We are having some problems when coating cast aluminum parts. These cast parts are usually decorative parts of handrail or fence systems. The problem with peeling is due to corrosion that starts under the casting. There are no peeling problems with the aluminum extrusions, just the castings that are on them. So far, these problems are only occurring in waterfront areas. I’m sure the salty air is the main cause, but are there any ideas how to coat these parts better? “
“Corrosion under the powder coating will cause the film to peel off the substrate. This corrosion is the direct result of improper cleaning and pretreatment of the raw parts before powder coating. If these casting are made from aluminum, then you will have aluminum oxide and salts on the surface that must be removed prior to coating. The approved method for pretreating powder coated aluminum parts that will be used in high corrosive outdoor environments (i.e. salt air) is as follows:
1. Clean the surface (organic soils) using and alkaline cleaner.
2. Rinse the surface.
3. Remove the inorganic (aluminum oxide) soils using a deoxidizer cleaner.
5. Apply a chromate conversion coating (Alodine).
In lieu of this aqueous method of pretreatment you can grit blast the surface to a white metal profile to remove all organic and inorganic soils before applying the powder coating. However, this methodology will not provide enhanced corrosion resistance as the Alodine system described above. “