Painting Iron Castings
We are having paint adhesion failures on the centrifuges’ rotating hub, which is made of iron. Our customers have been complaining that after a couple of weeks of operation, the paint peels off the hubs and then rust takes over. What can we do to correct this problem?
Q. Our company manufactures process machinery and centrifuges, among other items. We are having paint adhesion failures on the centrifuges’ rotating hub, which is made of iron. After machining, the hubs are dipped in a tank of toluene to remove the machining lubricant, coated with a catalyzed two-component epoxy primer and then painted with a two-component epoxy enamel.
The environment where these machines are used is often hot and humid, with temperatures sometimes reaching 160ºF. Our customers have been complaining that after a couple of weeks of operation, the paint peels off the hubs and then rust takes over. What can we do to correct this problem? V.M.
A. Improper surface preparation is the greatest single cause of paint adhesion failures. Although you’ve chosen an excellent primer for this application, its adhesion, as well as that of any other coating, depends on the cleanliness of the substrate.
Yours is a textbook case, because when you clean by solvent immersion, the oily soils removed from the part become dissolved in the solvent. Unless you change the solvent in the cleaning tank often, your parts are immersed in a blend of solvent and oil. After you remove the part from the cleaning tank, the oil-contaminated solvent evaporates, leaving a thin film of oil on the surface.
A better approach for cleaning the castings would be an alkaline cleaner. Or even better, following it with a zinc or iron phosphate pretreatment applied by immersion or power spray. As an alternative, using a steam spray cleaner/phosphatizer will provide an excellent surface for painting. Furthermore, using any the aforementioned aqueous surface pretreatments is safer than using a solvent tank for machining oil removal.
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