Removing Rust, Mill Scale And Oils In One Step

Q. We have a small job shop that powder coats. We do parts of varying sizes all the way up to 20 ft in length, made of steel or aluminum, occasional magnesium. The parts we receive, as stated before, are constantly different sizes and shape and some of it is made of HRS steel and has rust and corrosion on it, as well as oil. I always see the typical pretreatment stages to be a cleaner, rinse, phosphate, rinse, sealer, rinse or not. Nowhere do I see anything to deal with the rust and mill scale from the HRS steel. Is there a product that removes the rust, mill scale and oils in one step? Or are there more steps required, I can honestly say most everything I ever read about seems like a perfect world where the only thing the steel has on it is oils. This is not the case in my business. We have a manual spray washing system that we use due to our varied parts, but I can absolutely see the need for a dip tank being a useful tool as well for the smaller parts. J.C.

A. To address what you want to do generally requires acid cleaning of the surface for rust and scale removal. Another alternative is mechanical cleaning such as shot or bead blasting to remove contaminant prior to processing through your pretreatment and paint system. If using an acid, prior to the rust removal, the part should be adequately degreased such that the acid can uniformly remove the rust (otherwise it will be inhibited in the areas coated more heavily with oil).

You may be running into problems finding help from suppliers or others in the industry since the pretreatment and painting of hot rolled steel is not very common in the industry for all the reasons and problems you have described. Much of the industry would use a cold rolled steel in order to apply a pretreatment and powder coat to the surface. That would eliminate the problems you are experiencing. If you do a cost analysis of what you would need to do (capital, labor, and consumables), it is likely cheaper for you to go with the cold rolled steel in the long run rather than continue to try and make the hot rolled steel work in a process it was not designed for.