Industrial Surface Chemistry

Picking the Correct Pretreatment Before Powder Coating

Q: We make component parts for agricultural equipment. Some of the parts are small brackets and some are very large parts with bolted components, welds and different metal thicknesses. We want to convert from liquid coating to powder coating and we are sorting through the different options for part preparation. At present, we wash all the parts in a three-stage washer and coat them with a baking enamel. We have some problems with rust around the welds and the edges of the parts, and we need to get better in these areas. We expect the powder to hold up better, but we know we need good surface to prevent the rust and get good adhesion. We have talked to chemical suppliers and they all suggest a multiple treatment process that includes cleaning and probably treatment with zirconium oxide. We also know that some other companies make similar products and currently do powder coat and use a blast operation. Should we do a chemical treatment, should we blast or should we do both?

A: You have a number of challenges that you need to address in order to get rid of the rust. For starters, you use a lot hot-rolled-steel (HRS), which has a lot of organic impurities such as rust inhibitors and a number of inorganic substances such as mill scale. So the soils that you have on your parts will challenge any simple approach to cleaning and treatment. In addition, you add some challenges when you cut, weld and assemble the parts. Your pretreatment approach needs to address all of these challenges to get the rust to a better place.

First, you will want a good chemical system that can remove the organic soils and treat the surface. An acid cleaner may be useful to help break down the mill scale and weld smut. Zirconium makes sense, if you have some aluminum parts. It also is useful because it does not create the sludge that is normal with iron or zinc phosphate solutions, keeping your nozzles clean and reducing maintenance.

Chemical acid solutions have been used for the removal of scale, rust and other inorganic substances, but I am not a strong supporter of that. Some solutions are strong acids and require fairly high heat to be effective, which is not something that is nice to have in the system. Abrasion works better and does not require a hazardous solution. Of course, abrasion has its own challenges. If you sand and grind by hand, you will need a lot of labor and it is not easy labor. If you set up a manual pressure blast system, you will have a person suited up in an enclosure working a difficult job with potential health hazards. You can set up an inline wheel blast system that will work well. Just keep in mind that wheel blast systems do wear over time and need maintenance to keep them in good working order. Still, a wheel blast system may be your best addition to take care of the rust and scale. Also, the cutting will produce laser scale on the edges that will cause adhesion failure, so that is something the blast system can deal with too.

So the answer is you should consider doing both — a multistage washer and a blast system. This is the most expensive approach and it will require a lot of space, but it will be reliable for dealing with all of your soils and getting rid of the rust.

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