Picking the Correct Pretreatment Before Powder Coating
Powder coating expert Rodger Talbert explains to a user when they should do a chemical treatment, a blast or both to effectively pretreat.
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.
Q: Our parts have some sharp edges in the assembly and we have had some problems with corrosion due to light coverage in these areas. We work with our spray operators to get them to work harder at coverage in these critical areas, but we still have some rejects due to light coverage and we get some returns due to corrosion. Can you give us any tips on how to get better coverage in these areas?
A: Sharper edges in a product are difficult to cover because there is really no surface there to cover. The coating flows to the edge, but leaves a line of very thin coating. Some coatings will cover better than others, but all will leave less film build at the edge than on the flat area of the part. The options to get better coverage are to change powder, change the part surface or add two layers of coating.
Talk to your powder supplier to make sure you are using a product that has flow properties that will give you best possible coverage at the edge. This is not a perfect answer, but it can help. You can grind that sharp edge to create some radius and get much better coverage because the radius adds surface area that can be adequately coated. If that labor sounds like a problem (usually is) then you have to go to double coating. A second layer of coating is really the best way to confidently beat the edge coverage issue. The first layer flows to the edges and creates some radius. The second layer flows over that radius and gives you real coverage. If corrosion is the problem, as you describe, I would use an epoxy primer for the first layer to provide some additional resistance to moisture penetration.
Discoloration of Metallic Powder Color
Q: We make outdoor gates and other rail products, and have a powder coat finish for our railing products that has been discoloring after installation. Most of these metallic colors are silver or gold. The finish has been turning a darker color and rubs off after a while in the areas that are normally touched by people walking along the installation space. Should we look for a different supplier of these coatings?
A: Metallic powders contain aluminum or mica flakes to provide the striking, shining appearance that is enjoyed by many consumers. These coatings are typically not stable when exposed to sunlight and they can also be sensitive to handling damage or abrasion. Most suppliers of these products, along with the Powder Coating Institute and reputable experts in the industry, recommend the application of a clearcoat powder over the base coat. Talk to your powder supplier about this and their options for a suitable polyester clearcoat that is compatible with your metallic colors. To use the clearcoat, you will first apply the base and partially cure it (about 50-60%). Then apply the clear and cure the two coats together. Try some samples and be careful not to apply too heavy a coat with the clear to avoid loss of clarity of the base color. This will make your product perform better in sunlight and resistant to contact damage. Be aware that all organic coatings, even with a clearcoat over them, will lose some color and gloss over time. However, you should have much better results with these finishes after applying the clearcoat. The clearcoat will help protect the metallic coating and also give you an additional layer for better longevity of the finish.
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