Powder Coating

PC0818

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30 POWDER COATING, August 2018 By popular demand, we're bringing you a series of past pretreatment problem columns, representing the best of Brad B. Gruss, our colum- nist for more than 20 years, who has now retired as president of the con- sulting firm, Pretreatment & Pro- cess Consulting LLC. Brad special- ized in training, troubleshooting, and independent line audits for pretreatment processes. In his more than 30 years in the industry, he made numerous presentations on pretreatment for powder coating. Eliminating field failures with coated stainless steel Problem We've been powder coating stainless steel and mild steel products for more than 15 years. We have very few field problems with adhesion on mild steel, but several adhesion issues with stainless steel. We use an iron phosphate soap with manual application heated at 170°F to 200°F. ere is no visible separation during the rinse process. It appears to be very clean, but after being exposed to the weather for a time, we have some flaking off. We also scuff the part, but this still doesn't help very much. We use a urethane-based poly- ester top coat and an epoxy primer. Our soap is formulated to go down the drain, so any alternative would have to be very user-friendly. Is there anything else we can do? What would be a reasonable time to expect the paint to last before seeing field problems—one, two, three, or more years? I know stainless is tough to deal with, so any suggestions would be greatly appreciated. J.D., Bristol, Va. Solution Powder coating stainless steel is easy; obtaining great adhesion for numerous years of outdoor expo- sure is the difficult part. Based on the information you pro- vided, you should increase the me- chanical abrasion properties you de- scribe as scuffing. e reason I say that is an iron cleaner phosphate is offering no more than degreasing of the stain- less steel. An iron phosphate won't conversion coat (iron phosphatize) metal that has no iron in the alloy. In- creasing the mechanical profile of the substrate will provide more tooth or bonding sites. You did mention a rinse after the wand phosphate. The best rinse is deionized (DI) or reverse os- mosis (RO). Try an increased quality final DI or RO rinse with a total dis- solved solids (TDS) of no more than 10 to 25 micromhos. In summary, sand, glass bead, or stain- less steel shot blast to provide profile. en, do a good degreasing and oil re- moval, followed by a high-quality rinse and dry. Apply the epoxy primer and top coat, and then cure. Hope this helps you. Peeling powder coating from aluminum profiles Problem We're powder coaters here in Croatia. We have pretreatment in nine baths-dipping. Our problem is peeling of paint from aluminum pro- files. We're using what our supplier recommends. The oven is working perfectly. We tried some other prod- ucts from our supplier, but the result was the same. e paint is peeling, not on all the profiles, but we can find 10-15 profiles every day. We're work- ing 5,000 kilograms per day. Please, do you have some opinion about this problem? S.S., Donja Zelina, Croatia Solution I'm having a little diffi- culty with communication here, but we'll try to offer some direction. It sounds as if you've done the right thing by trying to work with your chemical supplier. What do they say is the problem? What is their follow-up from the last meeting when you tried other products or parameters? I'll for- ward this to the US branch of your supplier to see if they can assist from here as well. When you only have a few parts fail- ing with paint peeling, it's either sub- strate specific or process-control re- lated. Start off by conducting a sub- strate audit. Track the aluminum ma- terial back to the supplier to ensure that there are no differences from batch to batch or lot to lot of incom- ing substrate. Look for minor differ- ences. Look for existing corrosion or oxide, excessive die scratches, or ex- truding blemishes. Conduct a soil audit. Find out what oil or lubricant may be on the substrate. Determine if any soils remain on the parts for exces- sively longer intervals than whatever normal is. Ask the following ques- tions. What's the common lag time from manufacture of the substrate to your receipt of the material? Where's it stored? Under what conditions? How long do you store in house? Do you add any additional soils, lubri- cants, or processing solutions? Concerning process, what's the exact r e c o m m e n d e d p r o c e s s f r o m t h e chemical manufacturer? Do you fol- low it exactly the same each time? Does your line stop for any reason? Line stops plague many finishers, but Brad B. Gruss Pretreatment & Process Consulting LLC BEST OF PRETREATMENT PROBLEM SOLVER

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