Issue link: https://www.e-digitaleditions.com/i/665686
POWDER COATING, April 2016 17 By popular demand, we're bringing you a series of past pretreatment problem columns, representing the best of Brad B. Gruss, our columnist for more than 20 years, who has retired as president of the consulting firm, Pretreatment & Process Con- sulting LLC. Brad specialized in training, troubleshooting, and inde- pendent line audits for pretreatment processes. In his more than 30 years in the industry, he made numerous presentations on pretreatment for powder coating. Correcting adhesion failure from acid residue Problem We're having problems with a pretreatment system. Our com- pany's current process is as follows: Stage 1: sulfuric acid (laser oxide removal); Stage 2: rinse; Stage 3: iron phosphate; Stage 4: rinse; and Stage 5: nonchrome seal. We are having issues with large parts for industrial machinery. Acid is boil- ing out from carryover on these large parts, primarily from the inside of the open parts where rinsing is at a mini- mum. The dried-on acid residue causes adhesion failure with the pow- der coating. This is a major issue for us because we now have rejects going back more than a year. The problem has always been there; we just now dis- covered the rejects. First, do you have some ideas as to how we could set up the five-stage system to do a better job? Second, we are think- ing about adding two additional stages. I met with the equipment folks today. They, of course, would put in the stages; however, they aren't sure if it will solve the issue. Oil carryover and acid boil-out seem to be at the root of the adhesion loss. Any ideas about how to combat this? I have already sug- gested manually removing the laser oxide. Thanks for your input. M.B., Kent, Wash. Solution Laser scale is one of the toughest issues for finishers. Since the beginning, I've stated many times that mechanical removal is the best route. The reason? Why should you over- activate 98 percent of the surface area of hot-rolled pickled and oiled (HPO) steel when the scale is on 2 percent of the surface area? Acid descalers inter- fere with high-quality post-phosphatiz- ing. Most of the phosphate is streaky and powdery, and the over-activation creates flash rust because the pH is too low, which instantly creates excessive powder instead of phosphate. The problem is finishers get talked into the acid bath by aggressive chemical sales people, and no one puts in the ad- equate number of stages to do it cor- rectly. The acid source must be neu- tralized before the phosphate deposi- Brad B. Gruss Pretreatment & Process Consulting LLC BEST OF PRETREATMENT PROBLEM SOLVER