Issue link: http://www.e-digitaleditions.com/i/862794
Removing laser oxide scale from steel parts PROBLEM We're having a problem with micro-pinholing in our powder coated finish from laser scale. Our pre- treatment process is as follows: —Stage one: Sulfuric acid for laser oxide removal —Stage two: Rinse —Stage three: Iron phosphate —Stage four: Rinse —Stage five: Nonchrome seal The parts we're having issues with are for tractors. We're seeing acid boil out from carryover on these large steel parts. The issue is primarily on the inside of openings where rinsing is at a mini- mum. The dried acid residue causes ad- hesion failure with the powder coating. This is a major issue as we now have re- jects going back more than a year. The problem has always been there; we've just now discovered the rejects. We're entertaining the idea of two ad- ditional stages, but we're not sure if that will solve the issue. Carryover and boil-out of the acid seems to be at the root of adhesion loss. Any ideas on how to combat this problem? M.B., Salina, Kans. SOLUTION Laser scale is one of the toughest issues for finishers. I've stated many times that mechanical removal is the best route. Why should you over- activate 98 percent of the surface area of hot pickled and oiled (HPO) steel when the scale is on 2 percent of the surface area? Acid descalers interfere with high-quality post-phosphatizing. Most of the phosphate is streaky and powdery, and the over-activation cre- ates flash rust because the pH is too low, which instantly creates excessive powder instead of phosphate. Finishers get talked into the acid bath by aggressive chemical sales people, and hardly anyone puts in the ade- quate number of stages to do it cor- Brad B. Gruss Pretreatment & Process Consulting LLC BEST OF PRETREATMENT PROBLEM SOLVER POWDER COATING, August 2017 27 Take advantage of early registration discounts and free access to the show floor. pbepowdershow.com The Midwest Powder Show TM is the only event in 2017 dedicated to the powder and bulk industry. Unique learning. Exceptional value. Register now to attend the only event in 2017 offering an interactive look at the top brands, the ability to compare products side-by-side and get answers straight from the manufacturers. Visit with top suppliers on our show floor. You will discover the latest in powder bulk solids services and technology covering every stage of the manufacturing cycle from processing to storage. Enrich your knowledge. Get the latest updates in dust collection and regulatory changes that help keep your company operating safely and competitively while either attend ing our top industry leading educational program or walking our show floor. September 27-29, 2017 Greater Columbus Convention Center Columbus, Ohio