Issue link: https://www.e-digitaleditions.com/i/941907
34 POWDER COATING, February 2018 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. Evaluating pretreatment options PROBLEM Our product is the bar on a chain saw. We stamp the geometry, spot-weld components, induction- heat-treat wear surfaces, pretreat with phosphoric acid, then powder paint with epoxy. The pretreatment consists of phosphoric acid immersion, followed by two independent water rinses. Our immersion tanks are leaking and obviously need replacing. This is an op- portune time for us to evaluate the op- tions for pretreatment. Can I also con- sider mechanical (sand blast or wire brush) or no pretreatment at all? I don't know the risks to the powder paint quality. R.O., Portland, Ore. SOLUTION I'm trying to picture your raw part in my mind from your descrip- tion. It probably has heat scale from heat treat, possibly burnt on oil from spot welding and some discoloration. The phosphoric probably has worked well as you're dealing more with inorganic soils than organic (oil/soil issues). Did your old system work for you? Did you have any or many field complaints on quality or adhesion or premature rusting? If not, let's assume your old system pro- vided the quality necessary to satisfy your marketplace. If you had poor results, we could certainly expand your system into a higher quality process. Another question, did your process meet your production demands? I would really start there. Now to the basics. Powder generally re- quires a clean, oil-free, dry part to get powder to adhere, melt, and flow with- out any imperfections in the coating. So, I would be against recommending no pretreatment of any type. I've never had a bar that rusted, but the coating at least around the edges goes fast from abrasion. I would do some testing on pretreatment and powder chemistry. 1. Have several bars "blush-blasted," mechanically sanded, or steel shot blasted with a profile of no more than 2 mils. Immediately coat and cure with no time lag between the light blast and coating operations. 2. Have several bars run under your cur- rent system of acid pickle, but replace the last rinse with DI or RO water. For a test, you could secure demineralized water at a local grocery or hardware store. Dry, coat, and cure immediately. 3. Contact your current chemical sup- plier and ask for a technology called DIP, or dry-in-place, non-phos- phate conversion coatings. If the supplier has this, run an alkaline cleaner in one, rinse in two (requires a good rinse), and DIP in three. Dry under 300°F, coat, and cure. Measure the final powder thickness of the different pretreatment processes, and select two bars from each set that have the same powder coating thick- ness. Send off to an independent lab for salt-spray testing. Anything more than a couple of hundred hours for your type of part would be adequate (in my opin- ion, if you don't have a current spec). These three items cover the chemical or nonchemical technology, but we need to address your production demands. Should you be on the low side of meet- ing production numbers, I would con- sider contacting a turnkey finishing sys- tem house and getting a bid on a three- stage monorail spray system. In that testing, I would bump it to four stages (adding a second rinse) and also test the DIP technology. Should your parts re- ally be loaded with scale and inorganics, you may have to look at expanding the process to include an acid clean as well. Hope this helps. If I'm way off base, you'll need to provide more informa- tion. Good luck and do some testing! Adhering powder to aluminum castings PROBLEM We have a three-stage pre- treatment process: phosphate in the first stage, clean water rinse in the second stage, and nonchromic sealer in the third stage. We just dumped the tanks. Our pretreatment process temperature is 120°F; phosphate bath concentration is 5 to 6 ounces per gallon (6 to 8 drops solution turns from colorless to pink). Our pH range is from 5.5 to 5.7. We receive 6-inch bell shells that are alu- minum castings from China, and we just ran into an adhesion problem. Dur- ing the crosshatch test, the powder just flakes off the shell. Because the mold is old, the people in China have to grind and sand the part to remove excess ma- terial, which poured out onto the part. So, the parts come in with scratch marks. We tried running the parts through the pretreatment process twice, but it didn't solve the adhesion problem. We'd appreciate any advice and help. Thank you. S.C., Long Branch, N.J. SOLUTION You indicate you have a three-stage process. Many of the chem- ical companies have cleaner phosphates Brad B. Gruss Pretreatment & Process Consulting LLC BEST OF PRETREATMENT PROBLEM SOLVER