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16 POWDER COATING, April 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 re- tired 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. Preventing flash rust PROBLEM We're experiencing flash rusting between our wash stages at our break time, usually 10- to 15-minute breaks. We have a five-stage pretreat- ment system. We've been trying some different control for our chemistry. When we started out, we had a wash, a rinse, another wash, and two more rinse stages. The bath temp for our wash stages was 120°F. The first change we made was to lower our bath temp to 110°F. We also changed our stages and now have a wash followed by two rinse stages. We used to run our pH at 5.9 to 6.1 (normally stayed in that area) and our total acid at 7.5 to 8.5. We made some changes to those and are now running our pH at 5.8. We start out with a clean bath with a total acid (TA) of 4.0, and we raise that as we need to achieve a water-break-free part. We noticed that when we did our first tank cleaning, there was a lot of sludge. Some of it was so hard that we had to break it up with a shovel. We did some testing with the hard sludge and found that heat seemed to dissolve it. Our TA may go as high as 20.0 before we sched- ule another cleaning. We're going to try to hold our pH at 5.8 because we feel that the rust and sludge may be from a low pH. We just recently turned our temp up on our first bath to 125°F, which also will heat our third bath to 115°F (hoping this will also help dissolve the sludge). Well, I will end there, and ask if you see any problems with what we've done or see something else we should try to reach our goal to lower costs and still achieve clean parts with no flash rust- ing. Thanks for any help you may have. K.T., Spring Lake, Mich. SOLUTION When I read your ques- tions and comments I came away with two primary questions. 1. Why do we get flash rust? 2. W h a t d o w e d o t o p r e v e n t t h e sludge? Flash rust. Generally, flash rust can be caused by good cleaning and poor phosphate deposition, or good cleaning and allowing parts to begin drying be- fore phosphate application, in your case, the potential for lowering the ac- tivity of the bath or temperature. Cleaner iron phosphates and cleaner coaters are unique in that cleaning must take place first before the acidity, and accelerators can effectively begin the phosphate reaction with the steel. The most important part of any cleaner phosphate (in my opinion) is to have a good surfactant and wetting system ca- pable of removing or displacing any or- ganic soils on the metal. Once that bar- rier of soil is removed, the acidity and key components of the chemical can begin to slightly etch the metal and cre- ate the pH rise at the solution metal inter-phase and allow the iron conver- sion to take place. The important variables after achieving a clean surface are: a) Acidity source regarding TA/FA (free acid) b) Accelerator c) Temperature d) Contact time In your case, the lowering of the tem- perature may have reduced the activity of the conversion coating to the degree that you achieved the minute etching but not sufficient activity to deposit the phosphate. When that occurs, you have a slightly active surface with no passivation afforded by the conversion deposition, and thus, you have a very reactive piece of metal ready to oxidize or flash rust. If I understand your situation correctly, the break time of allowing parts to be in the washer in moist conditions would also promote flash rust. The best way to prevent substandard parts caused by breaks or shutdowns is to stagger your employee breaks to allow continuous production, or don't hang parts on the line if they won't make a continuous run through the process. In other words, leave a break in the line equal to the line speed times the minutes of break time. You may want to work with your chem- ical supplier to determine how to best shift the variables listed previously to re- duce energy costs. This may include longer contact time, greater chemical concentration, or reaching optimum TA/FA settings for this particular chemistry. Keep in mind that cleaning Brad B. Gruss Pretreatment & Process Consulting LLC BEST OF PRETREATMENT PROBLEM SOLVER