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40 POWDER COATING, June 2016 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 Consulting LLC. Brad specialized in training, trouble- shooting, 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. Adhering powder coating to aluminum Problem We've been experiencing problems with powder coating adhe- sion on aluminum material. We've raised our temperature in stage one to 140°F and have reduced our pH level to 4.6 in stage three at the recommen- dation of our chemical company. The aluminum that we use has oxidation. This is the grade of material that we use for our product. We run our oven temperature at 390°F. Do you have any recommendations? P.S., Green- wood, S.C. Solution Thanks for your question regarding aluminum pretreatment and poor adhesion. You mention that you have raised your temperature in stage one to 140°F and dropped your pH in the phosphate stage to 4.6. That sounds like a good direction but may not be good enough. Not so long ago, I wrote a two-part ar- ticle for the magazine titled "Alu- minum pretreatment: Old issues and new developments." You can find both parts by searching the online Ar- ticle Index at www.pcoating.com. In the article, I spell out everything you need to know. The key to aluminum pretreatment in- volves three factors: 1. The part must be cleaned or free of organics. 2. The part must be chemically or mechani- cally deoxidized or etched. 3. The part must go through a final rinse of high- quality water (deionized or reverse os- mosis). Those are the basic mini- mums. From there, you can add a high-quality conversion coating specif- ically designed for nonferrous metals. You don't mention this, but my guess is you're running both steel and alu- minum in the same line. It's impossi- ble to maximize pretreatment for both metal substrates in the same five-stage washer because your chemical choices will be more suited for either steel or aluminum. You also mention that you have severe oxidation, or corrosion, on the aluminum. If this is the case before cleaning, you may be forced to me- chanically deoxidize by blasting or sanding (DA or Timesave sanders). A better choice would be to determine why the substrate is in poor condition before pretreatment and then correct the incoming problem at its root. I hope this helps you to get started on improving the situation. Coating a magnesium casting for the automotive industry Problem We're looking to coat a magnesium casting for the automotive industry. We tried zinc phosphate plus e-coat plus powder coating, but got really poor adhesion and salt-spray test results. We heard a little bit about a specific company's chromate-based conversion coating to use instead of zinc phosphate. Is that the right one to use? M.O., Notre Dame Du Bon Conseil, Que. Solution Thanks for your question. Effectively treating magnesium seems to be very easy for some and very diffi- cult for others. You mention a company's chromate- based conversion coating process. I'm most familiar with that company's dichromate pretreatment, which is ex- cellent. Automotive requirements will specifically spell out the process you need to use in many cases. If they don't, then you'll at least be required to meet a salt-spray specification depend- ing on what the part is and where it goes on the automobile. Many custom coaters treat magnesium in a five-stage process that uses an alka- line ferrous type cleaner and a molyb- date accelerated iron phosphate. The pH control becomes important so as not to overreact or smut the magne- sium. I suggest you revisit your customer and the specifications the contract spells out before making any radical changes to the process. You may greatly im- prove the current part failure and not satisfy the requirements for the con- tract you have. Tweaking pretreatment for 100 percent powder coating coverage Problem We have a five-stage phos- phate washer system. Currently, we're run n in g our c hem icals a t a p H between 5.8 and 6.0, and total acid (TA) between 6.0 and 6.5. We're hav- ing some different parts coming into the powder-coat room that don't look 100 percent coated. Should we raise our TA up to 7.5? What effect could we have by running the TA at a higher level? Thanks for all the help now and in the past. K.T., Spring Lake, Mich. Brad B. Gruss Pretreatment & Process Consulting LLC BEST OF PRETREATMENT PROBLEM SOLVER