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POWDER COATING, November 2016 27 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, troubleshooting, and independent line audits for pretreatment processes. In his more than 30 years in the industry, he made numerous presentations on pretreat- ment for powder coating. Tracking conductivity rather than pH and TDS Problem I recently did an audit of a powder coating line in Mexico. Most areas of this operation are quite nice. I did have some concerns regarding their data control. Of course everything was in Spanish, which is not my first lan- guage, so I had to kind of decipher what it all meant. From their data sheets, it was apparent that they do not control pH and total dissolved solids (TDS) but rather con- ductivity, which has a parameter of less than 2,300. I indicated to them that this was not common practice, and I would prefer they contact their chemi- cal supplier to create a pH level and also have a TDS of less than 100 parts per million (ppm). Do you know why they would be controlling conductivity? I also found a yellow film on raw cold- rolled steel parts after the dry-off stage. I was told that this was an iron phosphate film and that their chemical supplier had indicated that this enhanced the ad- hesion. I was under the impression that a film always created an issue with adhe- sion. They also indicated to me that the film was only apparent on raw steel and not on pre-plate such as electro-galva- nized material. Could you take a mo- ment and explain to me if this is a valid statement? T.B., Las Vegas, Nev. Solution To assist you properly, it would be good to know the number of stages and what chemical was in each active stage. I can't tell from what you wrote if you're only talking about the last stage or all stages. Generally, I'm not a fan of monitoring TDS in alkaline cleaning or phosphatiz- ing stages as you're measuring "anything that is dissolved in the bath." This could mean you are NOT measuring active chemical ingredients, so you really would have no idea of the concentration. In a phosphate or conversion, you also must either know or understand pH as pH or an acid source is critical to starting the phosphatizing reaction with the metal. The best way to do that is to titrate the solution against a known standard as a minimum and also check pH. Some- times, the amount of active chemical in the last or seal rinse stage is so low that it's almost impossible to measure with wet titration. In these cases, the measuring of TDS and pH together is typically used. The yellowish film you noted? This makes no sense to me and sounds like an excuse. Are you sure it wasn't flash rust? To run a little test, you could take a part off the line as it exits the last stage and manually air blow it dry. If you didn't see a yellowish film, then the parts after dry off probably exhibit flash rust. The only thing that might make sense is that it's not a phosphatizing material and is more of a transition metal conversion of some type. I think you need more information about the process and the particular chemistry in use for a better understand- ing of what truly is happening here. Pretreating aluminum Problem Can you give me a quick primer on pretreatment for aluminum? I have 5052 alloy plate and aluminum castings. B.V., Chicago, Ill. Solution Your request for a quick primer on an all-aluminum process can't be quickly done without more information. With that said, I offer the following. The three most important criteria are as follows: 1. Aluminum must be clean (water- break-free). 2. Aluminum must be deoxidized (etched). 3. Aluminum requires a high-quality final rinse (reverse osmosis [RO]; deionized [DI]). Here are some issues to be aware of with aluminum pretreatment: 1. 5052 is straightforward, and it's rel- atively easy to achieve good results, if you follow the above. 2. Aluminum castings are all alloy depen- dent when it comes to performance. 3. The hours of salt spray required will dictate the number of steps in pre- treatment. The good news is that the line will be all nonferrous. Serious problems arise when it's a combination of ferrous and nonferrous. I've said for years that it's impossible to maximize performance for both substrates in one line. The simple process would be (for 300 to 1,000 hours ASTM1 B-117): • Stage one: Alkaline clean and par- tially deoxidize (low hydroxide alka- line cleaner); caution on pH, concen- tration, temperature, and contact time to ensure that no or low smut development occurs on castings. Brad B. Gruss Pretreatment & Process Consulting LLC BEST OF PRETREATMENT PROBLEM SOLVER