Issue link: http://www.e-digitaleditions.com/i/799392
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 now 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. Salt-spray testing of powder coating over galvanized parts PROBLEM I've recently received some salt-spray results of powder coating over galvanized material that is incon- sistent. We ran two parts at the same time through the same conditions. One part had good results, and the other did not. I've seen some old arti- cles indicating that salt spray isn't a good test for galvanized material. Is it still the thought that salt-spray testing shouldn't be used as a method of eval- uating the corrosion resistance of pow- der coat over zinc coatings? What test, if any, does give a good evaluation? L.A., Shelbyville, Ky. SOLUTION Salt spray is a good test for determining performance of pow- der over a galvanized surface. Inconsis- tent results between two pieces run at the same time is perplexing if the con- ditions were indeed the same. There can always be some variation in salt spray from one piece to another. I've seen cases in which you may have 50 to 75 hours difference in the same set of parts or panels. Usually, there is a dif- ference. Are you sure the gauges or metal thickness are the same? Are you sure the two pieces in question are from the same mill or steel supplier? Are you sure the two pieces are from the same run at the mill? Many finishers experience poor results over a galvanized surface. The reason is usually in the substrate or that the pre- treatment isn't specifically designed for a galvanized surface. If you're running both steel and nonferrous metals through the same pretreatment line, it's impossible to maximize salt-spray performance on both substrates at the same time. There are several new tran- sition metal conversion, or DIP, coat- ings designed for aluminum and zinc substrates. The ones I've witnessed were very impressive. One new test conversion coating I saw in a chemical company's lab provides 150 hours salt spray without a coating! You also have zinc-rich powder primers that offer outstanding salt-spray hours. Consider auditing your current process and investigating some of the new pre- treatment offerings available. Finally, do enough pre-testing to ensure that any claims made are indeed true. Rinse-water quality when running rework through the pretreatment system PROBLEM Can you comment on the quality (total dissolved solids) of rinse water as it may relate to recoating parts? T.G., Chicago, Ill. SOLUTION In response to running rework, the issues are: 1. Good intercoat adhesion 2. No contaminants showing through the coating 3. Maintenance of corrosion specifica- tions The problems typically are: 1. Poor adhesion 2. Softening of the existing coating by chemical attack 3. Hard-water minerals telegraphing through the second coating 4. Excessive mil build Mistakes are made, such as leaving the alkaline cleaner stage on, which softens or disrupts the first coating. Polyesters are subject to hydroxide attack. Sodium and potassium hydroxide are used in hot paint stripping. The second biggest mistake is that some coaters stop their lines too frequently when rework is in the washer. Any stopping of the line will encourage dry-down of either chemical or water minerals. Finally, you see re- work intermingled with virgin stock. I like to see a company first determine what the rework amount is to qualify how many parts we're dealing with. Then I can determine the primary cause of the failures. But first, work on the is- sues I've discussed previously to elimi- nate or minimize the double or triple cost of a sloppy process. In my opinion, when running rework, the cleaner stage should be off. There should be a final high-quality rinse of de- ionized (DI) or reverse osmosis (RO) water. The acid stage can be on or off; testing will determine this. At issue is whether the rinse impingement alone will be enough to remove sanding debris, and how much handling or resoiling took place during the time the substan- dard parts sat in an unclean environ- ment. Some finishers add a wetting agent in the final stage to minimize solids telegraphing through the second coating. This, much like a rinse aid in a dish- washer, does work, but how much dam- age will you do to coating specifications? Brad B. Gruss Pretreatment & Process Consulting LLC BEST OF PRETREATMENT PROBLEM SOLVER POWDER COATING, March 2017 27