Issue link: https://www.e-digitaleditions.com/i/787158
32 POWDER COATING, February 2017 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. Failing the water degradation test PROBLEM We're having a failure in one of our functional (salt spray/per- formance) tests. These tests include acetic acid salt spray (24 hours), neu- tral salt spray (96 hours), chemical test- ing, sand abrasion, water degradation, and so on. The test that's experiencing failures is water degradation. This test involves parts being submerged in de-ionized (DI) water at 100°F for 24 hours. When this test is performed on a part that's coated in a clear powder (trigly- cidyl isocyanurate [TGIC] based poly- ester), we're experiencing a failure at a rate of about 15 percent. That means about one out of seven parts in testing failed. Our other pigmented powders (both TGIC-based polyester and epoxy hybrid powders) don't seem to experience this failure…or at least we can't visually see the failure. The sub- strate the powder is applied to is a zinc or brass part that is plated with nickel, black nickel, and/or copper. After the part is removed from the water degradation testing and allowed to set, the coating separates from the part in a localized area. Note: There is no indication of the failure before the testing. The failure is a milky looking "stain" on the part. Additionally, the "stain" appears to have concentric rings (like evaporation rings). This "stain" is actually where the coating has separated from the substrate in a local area. This occurs in an area of the part in which water would last drip from the part. The following is a description of the process these parts go through on our powder coating line: Before the parts reach the powder coating line, the sur- face is distressed/relieved. The parts then go through the following pre- treatment process. I have noted the typical conductivity readings (in mi- crosiemens) of the rinse tanks in italics. • Alkaline cleaner • Two counter-flow rinse tanks (city water) 900, 500 • City water halo rinse • Acid tank (sulfuric acid/city water with PH of ~2) • Two counter-flow rinse tanks (city water) 350, 300 • City water halo rinse • Empty process (city water) 300 • Two counter-flow rinse tanks (city water) 275, 275 • City water halo rinse • DI tank (reused/recycled DI water) 15 • DI halo 5 • ~2-minute drip time • DI halo 5 • 350°F dry-off oven • Heat dissipation tunnel and on to powder coating (part temperature less than 100°F) What are some thoughts on a possible solution for this failure? Thanks in ad- vance. M.D., Sanford, N.C. SOLUTION Thank you for the ques- tion and for the very detailed part and process description. Obviously, you're an experienced metal finisher with a well-designed process. This makes troubleshooting more difficult because you probably have looked at the obvi- ous. Four things stand out in your de- scription, after reading it three times: 1. Zinc or brass part 2. Clear TGIC vs. epoxy and pig- mented TGIC 3. Plating 4. Failure point being the drip line I don't have the answer, but offer the following: Zinc and yellow metals each have peculiar issues. Zinc has porosity, and both are subject to oxidation and chemical attack. What can you say to the consistency of the substrate? Does it happen more on zinc or more on brass? If zinc, I would investigate cast- ing consistency and porosity issues. I've seen outgassing or organic perme- ation from castings, which in severe cases causes delamination, especially after DI water boil testing. A minor outgas occurrence may not disrupt the coating but allow minor oxidation. In both substrate cases, do you use a hy- droxide cleaner, either sodium or potas- sium? Is there any chance of part dry- down during indexing of the plating se- quence? I understand that with an acid pickle in that process you should re- Brad B. Gruss Pretreatment & Process Consulting LLC BEST OF PRETREATMENT PROBLEM SOLVER