Issue link: http://www.e-digitaleditions.com/i/901126
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 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. Prepping galvanized steel tubing PROBLEM My pretreatment question pertains to galvanized steel tubing preparation and painting. Our first at- tempt was to treat this part like all other parts, so we ran it through the wash sys- tem, assured that it was dry, and then painted and cured it. The surface finish was terrible because of the bubbling of the galvanized finish. Since then, we've used muriatic acid to remove the galva- nized finish and painting with accept- able results. Is there any other method or product that can be used besides muriatic acid? D.B., Hebron, Ky. SOLUTION Zinc-coated substrates can be difficult to deal with. The problem you had with bubbling of the galvanized finish could be referred to as outgassing. You should try to determine if the gal- vanized part was hot-dipped or an elec- trocoated version. Generally, hot- dipped materials are of far greater thickness than electrocoated steel sub- strates. They're prone to liberate gas, moisture, and sometimes organic soils. People often prefer galvanized materi- als because of their sacrificial nature of protecting the steel. Try pre-baking the part at a tempera- ture that is hotter and longer than the scheduled cure temperature of the pow- der. This sometimes liberates or forces out the contaminants that produce the bubbly finish you described. Most non- ferrous substrates require the following as a minimum for pretreatment: 1. Removal of all organic soils 2. A light chemical or mechanical deox- idation 3. RO or DI water as a final rinse There are some new chemistries available for zinc and zinc-coated steel. You can search for them in this magazine's On- line Reference & Buyer's Resource on the home page of www.pcoating.com. Check the listings under Conversion coatings, Phosphate & chrome free. Hope this leads you to some answers! Testing techniques to determine salt-spray hours PROBLEM We're a leading compres- sor manufacturer in India. We use both powder coating and liquid coat- ing on our compressors. We're exam- ining the salt-spray test for the powder- coated panels as well as the liquid- coated panels. We're doing X mark- ings on the panels and keeping the panels inside the chamber for exposure to the salt spray. We're getting corro- sion on creeps within 150 hours, but other areas are good. What should be the extent of corrosion on creeps, that is, on X marks? C.K., Coimbatore, Tamil Nadu, India SOLUTION It sounds as if your prod- uct is good for 150 hours of salt spray. What should it be? I don't know. First, start out by determining if your prod- uct is an indoor or an outdoor product. Ask yourself, "Where does this product go or reside during its useful product life?" What do the customers of your company expect in product perfor- mance? Generally, indoor products, those that are in a controlled or moder- ately controlled environment in terms of moisture and the elements, should have somewhere between 200 to 400 hours of salt spray with standard pre- treatment and an off-the-shelf powder. Exterior, or outside, products generally start at 500 to 1,000 or 3,000 hours in salt spray, according to the American Society for Testing and Materials (ASTM) B-117 standard. Creepage from scribe is spelled out under the ASTM standard. I like to first define what is failure and then develop a test that most closely assimilates the use of the product and the conditions it's ex- posed to in the field. When you deter- mine what performance is adequate to satisfy your customers, you can then develop a pretreatment process and coating selection that meets or exceeds customer expectations. Consider meeting with your pretreat- ment and powder coating suppliers to ask them to help you set a standard. Ask them what process modifications or product choices are available that will help you develop the process and per- formance that you seek. Should you think your current 150 hours is sub- standard, start at the beginning and make sure your parts are 100 percent cleaned (water-break-free) and fully cured. With powder coating, it all starts with good cleaning and complete cure. I hope this points you in the right direc- tion. Good luck in your testing! Cleaning steel and aluminum, and red herrings PROBLEM I've heard throughout the coating industry that the steel and alu- minum currently used by many differ- Brad B. Gruss Pretreatment & Process Consulting LLC POWDER COATING, November 2017 27 BEST OF PRETREATMENT PROBLEM SOLVER