Issue link: http://www.e-digitaleditions.com/i/715656
POWDER COATING, August 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 indepen- dent line audits for pretreatment processes. In his more than 30 years in the industry, he made numerous presentations on pre- treatment for powder coating. Adhering powder to aluminum castings Problem We have a three-stage pretreatment process: phosphate in the first stage, clean water rinse in the sec- ond stage, and nonchromic sealer in the third stage. We just dumped the tanks. Our pretreatment process temperature is 120°F; phosphate bath concentration is 5 to 6 ounces per gallon (6 to 8 drops solution turns from colorless to pink). Our pH range is from 5.5 to 5.7. We receive 6-inch bell shells that are aluminum castings from China, and we just ran into an adhesion problem. During the crosshatch test, the powder just flakes off the shell. Because the mold is old, the people in China have to grind and sand the part to remove excess material, which poured out onto the part. So, the parts come in with scratch marks. We tried running the parts through the pretreatment process twice, but it didn't solve the adhesion problem. We'd appreciate any advice and help. Thank you. S.C., Long Branch, N.J. Solution You indicate you have a three-stage process. Many of the chem- ical companies have cleaner phosphates designed for multi-metal lines. When you're using a cleaner phosphate, it's imperative that the formula incorpo- rate a fluoride source. Fluoride is criti- cally important to help in deoxidizing or microetching the aluminum sub- strate. You indicated that the pH of your bath is from 5.5 to 5.7. This tells me that it probably doesn't have a flu- oride source. Usually, these materials run somewhere between 3.75 and 4.50 in pH. Please check with your chemi- cal providers to see what upgrades they can make to be more aggressive on the aluminum. The three nonnegotiable standards for nonferrous preparation are as follows: 1. Remove all organics, casting release agents, waxes, and any processing oil from the parts. 2. Provide deoxidation or etch to the aluminum. This can be accom- plished chemically or mechanically. 3. Provide a high-quality final rinse. Deionized or reverse osmosis is pre- ferred. Here are some questions for you: • Are you sure your current process yields a water-break-free part as it exits the last process stage? • Have you checked your parts for water-break-free surfaces after the dry-off stage? • A r e y o u c o n f i d e n t t h a t y o u a r e achieving full cure on the parts? • Where is the delamination? Is it uni- formly poor, or is it on the sanded or abraded areas? With this information, I can narrow down a probable cause and pinpoint areas to attack first. Other issues that may affect product adhesion include line stops, part drying, contaminated rinse, poor-condition final rinse, or too much dissolved solids. Transfer, or lag time, is- sues between pretreatment and dry off can be related, as well as excessive dry-off temperature. I hope this gets you on the right track. Please feel free to contact me with additional information. Powder coating adhesion on chrome molly Problem I have clients who are stating they will NOT sandblast as it changes the temper and molecular struc ture of the c hrome molly ( c hrom ium m olybdenum, o r chromium steel) used in sprint car frames. They will only have them soda blasted. I am concerned about the adhesion of the powder coating with this method and the additional prepa- ration required when using this method. I appreciate any insight or information you can provide. E.D., Billings, Mont. Solution Thank you for the question. This question deserves a response from a metallurgist, but here are my thoughts and opinions. First, let me say that thor- ough testing should be conducted to either prove or disprove both my opinion and your customer's opinion. Soda blasting is definitely less aggres- sive than sandblasting. Both ap- proaches provide a mechanical bond, or profile, for the powder coating. In either case, a postaqueous pretreat- ment of iron phosphatizing and a final improved water source—deionized (DI) or reverse osmosis (RO)—could be used. A chrome-molly substrate, de- pending on the alloy, may or may not have some ferrous ingredients. If it does, you may receive value via phos- phatizing; if not, you are at least re- moving any organics, blast debris, fin- gerprints, and such. The final DI or RO water rinse provides a better, less contaminated surface between the substrate and the coating than a regu- lar water rinse. Brad B. Gruss Pretreatment & Process Consulting LLC BEST OF PRETREATMENT PROBLEM SOLVER