Wet Coat Paint or Powder coating onto Zinc/yellow chromate finish
Q. We are a small company in New Dundee Ontario - and we are experiencing bondage problems and thought you may be able to help. We manufacture CRS racks for military applications. Our customer has requested that we Zinc plate with Yellow Chromate for paint adhesion. Then Vinyl wash primer and yellow Zinc Chromate primer on top prior to wet coat baking enamel. On small test samples we appears to achieve proper bondage. However, on actual racks, we loose bondage around bolt holes and pressure points. We have attempted to test with powder paint after the Yellow Chromate with no better results. In both cases, the yellow Chromate dip separates from the zinc plating and remains adhered to the paint flakes. Is temperature a concern when working with paint curing and yellow Chromate? Or time in the curing oven a factor? In all cases, we have let the plating sit for 1 week prior to painting. Is this an area of concern also?Russ Godfrey
electronics - New Dundee, Ontario, Canada
A. Dear Russ, After reading your problem you don't mention what pre-treatment you were using prior to your customer's suggestion. The treatment mostly used in the racking industry for steel substrates is iron phosphate prior to paint whether for powder or liquid. Is there a specification you are working to from your supplier, i.e., Salt Spray, Chemical Resistance or Humidity, etc.? What type of curing process are you using? Please supply more information Regards John Underwoodjohn underwood
- burlington Ontario, Canada
A. I wonder if you need a 2% to 5% nitric acid dip before chromating since you are experiencing chromate separation from zinc.Mandar Sunthankar
- Fort Collins, Colorado
A. We always use yellow chromate before coating our parts. These parts are used in cars like BMW and Mercedes. There are no complaints. What about rinsing? We rinse in five steps and at last with deionised water. Then drying with max. 60 °C. If you wipe the surface with a white napkin this napkin has to remain absolutely clean.Klaus-Martin Hentrich
- Velbert, Germany
Q. Further to our letter # 2922, we are still experiencing problems with bonding. We are now trying to coat over zinc plating with slightly better results, however not satisfactory. We've stopped using the Yellow Chromate over the zinc plating, as we found it was breaking bondage with the zinc when baking the powder paint at 400 °F.
We are using an iron phosphate wash after zinc plating and prior to powder painting. Should we be using a zinc phosphate wash? Is there a different plating process that could give us the same rust protection and increase paint bondage at the same time? Or is there an etch the platers could put parts in after plating to help with adhesion of the paint?Gary A Ritz
electronics - New Dundee, Ontario, Canada
A. "Our customer has requested that we Zinc plate with Yellow Chromate, Vinyl wash primer, yellow Zinc Chromate primer, wet coat baking enamel. On small test samples we appears to achieve proper bondage. However, on actual racks, we loose bondage around bolt holes and pressure points."
At this point, you need to see if this coating system is reasonable. Ask to see the military specification or some other basis for this coating system. It can't be a list of mil specs for each individual coating, it must be for the system. Your vendors of coatings can help, but you have to work from the top coat backwards to the primer, then wash, then chromate. Unless you formulate your own paint, you can get a recommendation for compatible bottom coats.
Q. We zinc plate .001" then yellow chromate a part for powder coating. The plate adhesion is good. However the powder coating supplier is having a bubbling problem. The bubbling is only the powder coat not the plate. Can the chromate cause a problem to the powder coater? The powder coater is concerned with the plate thickness in threaded areas as being the problem. Is there a minimum thickness required for zinc plate before powder coat?
- No. Andover, Massachusetts
A. Generally the chromates if properly applied over clean surfaces will provide excellent adhesion and no bubbling. Without actually putting the parts through a battery of tests myself, the only thing I can add is that a high percent of the failures that I have seen were caused by impurities in the plating, not the chromate. I also have found that you can usually see if the plating is bad if you see cloudiness or uneven coloring on the surface.
Just a shot to look at...good luck.
A Guide to High Performance Powder Coating
by Bob Utech [link is to Amazon]
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