Galvanneal Off-Gassing Problem
Q. Our company is a small job shop that surface coats using electrocoat and powder paint. We have recently started powder painting automotive window frames made from galvanneal (A40) . System consists of : 7 stage washer (PPG vendor) of which 6 are used for powder, a dry off oven which heats parts to 185 °F, powder paint booth (Nordsen) which uses an epoxy based powder (Morton) for these parts as per automotive spec, and cure oven which heats metal temperature to 350 °F.
Problem is that we have two different batches of galvanneal from the same vendor. One produces a 2% reject rate and the other up to 90% reject rate. I have thought that the cratering was due to off gassing of galvanneal. This part is a channeled rectangular part. Multiple things have been tried including preheating parts to 450 °F in cure oven prior to painting, and burning off raw parts at 850 °F prior to painting. Craters are concentrated around the corners and the 90 degree draws of the channel.
It is hard to believe that any gas could remain trapped after 1 hour at 850 degrees and then crater at 350. What am I overlooking? Is this a chemical reaction between powder paint and substrate on draw areas ?
A. Robert, I would expect that if the part was preheated to 850 then only exposed to 350 for the cure that would in most cases rule out any outgassing concerns. I would try a different powder on a couple of the problematic pieces. I am not sure if your line would permit this, I would be glad to test these parts for you at our facility to see what results we get. I am in the testing department and we have a really nice test facility and close ties with many powder and liquid suppliers. If I could help at all with your problem please call me.Tim Doscher
A. Galvanneal can be brittle compared to other galvanized coatings. Maybe there is some flaking or the coating at the bend sites you mention. If so, this will allow for paint to get beneath some of the galvanneal and disrupt the surface when it is cured.Dennis Kaegi
A. Dear Robert,
Your problem seems to be a typical outgassing problem, that can generally be cured as you already know by degassing or preheating the workpieces; however, you are forgetting one thing. Once you preheated the part the gases and oils might bake out of the substrate; however, what you have left in the cavities is air, that is very hard to get rid off.
As a next step I would try to use so called OGF or Outgassing forgiving powder coatings. Due to Gel time extenders in their formulations, they allow the gases to escape and flow back together to cover up craters and pinholes. Several powder manufacturers offer this kind of technology. If you need any further help please feel free to contact me.
RegardsThomas F. Gratz
St. Charles, Illinois
A. I would suspect that the chemical in your pretreatment may be softening the galvanized coating. This is a common occurance with galvanized materials. If your metal supplier does not have QA control between batches or their products, you will have different softening rates on your product. This could be why some of your material is off-gassing at a different rate than another. I would try to reduce your pH level to lessen the caustic nature of the acid attacking your substrate. It worked for me.
A Guide to High Performance Powder Coating
by Bob Utech [link is to Amazon]
A. Have you measured for the presence of mercury in the metal? if mercury salts were used in the pickling brine, this would change the characteristics of the composition. The first heat would cause a slow-acting breakdown of the mercury that would be manifest in the second heat.kris bancroft
- Tallahassee, Florida
Is galvanized steel off-gassing dangerousNovember 5, 2014
Q. I am wanting to purchase a galvanized steel spiral staircase . The website states not to paint it for 6 months due to off-gassing. The staircase will be in my house, (a small house) would the off-gassing be dangerous to my health?
- Alma Colorado USA
A. Hi Julie. Galvanized coatings are basically zinc, which is an essential nutrient (a "mineral"). I personally don't think there's any danger. There is some validity to the "wait 6 months" idea, but it also keeps you off their back for 6 months ... which is probably a good chunk of the warranty period.
I can't advise you to do differently than the vendor suggests, but some people scrub galvanized surfaces with TSP, then wash with diluted vinegar, then use a self-etching primer. The basic issue is that paint doesn't adhere very well to galvanizing.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
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