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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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White rust after powder coating hot dip galvanized steel
January 31, 2009
Q. We supply and install balustrades fabricated from mild steel. These balustrades are hot dip galvanized and polyester resin powder coated when installed in exterior applications.
The balustrades are always powder coated within 5 days of hot dip galvanising (air cooled not chromate dipped)
We have an occasional problem with white rust residue appearing on the powder coat after around 6 months installed. This is always very extensive and occurs regardless of whether the balustrade receives regular rain exposure or is in a sheltered environment.
This phenomenon is causing us to loose faith in the use of powder coat over galvanised steel as it has occurred with items from several different powder coat applicators.
To date our remedy has been to remove the balustrade, abrasive blast paint strip, re-galvanize and repaint the item (usually with a two pack epoxy base coat and urethane top coat). This is very expensive and time consuming.
Your comments will be much appreciated.
Fabricator,Installer - Auckland New Zealand
A. Powder coating over galvanizing is notoriously difficult, but only because people often skip the appropriate pretreatment.
Without a competent pretreatment it is a disaster waiting to happen, often ending up as you describe - an expensive re-work.
We do both operations in one plant. We do chromate quench passivate, but that's for the convenience of the galv plant, not a requirement. Its not a problem to do it, provided that chromate is removed.
A good pretreatment will include typically a degreasing in perhaps NaOH, acid etching in sulfuric, phosphoric (or a mixture of the two) acids. At least 2 stages of rinsing between each of these, and after. Then some options: Chromate, Zinc Phosphate, or one of the newer Titanium, Manganese or silane based chemicals. Possibly 2-3 rinses afterward including a demin rinse.
Much less than this is risky. Pretreatment like this should give 30 years maintenance free service or more, even in arduous environments.
If you are having failures its unlikely to be the powder, more likely the application or curing, and quite likely the pretreatment (or lack of it). Ask your powder coater what pretreatment. Or go see it. As a customer you should be entitled to see that they are doing it well. If they hesitate, be suspicious.
Incidentally, do they do any linishing of the galv (to smooth typical lumps, spikes roughness etc) between galv and powder coating? Or do you do that?
Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
February 2, 2009
Black & White Corrosion on powder coated salt-spray test panels
Q. I have tested powder coated panels for salt spray to 500 hrs. The result is Black & White Corrosion found for 15% area after 500 hrs. I wanted to know if the result can be taken on a positive side because red rust has not been evidenced?
Ram Mohan- Chennai, India
November 18, 2015
A. Hi Ram. Although 500 hours to white rust sounds reasonable, generally the finishing specification defines failure. What spec are you processing to? If you are seeing white rust but no red rust on steel parts, we assume that they are zinc plated or galvanized?
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
November 2015
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