Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Pre treatments that have ill effects on galvanneal coating prior to powder coat
I supply an "A40" product to an automotive stamper. They seem to complain about the coating "flaking" off and gumming up the press as they run the material. They also complain that the parts cannot sufficiently be powder coated because of the flaking.
Are there certain chemicals that will cause the coating to flake during processing. For example the coolant durning stamping or the pre treatment for powder coating.
Obviously the first to be blamed is the steel supply. I felt there may be some other "evil" at work.
Your thoughts ?
Tami Brewer
Material Manager - Battle Creek, Michigan
2007
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2007
Dear Tami:
I have only worked for a few sheet galvanizers so please consider what I say as somewhat remote to your problem. The worst case of galvanizing pealing that I have experienced was on re-phosphorized aluminum semi-killed steel. While working for one USA sheet galvanizer whose product was used for deep drawing in automobile manufacture, one of the biggest problems was flux control. I do not know the source of your A40. If it was from a hydrogen/inert gas reduction of surface iron oxide then there is no flux.
With galvanneal the product is heated long enough (after galvanizing) that the entire coating is a zinc-iron alloy and this alloy I think is primarily 6% iron/94% zinc. This coating is bonded in a perpendicular direction from the steel surface and should not peal.
In a general way the following should be evaluated: Steel Chemistry; Zinc Bath Chemistry; Flux Formulation (if one is used); Passivation Chemistry (if used); Post Treatments (e.g. waxes or lubricants); any other parameter which with production is likely to change (outside normal ranges).
I think if you post more information on your process, you may get a response from a more knowledgable person on this subject.
In the past several years steel chemistry (due to recycling and steel making changes) has gone "crazy" and has adversely affected many, many galvanizers.
Regards,
Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA
Galvanneal sometimes has the unfortunate effect that the coating is laminated, and the top of it can separate in working, leaving a flake of zinc peeling off, and a layer of zinc (thinner) left behind.
This might be your problem?
Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
2007
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