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Powder coat adhesion problem on zinc plate

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Q. We have a powder coat adhesion problem when applied over zinc plating. I see from reading your site that we are not the only people with this type of problem. Some batches of plated parts powder coat fine. And on some it falls off in great patches. I have spoken to the platers but they offer little help. We only have 2 operators in our area to choose from and I have tried both. I personally know little about the zinc plating process so I need to know what questions to ask when I next meet with them.

Mike Owens
Sheetmetal Engineering/Powder coating - Christchurch, Canterbury, New Zealand


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A. The curing of powder paint over zinc electroplate outgasses and blows thru the partially cured powder. Organic additives in the zinc electroplate (worse with Acid Chloride formulations) explode thru the powder.

The same is true for the chromate conversion coating. The chromate should be very thin because the thicker films will separate from themselves.

Best case: Alkaline zinc with least amount of brightener held on the slightly less than bright side, then a very thin chromate, then ramp up the baking very slowly.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Editor's note:    
   Mr. Probert is the
   author of


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A. First let me ask you why you used zinc plating in your product and then give it powder coating? Surely you will face many problems by this method. The adhesion of powder coating after curing with zinc plate surface is very poor and the surface finish not good because some gas and organic additive will be evaporate during curing of powder. So you must change the finish of surface before you paint it powder like steel and make phosphate for it before powder coating by these method you will increase the adhesion and improve the finish of surface after curing. If it is very difficult so, you must accept alkaline zinc plate with least amount of brightener may be not bad.

Aly Gomaa
- Cairo, Egypt


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A. The key to getting adhesion of powder coating on galvanizing or Zn plate (we do both here), is the PC pretreatment. Degreaser Acid etch Zinc phosphate Lots of rinses in between. We get excellent adhesion on Zn plate and on hot dip galvanized. For thick HDG coatings, degassing is required (heat to 210 °C for about 10 mins) then pretreat.

Geoff Crowley Geoff Crowley
galvanizing &
   powder coating shop
Glasgow, Scotland


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Dear Sirs,

If you can find a local metal finisher who will develop a waterborne primer, then you may find that adhesion and corrosion resistance will be an improvement over a one coat powder system. We have finished such a product for 4 years now and it shows no sign of failure as a system primer/powder (galvanized crash barriers after 4 years in a hostile environment).

Terry Hickling
Birmingham, United Kingdom

September 22, 2012

Q. Dear sir,

Can we do powder coating on galvanised components as we are facing internal corrosion if mild steel tubes are used. If yes, then what is the process?
Thanking you,
joshi

y.b.joshi
automobiles - honda sattari goa india


September 23, 2012

A. Hi y.b.

Geoff has described the traditional process for this situation: zinc phosphating in between the galvanizing and the powder coating. Good luck.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

Phosphating Metal Pretreatment

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