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-----Durability of zinc plated steel vs. hot dip galv steel with powdercoat
Could you help us with a problem that is constantly debated in our industry (metal fabrication / security work) : For mild steel metalwork that is finished in polyester powdercoat and installed in exposed situations, that are within 10 kms of the ocean (Sydney): is hot dip galvanised better than electrogalvanised (zinc plated finish)
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We have a constant problem with hot dip galvanised work that has a white powder break through the powdercoat and disfigure new work sometimes within 12 months, especially when very close to the sea.
The hot dip work doesn't actually rust but it looks terrible, We have recently started using electroplated process with very good results as far as quality of finish is concerned, but are concerned that the much thinner coating of zinc under the powder may cause the metal to rust relatively quickly (within about 5 years)
We realise electro plate process doesn't get into joints and internal surfaces of tubes but we can design most work accordingly. Also what is the recommended powdercoat pretreatment for electro plated steel.
- Sydney, NSW, Australia
1998
A good zinc phosphate pretreatment before powdercoating should take care of the problem. To get a good phosphate, they may have to pickle the galvanize slightly. Galvanize, being many times thicker, should work better than plate for your application. Both it and plate should be phosphated properly before powdercoat.
James Watts- Navarre, Florida
1998
I agree with Jim. The missing component appears to be a good quality zinc phos. You need the extra white corrosion protection that it will give you under the powder. You are getting salt moisture migration through the coating which is causing the hot dip galv. to start white rusting. Once it starts, there's no stopping it.
You needd the isoelectric effect of the zinc phos to insulate the zinc from the outside saline atmosphere.
A spray zinc, 250-350 mg/ft. sq should be adequate. Tell us more about your current pretreatment system.
Craig Burkart
- Naperville, Illinois
1998
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