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Powder coating pre treatment process after galvanizing

Dear Sir,

I have to powdercoat a steel section of 6mm thick after hot dip galvanizing. After Gal process I do not do water or chromate quenching and within 24 hrs I do the powder coating (Pure polyester) and I want the coat to withstand 1000 hrs salt spray. PL guide me to the pre treatment process I should go for before powder coating. I hv been confused by a lot of people please inform me the correct method.

Venky
Steel section manufacturer - Chennai, TN, India
2004

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First of two simultaneous responses -- 2004

Dear Sir.,

We have used the following process for a long time with no problem and a good performance.

1. Alkali degreasing
2. Rinse Water
3. Pre phosphate rinse
4. Zinc Phosphate
5. Phosphate rinse
6. Sealer.

To guarantee that you will get 1000 hours will depend on the chemistry that you use in your plant, and the control that you take on the pre treatment stages.

I will recommend you to read a couple of good articles on this on the following web sites: https://www.galvanizeit.org/showContent,178,220.cfm#profiling http://www.corp.indgalv.com.au/technical/powcoat.htm

As you will find from the readings the use of Zinc Phosphate will perform best.

Juan Carlos Sanin
- Medellin, Colombia

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Second of two simultaneous responses -- 2004

There are only really three pretreatments for powder coating galva steel. 1. Light shotblasting. Re-profiles the surface to allow adhesion to the zinc, but has disadvantage of removing quite a lot of zinc, and if human dependent on how much is removed. 2. Zinc phosphate preceded by degreasing, and acid etching with plenty of rinsing in between. 3. Chromate also with degreasing, acid etching and lots of rinsing.

Galv needs to be etched (or re-profiled) to get adhesion, but the zinc phosphate is best in my opinion. We get 2000 hrs with this.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK