Letter 5005

Galvanizing and powder coating steel and aluminum for salt exposure 

.

I'm about to have steel and aluminum parts powder coated. They are bases for outdoor, coin-operated telescopes. Since they are often installed near the ocean and should last 20 years or more, corrosion is a significant issue.

The head of the telescope is cast aluminum. I plan to have it phosphate coated and TGIC polyester powder coated, as I've heard this is the best approach for aluminum parts.

The welded base enclosure and various small parts are mild steel. I plan to have them galvanized and polyester powder coated. I'm not sure what type of galvanizing to spec for the base, though; it has a 4" pipe, 50" long, welded into a ~18" steel cube with a door. (I would be happy to provide drawings to anyone.)

Can these bases be spray galvanized before powder coating? I'm currently dealing with prototypes that have already been welded up, but should the production parts be galvanized before welding?

I would appreciate any advice on these issues.

I would also appreciate recommendations on galvanizing and powder coating shops in the San Francisco Bay Area.

Thanks, Wallace

Wallace W [name deleted for privacy]
- San Francisco, CA, USA

----
Ed. note: Readers please reply to the technical elements of the question. Our supporting advertisers will respond in private regarding the sourcing issues.

You should also consider Enameling, or porcelain coating these things. It would be much more expensive, but the resulting coating would be WAY more likely to last your twenty years without failure.

Jeff Watson - Pearland, TX

Aluminium alloys require hexavalent chromate conversion coating for max performance. Phosphating or phosphate-chromate (green chromate) are v. poor seconds. Zinc coating must be applied after welding; or it will be destroyed by the heat. Steel can be flame sprayed with zinc or even aluminium. The alternative is true hot dip galvanising of the welded assemblies.

With the latter you can get away with a LIGHT abrasive blast and powder coat, but again a hexavalent Cr process is best (not the one for Al!). 300 gm/sq.meter Zn should be more than adequate for longterm durability. If you want the paint finish to last the correct pretreat is essential Roger

Roger Bridger
- Croydon UK

Jeff - I'll look into the enameling. Thanks. Roger - Thank you very much for the information. I have a couple more questions:

(1) What kind of vendor would offer hexavalent chromate coatings for aluminum and steel? I'm not familiar with those finishes. An electroplating shop?

I have now had our prototype steel parts hot-dip galvanized, and the surface finish leaves much to be desired. It is extremely uneven, with ~1/16" ridges of zinc in some places and large lumps of zinc in others. I suspect that hot dip is the most effective type of zinc coating for the welded steel base, since it would be difficult to spray its interior. The finish on these parts, however, was unacceptable. 300 gm/m^2 would have meant nothing to this shop; they do not have anything near that degree of control. Any tips on finding a hot dip vendor that would be more like what I'm looking for?

Thanks again, Wallace

Wallace W [name deleted for privacy]
- San Francisco, CA

Although zinc electroplating by itself would not stand up to this service, zinc electroplating followed by chromate conversion coating and powder coating might do well. And the smothness/brightness of the finish would be far better than is possible from galvanizing. Good luck.

Ted Mooney, P.E.  finishing.com Brick, NJ

Kynar gives 20 year warranty protection on Aluminum. Electrocoats will fade and change color.
Do it with Kynar

Bob Baum
- Flint, MI USA

Yes, Kynar sounds like a good possibility. I didn't see anyone suggest electrocoats though.

Ted Mooney, P.E. 
finishing.com
Brick, NJ

I want to post a question or inquiry of my own.

 

I want to answer or follow-up on this question publicly (in non-commercial fashion).