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"How does galvanized steel hold up to stainless in coastal environments?"



2005

I'm trying to determine the best possible means (read: cheap) for protecting a set of coastal steel electrical enclosures. The other requirement, besides cost, is a design life of 15 years. We're thinking about a nylonic resin coating over a polyester powder coat. This option will require periodic recoating, which may prove unattractive to the end-user. Also being considered is galvanized with a polyester powder coat. This last method seems less expensive in the long run, though I've had trouble locating information on the expected lifetime of this option. Stainless is considered too expensive for this project. What are the relative merits of these three approaches, including cost? I realize galvanized doesn't hold up to salt-spray tests, but hear that it performs well in the real world.

Aaron Bailey
Engineering - Portland, OR, US
^


2005

Dear Aaron,

I would never recommend you the use of galvanized steel unless you intend to apply hot-dip galvanizing after manufacturing of the half-finished product. Using pre-electrolytically galvanized steel sheet will lead in corrosion problems in the sharp edges, since Zn is not able to cover completely those edges and the paint will be much thinner there. After a couple of years the remote protection effect of Zn will not be enough to secure from rust apparition. Same problems could arise if welded construction is used.
Anyway, if your design is free of welded areas and sharp-edges exposed to the coastal atmosphere, you should make sure that the thickness of the Zn-layer is at least 20 micrometer (275 gr/m2 double side) if you want to guaranty a design life of 15 years. Of course it depends on the final location; temperature and atmospheric humidity make a difference. Where will you place your products?

Polyester powder coat is ok, but pay attention to a correct pretreatment in order to ensure against adhesion problems.

I don't consider periodic recoating a good idea.

Have you contemplated using Al? When properly used, powder coated AlMg and AlMgSi alloys feature a good behavior near sea, and they aren't much more expensive than galvanized steel.

Best regards,

Ana Alvarez
- San Sebastian, Spain
^

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