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Corrosion of galvanized steel in a Marine Environment


Q. How far away does a galvanized steel exterior product need to be located from the ocean before the rusting effects become negligible?

I manufacture a galvanized steel rail that is polyester powder coated.

And what is meant by MARINE grade powder coat? Is this actually in the paint or in the pretreatment of the metal?

Thank you.

Joe Garcia


A. Hello.

My understanding is that you have selected galvanising as your primary source of protection (galvanic) for your steel substrate. You have then chosen powder coat as your aesthetic coating to make it look nice. All zinc coated steel (galvanized steel) will react with oxygen and moisture to allow the zinc to become sacrificial to the steel and thus form zinc oxide. With a powder topcoat this reaction will still occur, only it will happen under the powder. This will continue relatively unnoticed for a period of time, after which two things will occur. The zinc oxide (also known as white rust) will form on the powder surface making it unsightly. The second part is that the powder will eventually lift and peel. Here is where you need to consider both pretreatment and choice of powder for marine applications. To diminish the formation of zinc oxide the article should be pretreated with either a chromate conversion or zinc phosphate prior to powder application. Whilst there are different grades of powder it is important to select a powder that has a higher resin quality and quantity (these may be known as marine grade). A further step would be to apply a powder primer, based on epoxy technology, underneath the polyester powder topcoat to further slow the ingress of moisture to the substrate.

To provide a short answer to your question, yes you can get a marine grade powder that will perform better, but it will only ever perform as well as the pretreatment applied.


Michael Lind

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