Phosphating vs. galvanising for roofing sheets

We have a very difficult situation where we have supplied a Galvanised sheet for roofing where concreting is to be done. The customer requirement is it should be metal sheet with Phosphate coating. I am really confused because the purpose of both is corrossion prevention & I think galvanising is better than phosphating. Kindly explain the difference between two.
Thanks

Hi, Dinesh. Phosphatizing is usually a pretreatment for paint or powder coating, whereas galvanizing is a heavy coating of hot-dipped zinc. A galvanized surface is far more corrosion resistant than an unpainted phosphatized surface, but can be difficult to paint. Are you sure there will be concrete on both sides of the sheet and no painting will be done? I thought it was common to paint the bottom. Are you sure there is no incompatibility between galvanizing and the particular concrete or grout that will be used? Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Could be they want both. Its common to phosphate treat galvanized steel in order to get adhesion of a decorative coating.
Immersion in a phosphate solution to get this finish.

adv.
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