Letter 14098

Phosphate pretreatment  

Hello,

I need some information on phosphate pretreatment on carbon steel. I need to know what to expect as far as appearance, rust inhibiting properties and the conductivity of the coating.

Thanks in advance,

Tim Allen
- Lawrenceville, New Jersey, USA

Hi Tim,

Your questions need to be more specific if you want useful answers.

The color, rust-inhibiting properties and conductivity of a phosphate coating are entirely dependent on the type of phosphate coating that you are applying.

There are many types of phosphate coatings and the choice of which one to use depends on the application. For pre-paint applications, iron phosphate and zinc phosphate are the most popular. Heavy zinc phosphate is used in wire drawing. Manganese phosphates are employed for giving a surface improved lubricity.

George Gorecki
- Naperville, Illinois

Hi, Tim. It's a broad question. Samuel Spring's "Preparation of Metal for Painting" [link is to info about the book at Amazon] is a good book on phosphating. Usually phosphate coatings are used as a paint base, but they may be simply oiled as well. There are many different types, the most popular being iron phosphate (used as a paint base), manganese phosphate (used as a machinery break-in coating), and zinc phosphate (used both as a paint base and as an oiled finish).

Phos and oil offers very limited corrosion resistance, but phos under paint is a very highly corrosion resistant finish. They are non conductive, but I wouldn't call them a robust insulator. Good luck.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

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