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Coating options for plumbing fittings (no contact w/ drinking water)

I am looking for a corrosion strategy for use with our plumbing connection system. This system uses PEXa plastic tubing that is connected together using various fittings. The tubing is installed onto the fitting and a compression sleeve is pressed over the tubing and onto the fitting. Such a connection system is used in plumbing and radiant floor heating applications.

The environment of the plumbing fitting is typically behind walls, and never directly exposed to the outside environment. The fitting when use in radiant floor heating is wrapped in electrical tape prior to the pouring of concrete or gypcrete. These compression sleeves are sold in sizes from 3/8 inch up to 2-inch size. Total numbers of these compression sleeves (all sizes) sold are on the order of 5 million pieces per year.

For years, this compression sleeve has been machined out of brass. We are now exploring the use of 1020 carbon steel and corrosion protection is a primary concern. Condensation on the compression sleeve is to be assumed in the plumbing application for the 25-year life of the product. I am most interested in a Dacromet® coating system that will give a high quality black appearance. The fitting will subsequently be printed with the companyname, size, etc.

I have considered hot dip galvanized, zinc chromate, black oxide, e-coat but it is difficult to differentiate and optimize, I need very good protection - cheaply.

Any tips would be greatly appreciated.

John Kimball
- Leesburg, Virginia
2004

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First of two simultaneous responses --

Sounds like a real problem. Even if you find a 25-year coating, how could you ever know that it wasn't damaged during installation? Electroless nickel might work if thick enough (.001"+), but might cost as much as the tiny savings between brass and steel. As for me, I'd rather spend the extra money (What is it, $10 or $20 per house?), than to risk a behind wall leak down the road. Penny wise/pound foolish?

Jeffrey Holmes, CEF
Spartanburg, South Carolina
2004

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Second of two simultaneous responses -- 2004

Don't consider galvanizing, it is designed to be sacrificial over time and will eventually fail. Look at coating systems like Autodeposition (R) from Henkel, or a pipeline type e-coat from Valspar, Dupont or PPG. All of those have a good chance of doing the job. A functional powder coating would do it too, but isn't a good choice for these small parts.

Neither Autodep nor e-coat will affect the dimensions of the part enough to worry about it, but putting the part together will scratch some of any coating off, so possibly you may want to "grease" these fittings with a good rust preventive after coating.

If all that washes out the money then look at several plastics, like Nylon. They are cheaper to mold to shape and are much easier on tooling than metals, so your machining cost will be lower. Be sure you pick one that will not degrade with age, or alkalinity (concrete).

Jeff Watson
- Pearland, Texas