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'Basic' dimensionless coating needed for 4' dia. steel rings

We manufacture ring shaped precision machinings, 36 to 72 inches outside diameter, 2 - 4 inches thick. Each weighs somewhere around 50 to 400 lbs. Low carbon steel. We currently have the rings coated with either (a) black oxide or (b) zinc chromate. Black oxide is nice because it doesn't affect the finished dimensions which are held to close tolerances, but the corrosion protection is very poor. Zinc Chromate gives a nicer finish and far better corrosion protection, but causes problems due to the thickness. The rings are not subject to harsh environment but they need to be coated just to avoid corrosion from airborne moisture.

Is there any coating similar to black oxide we could use, that will not add appreciable thickness, but will give better corrosion protection.

Steve Churchley
Design Engineer - Muskegon, Michigan, USA
May 20, 2011

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Hi, Steve.

Just a semantic point, but I don't think there is such a thing as a coating that doesn't add thickness. Processes that don't add thickness impart corrosion resistance by changing the steel rather than coating it. You might consider salt bath nitriding; it does markedly improve the corrosion resistance and I don't think it changes the dimensions. But it may be too expensive, especially if it needs polishing, or the increased hardness may be a problem for the application.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 22, 2011