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Black Oxide rust

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Q. I'm a manufacturing engineer with a plating problem. We have a long acme threaded screw which is currently finished via the zinc-iron phosphate conversion process. We are looking into the possibility of moving the manufacture of this part off-shore and are finding that zinc-iron phosphate is not a common process offshore. Black oxide is more common. How does the corrosion resistance of black oxide compare to zinc-iron phosphate. The part is used in medium voltage electrical enclosures. The part is protected from the elements, and thermostats keep the internal cubicle temperature at 4 °C above the ambient temperature to control humidity.

Bob Glowienka
Manufacturer of Medium Voltage Switchgear - Wendell, North Carolina, USA ++++

A. This sounds like a benign environment where the very limited corrosion resistance of black oxide won't be a problem. But this Acme screw sounds like a moving part that requires lubricity in the finish, and I think that phosphatization offers substantially more than black oxide. I doubt that a black oxided ball screw will be a good idea. But black oxide may be what you get anyway, regardless of what you specify, if you're not there to ensure compliance.

Ted Mooney, P.E. RET
Pine Beach, New Jersey++++

A. Dear Bob,

Black Oxidising on ferrous metal, also called as blackodising, is a decorative finish and has very limited corrosion resistant properties. A suitable grade of Rust Preventive oil will increase the life. If a dry film as top coat is permissible, a thin film of clear PU, epoxy or other lacquer will give far better life.

Zinc-iron, zinc-calcium, manganese-iron phosphating with suitable anti-rust oil will give longer life. Manganese phosphating produces a dark grey finish. Alternatively, zinc plating with black passivation will serve the purpose.

- Kanpur, U.P., India(2007)
Read electro-mechanical engineering blog
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