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Need black anti corrosion finishing for steel screws

2003

We use Screws St.St. with Black oxide for our outdoor products. Now we have to change two screws by security ones. We have found Security Screws of Steel with galvanized finishing (white color) and want to add Black oxide.

Questions:

1. How can we keep the Zinc finish properties to prevent corrosion and to add black oxide?
2. Is it prefer to add black passivation instead of Black oxide?
3. Is it right that Black oxide is used mostly for decor and to prevent light reflex and is not good to prevent corrosion?

Jane Vasilevsky
unwired communications designer - Lod, Israel

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Black Oxide and "so-called" black oxide depends on a top coat of oil, wax, or lacquer for corrosion protection, hence not suitable for outside exposure.

Black Chromate which is expensive because it contains silver metal will take over freshly plated zinc or cadmium electroplate (not hot dip galvanize). This is probably your best answer.

Black Chromate (from a formic acid bearing olive drab chromate) over zinc-nickel or zinc-cobalt or even zinc-iron, will give you the best protection but you may have some difficulty finding a job shop that can apply this system.

Many suppliers listed on finishing.com offer silver black chromate and black chromate for alloy zincs. All of my advice applies to freshly plated zinc or cadmium.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2003