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Is Passivation or Black Oxide Finish better for Stainless Steel?


A discussion started in 2007 and continuing through 2017 . . .

(2007)

Q. I am trying to evaluate two supplied metal threaded studs. Both are manufactured using 'CRES' (Corrosion Resistant Steel?). The stated finish on one is 'passivated' the other is 'black oxide'

My questions are:

What are relative advantages or disadvantages of using one finish over the other?

Is there a non-destructive test to confirm passivation has been performed? (Is visual sufficient?)

Richard Shamblin
Consumer - Knoxville, Tennessee


simultaneous (2007)

A. The passivated one will be more corrosion resistant, unless the black oxide is oiled or waxed.
Salt spray testing would be a cheap and reasonable test for rusting resistance.

James Watts
- Navarre, Florida


(2007)

A. No definite answer. Depends upon the particular CRES alloy, supplemental treatment given the black oxide, usage and corrosive environment.

A black oxided surface is used with liquid or dry film lubricants (which may incorporate anti-corrosives) which should be chosen for the particular usage. Black oxide is probably better for wear or galling resistance. Passivation may give better corrosion resistance in exposed locations which would wash away the black oxide's supplemental treatment.

Visual inspection cannot confirm passivation (although absence might be detected by rusting in humid environs). A quick and easy method not requiring test equipment is the copper sulfate test (method 102 of Mil-STD-753 [link is to free spec at Defense Logistics Agency, dla.mil] [Ed. note: replaced by AMS-STD-753 [link by ed. to spec at TechStreet]] ). Limitations: 304 & 316 may pass this test even without passivation (provided they've been properly handled), whereas some 400 series & PH SS may fail even if passivated. Very good indicator of non-passivation when 304 or 316 fail, though.

Ken Vlach
- Goleta, California
contributor of the year

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October 24, 2016

Q. If one of these two processes were to be used in an interior of a vehicle on a very small piece of SS (430L or 434L) what would the best process be? This small piece is made to blend with other black items in the interior of a vehicle.

Thank you

Katherine Seefeldt
- Janesville, Wisconsin USA


October 25, 2016

A. The main thing I try to point out in these situations is, if you want a coating anyway (black oxide, paint, powder coat, etc.), why use stainless steel in the first place? You may as well save on material costs and use mild steel.

The main point of stainless steel is the surface is more corrosion resistant as the bare metal (usually with passivation, of course).

If you must, then stainless can be painted or powder coated without too much trouble, but processes like black oxide and electroplating typically require stainless to be activated using a very harsh acid mix.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois


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