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Does Sanding a Passivated Surface Mean that It Is No Longer Passivated?

November 24, 2010

I work in the quality department of a machine shop which makes metal plates. One side of the plate is flat and smooth (about 4 inches by 6 inches), and the customer insists that there be zero scratches on that flat surface.

We sent the parts out to an outside processor for passivation, and they returned the parts passivated but scratched.

My colleagues insist that we can sand away the scratches, and not have to worry about re-passivating the plates. Are they correct, or do we need to have them re-passivated?


Other details:
-The material is 303 Steel
-The passivation spec on the print is AMSQQP35 [canceled], type II
-The passivation spec used (because the one on the print is superseded) is AMS2700
-The maximum thickness of the plates are .225 inches, the minimum thickness is .035 inches, but at most points it is about .100 inches thick.
-We make about 600 of these parts a year, shipped sporadically whenever the customer orders them.
-There are 17 pieces which will be re-worked in this case, but we may have to face the same situation at a later date.
-This is an aerospace job, so I don't want to take any chances.

-Ben

P.S. If it is a matter of the depth of the chromium enrichment (a term I only learned when trying to research this question) please give me a hint of how deep we can sand.

James McCracken
Machine Shop Quality Department - Long Beach, California, USA

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First of three simultaneous responses -- November 26, 2010

Passivation removes free iron (and contaminants) from the surface. Any sanding at all will destroy the passivated layer as it is very thin.

Tell your passivator to stop scratching your parts or find someone else.

Jeffrey Holmes, CEF
Spartanburg, South Carolina

Any treatment chemical or mechanical,will decrease the effect of passivation.

James Watts
- Navarre, Florida
Second of three simultaneous responses -- November 26, 2010

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James,

Well, passivation is extremely thin, so much so that you cannot measure it in any reasonable sense of the word (we are talking nanometers or even Angstroms here). Any abrasion to the surface will negate any positive effects of the passivation process. If you need to abrade then you will need to re-passivate.

Brian Terry
Aerospace - Yeovil, Somerset, UK
Third of three simultaneous responses -- November 29, 2010

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November 29, 2010

Thank you.

You have given me the backup I needed to convince my colleagues.

The plates will be re-passivated.

James McCracken
- Long Beach, California, USA