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Do you pickle machined surfaces?

In our line of work, we manufacture stainless fittings. The majority of these are "fully machined". We have been questioned about pickling and have had a few clients recently insist on pickling even though the pieces are fully machined.

Is there any advantage or anything to be gained by pickling a part that has been fully machined? School of thought has been that machining would remove any surface impurities.

Lynn Pye, Sr.
Fitting Manufacturer - Houston, Texas
January 7, 2008

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First of three simultaneous responses -- January 10, 2008

Pickling of a machined part can serve a useful purpose, removal of some of the contaminants from the machined part surface (especially iron particles).

These guys have a good paper:

http://www.outokumpu.com/applications/upload/pubs_113142858.pdf

Terry Natale
- Beachwood, Ohio, USA

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Second of three simultaneous responses --

Someone may be using a wrong word and really mean passivation. Since both processes are an acid dip, it is probably the cause of the misuse of the work pickle.
Passivation can remove smeared iron from contact with the metal working machinery. It also improves the oxide coating on the SS parts, giving them more corrosion resistance.

James Watts
- Navarre, Florida
January 10, 2008

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Third of three simultaneous responses -- January 11, 2008

Lynn,

Do you mean pickling or passivation? Pickling is really only of use if you wanted to clean up something like heat treatment scale. Passivation on the other hand is used to improve the corrosion resistance of the stainless steel by removing free iron from the surface and enhancing the passive oxide layer.

So it all depends on what your customer really wants.

Brian Terry
Aerospace - Yeovil, Somerset, UK

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The other responses are correct. I have often heard the terms pickling and passivation used interchangeably by people not fully familiar with the two processes. It probably also stems from the fact that nitric acid based baths can be used for both. However, when passivation and not pickling is what is required, citric acid based formulas may also be used. Let us know if we can help.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois

January 17, 2008