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Pickling 17-4 SS -- specification

I have run across an older part which is made of cast 17-4SS.

The print specifically call out for the part to be pickled and cleaned using a 2% hydrofluoric acid solution.
My current vendor will not perform this and says that he would use a solution of Nitric Ammomium Bifluoride.

Will this produce the equivalent result?
Why would any specify one process over the other?

Dan Greenier
product designer - Erie, PA USA
September 21, 2009

Hi, Dan. Hydrofluoric acid is truly dangerous in terms of acid burns. It dissolves bone, and sometimes has an anesthesizing effect where people do not realize a serious burn is in progress. For that reason some shops will refuse to use it, and try to use a fluoride product that is somewhat less dangerous.

So it depends on what the part is. If it's a critical flight component, you can't process it except in full accord with the spec. If it's not critical, make sure everyone understands that you are changing the plan -- especially the original designer if he's still on board.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 21, 2009

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70% HF is bad beyond understanding by anyone that has not used it.
Since you want a 2% solution, the HF could be purchased in a rather dilute form which is not bad. The 2% final solution is a pussycat that is not particularly worse than a 2% nitric solution. Buying it in a dilute form will be considerably more expensive as you may have to buy it from a company like Fisher or Baxter that carries high purity materials.

James Watts
- Navarre, Florida
September 22, 2009

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October 7, 2009

I always like to clarify the lexicon, then question the intention of the process being called for.

Picking in general is used when a part has heavy scale, heavy oxides, or bad surface discoloration that cannot be removed with any chemical sort of something hazardous.

Cleaning can be as simple as going at something with soapy water and a brush or rag, on up to using an ultrasonic cleaning tank.

Of course since we are talking about stainless steel, let us not forget passivation, which removes iron from the surface in order to help enable the formation of a corrosion resistant passive layer.

And since you mention that this is an older part, I will mention rust removal, which can be done effectively with chemicals far less hazardous than those used for pickling.

I say pickle only if it needs it, otherwise do whatever rust removal and cleaning it takes to make it look the way you want it to look, then passivate it and call it done.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois