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Optimum amount of solution iron in a nitric/hydrofluoric pickle bath

We are trying to determine what is the optimum allowable amount of solution iron in a nitric hydrofluoric pickling bath for grade 304 and 316 stainless deep drawn components.
We are removing annealing scale and want to know at what concentration does the solution iron begin to precipitate onto the surface of the items you are pickling.

John Braly
oem - Waynesboro, GA, USA
July 16, 2009

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That should not be any problem. The problem should be that at some level the iron will cause stainless to pit. If there is enough nitric in the bath, it will serve as an oxidizing agent and significantly retard the pitting.

A new bath is extremely aggressive, but as it ages it will slow down to a point of being not cost effective. Adding acid helps, but eventually becomes non effective. Find a chemistry book and look up the math formula for Ksp and you will see why.

James Watts
- Navarre, Florida
July 20, 2009