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Pickling SS for consistent architectural finish

I am fabricating a stainless steel job, where I am welding pipe & plate, followed by pickling. The pickling is very important in that it must present a consistent unblemished finish for non glare, decorative purposes. I cannot accept with this job residue of scum or similar staining or discolouration from the bath solution. Can you offer some advice please? Is there an article in the archives on establishing a pickling bath. This bath would be approximately 21' long x 4' width.

Best Regards

Robert Grace
stainless steel fabricator - Perth, W.A., Australia
2004

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Pickling in a mixture of 25% nitric acid + 3% hydrofluoric acid (or 6-8 oz/gal ammonium bifluoride) will produce a non-reflective surface. Obtaining consistent results will require control of time, temperature, bath chemistry (esp iron concentration). You should be able to get reproducible results day to day. Different alloys will react differently, and you may not get a perfect match between alloys. Weld materials will etch differently, and welds will be visible even if ground perfectly smooth.

Jeffrey Holmes, CEF
Spartanburg, South Carolina
2004

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I would add that you need a high pressure rinse with ultra-pure water to produce an unblemished finish. Any contaminants in the rinse water will show up on the dried surface. The post passivation (or pickling) rinse is an extremely critical part of the process. All the effort you put in to pickle a piece can be negated with a rinse with "city" water.

Joseph Lockrem
- Indianapolis, Indiana, USA
2004

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2004

Thank you, to Joe & Jeff.

This job is for structural supports, with a decorative style, in a large Asian Airport, where it is most important that there is no glare. The pickled finish should achieve the non glare appearance, however in sample pickling we noticed blemishing.

Your advice will help in producing a blemish free finish.

Regards

Robert J Grace
- Perth, W.A. Australia

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First of two simultaneous responses -- 2004

Not clear what you mean by "blemish". A few suggestions:

1. Clean to remove all grease, oil, dirt before pickling.

2. Pickle for long enough time, perhaps 1 to 2 hours at room temp.

3. Different alloys, welds and different fabrication methods may cause difference in appearance.

4. Possibly you will need to sand or glass bead blast prior to pickling.

Jeffrey Holmes, CEF
Spartanburg, South Carolina

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Second of two simultaneous responses --

Robert,c ould the water used after pickling be contaminated, I had a similar problem & it turned out the customer was using bore water with a high iron content.

Regards

Bruce Watson
- Sydney, Australia
2004