Metal finishing Q&As since 1989

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Specifying Pickling Solutions/Procedure

 

We manufacture freezing equipment for the food industry. One of the processes we utilize is pickling after fabrication and welding; between the mill and suppliers bad practices lead to much iron contamination by the time it get to the plant, it's amazing some of the things done with stainless materials. Most of our equipment is large and the materials are mostly structural types. We only use "L" grades of 304 and 316 stainless steel, to minimize sensitization. Rust is becoming more of an issue with USDA, so we pickle our assemblies to put equipment in service, in it's best condition from a corrosion standpoint. We have used different finishing shops to do this for us, and get different results, from each one. I specify parts shall be pickled per ASTM A380 , followed by a ferroxyl test performed by an independent party. After a job shop proves it's process I don't require the ferroxyl test to save some money. Pickling is used to descale stainless, my question is, since ASTM A380 has so many variables, can there be a concentration of Nitric/Ammonium BiFluoride specified, that will ensure heat tint and scale from welding, flame straightening, plasma cutting etc. will be removed. I just had a "lot" of parts come back from a finisher that still have scale and some rust, we used them before three times with good results, but they had to pass the ferroxyl test then, this time they did not, results not so good either.

The best pickling results I have seen have been outside of the USA, New Zealand and Brazil. In those facilities they were using concentrations on the high end of the ASTM A380 specification, good pickle was achieved in 30 minutes or less. Is there a concentration/time/temperature I can specify to get uniform results in scale removal. I very much appreciate all comment.

Thank you in advance,

Bradford P. Maas
- Northfield, Minnesota

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Your plasma cutting, welding and flame hardening scale and discoloration are not consistent and controlled.Why bother to just quote a controlled process standard for cleaning and passivation. It will lead to disappointment. Instead specify the desired result per cleaning process ASTM or MIL spec.For example parts to have a clean scale free surface per MIL spec 321 and pass feryloxide test per spec ASTM A123 . You pricing may vary depending on the tenacity of the scale and discoloration but your quality will be consistent.

Todd Osmolski
- Charlotte, North Carolina, USA
 

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Hi, Brad. Good to hear from you again.

The problem is that ASTM ASTM A380 is NOT a specification. It is a recommended practice. It will always be in general terms rather than specifics.

We straightened out the passivation end of it with ASTM A967, but the pickling end needs to be looked at. I will see what we can do at the May ASTM A1-14 meeting. One of the problems that you face is that the amount of scale on welds varies considerably, so it may be hard to specify ONE parameter. We probably need several degrees of pickling, like we do in passivation. Then you can specify which you want to your job shop. Anyone who has ideas for a proposed specification, please let us know so that we can give it to ASTM.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois