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Alkaline clean prior to pickle/etch

 

I have a question about the necessity of an alkaline clean before an acid pickle/etch step. We currently bring in pickled and oiled steel from our steel provider for a good finish (powder coat and epoxy/acrylic coat following Zinc Phosphate) on the final product. However this is expensive. We tried using steel with mill scale, and our pickle process removes the rust, but leaves the surface pocked after final coating because we are not removing enough material to leave a smooth surface. We are looking into installing our own pickle/etch and oil line so we can accept steel with mill scale on it, and process it ourselves.

Our space is limited, and I was wondering if it is necessary to have an alkaline clean in front of an acid pickle/etch step removing mill scale from steel prior to oiling. If we can skip the alkaline clean we will have room for the installation.

Thank you for your time!

James Hanley
- Seattle, Washington

It's probably not a good idea to go without the alkaline cleaning step as this is needed to remove the oil. If allowed to remain on the surface, the oil will most likely interfere with the pickling process as well as contaminate the pickle bath.

Patrick Patton
- Westlake, Ohio, USA

James, typically the purpose of an alkaline cleaner prior to acid pickling is to remove organic soils. I believe the steel you want to pickle is hot rolled material which should be relatively free of organic soils. If that is the case, alkaline cleaning would not be needed.

Acid pickling can be done even if there is a limited amount of organic soils on the steel if a surfactant is added to the acid pickling solution.

I recommend testing the process you choice with a bench top simulation to confirm that it produces the desired results before commencing with the construction of a new pickling line. If you need help with this simulation, most pretreatment suppliers can do this in their labs.

Roy Nuss
Trevose, Pennsylvania, USA

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Are you sure that acid pickling will get the result you want?

Try a sample as per the last reply to see that you can get the surface smooth enough by pickling.

Things you could do.

Put surfactant in acid.

Use an airless eductor system to move the acid about, think carefully about size of system as they can warm solutions up a bit, which would be not good for a pickle.

Pressure wash the steel before pickling, check where any run off from this will go as it may give you a fine if it runs into a river, if it runs to a sewer you may need a permit to discharge to the sewer.

As the answers before have said the best answer is to have the cleaner, you know that, but you can try the above to get you by.

Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England

Is pickle an acid or an alkali?

Josh Smith
- USA
March 5, 2012

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March 6, 2012

Hi, Josh. The term "acid pickle" was used 6 times on this page, so it's certainly acid in this context. I'd say it means acidic in all metal finishing contexts that I can think of. In a food context, pickled cucumbers are also acidic (pH < 4.6).

Regards,

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