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Fe Saturation point for effective HCl pickling of Steel?

2005

I am a chemical student attending McMaster University and had a short summer job at a local Steel company. I had the priviledge of touring much of the plant. When testing some HCl acid for iron concentrations I asked a question that was not answered:
"At what concentration of Fe is the pickling effectiveness reduced?"
I had heard that there is a point, ~20%, that the iron is of such high concentration that it can 'freeze up' in the pipes; but is the surface FeO2 removal affected?

Gina Mutch
student - Hamilton, Ontario, Canada

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2005

Absolutely the rate is affected. The answer to your question involves determining what material goes into the reaction and what material is formed. To this, you need a physical chemistry calculation and an appropriate constant.
The simple answer is the rate is dependent on how much you have on the right side divided by how much you have on the left side. Some products and reactants ions may be squared or cubed, so you need the correct formula. Really nasty reactions go thru a middle product which reacts to go to the final product.
The practical answer is that the reaction begins to slow down with the first molecule (ion) formed and slows down more with each additional ion formed. It eventually reaches a point that while the reaction will still proceed, it does it so slow that it is cost effective to dump, waste treat and make up a new tank solution.

James Watts
- Navarre, Florida

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2005

The saturation point of iron in Hydrochloric acid depends on hydrochloric acid content and temperature.
There are graphs in literature with this relation.

At saturation the bath freezes indeed and will be filled up with green iron chloride crystals.

In companies with pickling bathes in unheated areas this can happen with a sudden temperature drop in winter.
That is why several galvanizing plants in western Europe use heating to overcome this problem.

Dolf van den Berg
- Leusden, The Netherlands