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Role of ferric chloride in pickling process
November 26, 2009
We are doing nickel plating on automobile components, base material - MS.
I would like to know what is the role of ferric chloride [on
eBay or
Amazon] in pickling?
We maintain HCl concentration of 230-250 g/L.
Plating shop employee - India
December 1, 2009
A. Sir:
I do not think there is much ferric(+3) chloride in HCl pickle as it is a "reducing acid," which means the dissolved iron is mostly in the
+2 state (ous). As the Fe+2 increases, the pickling rate increases. This continues until the "free" HCl is depleted. A proper inhibitor allows the acid to be used to pickle more tons of steel.
Regards,
Galvanizing Consultant - Hot Springs, South Dakota, USA
December 7, 2009
Q. Can we completely eliminate ferric chloride[on
eBay or
Amazon]in our process?
Will it any way affect our nickel plating process?
- India
Q. Dear Sir:
Would you be kind enough to explain what happens to the pickling rate when the HCl pickle gets saturated with ferrous chloride? Does it outweigh the catalytic effect of ferrous chloride when it is within appreciable levels?
Thank you and regards.
Barlow Campano
galvanizing chemist - Jeddah, KSA
December 18, 2012
A. Dear Barlow,
I have not seen any HCl pickling bath really "saturated" with ferrous chloride, but I have seen pickling baths with more than 120 g/l of iron, and the catalytic effect of ferrous chloride is so high, that some parts are not even descaled, but pitted. The ferrous chloride makes the acid stronger to the steel but not for ferrous/ferric oxide or scale, so the pickling bath does not work well...
I hope I could answer your question! Regards,
- Cañuelas, Buenos Aires, Argentina
December 27, 2012
December 30, 2012
Q. Dear Mr. Daniel,
I really appreciate your response. Nevertheless, what I actually wanted to see is a pickling time comparison between a fresh hydrochloric pickle acid without iron content and a regenerated spent acid both at the same initial concentration.
If we were to follow the idea that ferrous/ferric chloride [on
eBay or
Amazon]s has significant effect on the pickling time over regular free HCl content, then I suppose all the published pickling time graphs would show shorter periods as HCl concentration decreases because vis-a-vis, ferrous/ferric chloride [on
eBay or
Amazon]s also increase. This is not the case though.
Also we have this common-ion effect (Le Chatelier's principle), wherein the solubility of a sparingly soluble salt is reduced in a solution that contains an ion in common with that salt. Thus, adding more HCl to a pickle solution with ferrous/ferric chloride[on
eBay or
Amazon]supposedly will reduce the solubility of the iron chlorides and thus precipitate it or reduce the ferric ions into ferrous form.
On the other hand, as the ferrous/ferric chloride [on
eBay or
Amazon]s in the system increase, the solubility or should I say the dissolution of iron and iron oxides from scale in the acid should also reduce because of the excess in iron ions.
Rate of reaction wise, you need to add a lot more HCl to compensate for its loss in reacting with ferrous/ferric chloride [on
eBay or
Amazon], I mean the chlorides do not act as catalysts because they react with the fresh acid.
Noteworthy I guess is the fact that materials pickled in very high iron solution have iron products clinging to it that cannot be removed easily even by double rinsing. You know that it's there if you scrub it after raising the material from the pickle solution and the silvery base metal is revealed. This tenacious film rusts easily and adds up to dross and ash formation in the zinc bath and could also cause coating problems.
HCl is a pickler of choice because it reacts with the ferrous/ferric oxide scale and the base metal creating a smooth surface after pickling. It must be that the "pittings" we are seeing on a material pickled in a regenerated acid are actually signs of inappropriate pickling.
If someone could only confirm my result of about 25% higher pickling rate of fresh acid compared to a regenerated acid then it's time to rest my case.
Barlow Campano
galvanizing chemist - Jeddah, KSA
A. Dear Mr Barlow,
I believe that the Kleingarn Graph is what you are looking for. It shows the relationship between iron content, HCl concentration and the optimum pickling rate.
regards,
- Selangor, Malaysia
March 7, 2013
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