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I was following a recipe for a seasoning containing rice vinegar. The directions noted that a metal saucepan should NOT be to used to heat the seasoning because it would react with the vinegar.

What would happen, chemically, if I used a metal (stainless steel) pan?

My thought is that the acetic acid and Iron could react to form Iron(III) acetate. Would this complex be in a concentration high enough to warrant the, "do not put in a metal pan," warning or would the reaction be something else?

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Vinegar was used as one of the main household cleaning and odor removal agents for a good reason. I think in general you can get numerous iron (III) and chromium (III) (which are less pleasant) salts and complexes aside from acetate, e.g. chloride/iodide if you use iodised salt or hypochlorite from chlorinated water.

Diluted acetic acid (vinegar) etches majority of steels quicker than the glacial one, dissolving its passivating $\ce{Cr2O3}$ layer. Aside from concentration, reactivity also greatly depends on the steel type and acetic acid purity [1]:

Carbon and Alloy Steels. Steel is attacked quite rapidly by all concentrations of acetic acid, even at room temperature. Glacial $\ce{CH3COOH}$ at room temperature is less aggressive than aqueous solutions of the acid, but still gives a rate of attack of $\pu{0.8 to 1.3 mm/yr}$ ... Therefore, steel is normally unacceptable for use in $\ce{CH3COOH}$ service.

Stainless Steels. The chromium stainless steels of the 400 series occasionally exhibit low corrosion rates in laboratory tests in dilute acetic acid. However, because field experience with these materials indicates high corrosion rates and pitting attack, they are rarely used for $\ce{CH3COOH}$ production equipment.
Impurities present in the manufacture of acetic acid, such as acetaldehyde, formic acid, chlorides, and propionic acid, are expected to increase the attack of stainless steels. Contamination with chloride can cause pitting,rapid stress-corrosion cracking, and accelerated corrosion of type 316 stainless steel. Up to $\pu{20 ppm}$ of chloride can be tolerated, but higher concentrations are likely to cause rapid equipment failure.
Transferring heat through a metal wall, as in heat exchangers, can drastically alter the corrosion characteristics of the metal.

If you know the exact steel type used, you can check its stability against various acidic media in several corrosion handbooks, e.g. [1] contains cumulative tables for corrosion behavior of manufactured steels tested under various conditions.

  1. Craig, B. D.; Anderson, D. S. Handbook of Corrosion Data; ASM International, 1994. ISBN 978-0-87170-518-1.
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