optical question

I have heard of this before. Any thoughts on the chemical equations? Red rust (magnetite) is Fe3O4, hematite is Fe2O3, acetic acid is CH3COOH (various spellings). How do you convert rust to iron oxide with vinegar and what do you really end up with?

Probably finer mixed oxides and hydroxides, which do well enough to look black. Both ferrous and ferric acetate are real things, and their decomposition will lead to both oxides, with products including CO2, ethane or acetone, I think. The reducing effect of these gasses may prefer Fe3O4. Decomposition temperature is over 150C, I guess.

Tim

Thanks, Tim.

From the op, this is on a hot stove, so the temperature is already well over 150C. Now that you have given some keywords for a search, I found Iron(II) acetate, with the following reaction:

"Iron powder reacts with hot acetic acid to give the product:"

Fe + 2CH3CO2H --> Fe(CH3CO2)2 + H2

I also found the following: ferrous acetate

"Soluble green crystals, soluble in water and alcohol, that are combustible and that oxidize to basic ferric acetate in air; used as textile dyeing mordant, as wood preservative, and in medicine. Also known as iron acetate."

It doesn't look like this would withstand a hot stove either.

There are zillions of web pages and Youtube videos on removing rust with vinegar. The reaction is usually slow and means immersing the piece in vinegar overnight then brushing vigorously. Many of these mention you have to protect the surface with oil or it will start rusting again immediately.

But this seems to be a different reaction, where the vinegar is applied to rust on a hot surface. The reaction has to be very quick, since the vinegar will evaporate fast. I know iron makes a lot of different compounds, but I hadn't heard of this one before so I was curious to find the reaction.

Thanks for your reply.

Yeah, it's not going to be anything remarkable. If you want a more robust corrosion and heat resistant layer, you might try phosphating. Iron(II) phosphate is black, and highly insoluble.

Standard surface prep applies -- degrease, strip the rust, convert it (something phosphoric acid is already good at, e.g. Naval Jelly), and if that doesn't do it already (I haven't used it myself), clean the surface in preparation for a process like Parkerizing.

Of course, for temperatures red hot and up, it's hard to beat good old fashioned enamel, the kind made with ceramic and glass particles.

Tim

Yes, I discovered phosphate a while ago. I live in the rust belt in Ontario, and I have never seen such problems with rust. I tried all the rust preventers but they only lasted a short while then disappeared.

We have a product at Canadian Tire called Rust Converter. It is 30% phosphoric acid, 30% Tannic Acid and 30% isopropyl alcohol. I tried it on my car and it is simply amazing. Wherever the rust starts, it quickly lifts the paint and continues underneath. Soon there is a huge hole that costs a fortune to fix.

Not with this rust converter. It takes only a few seconds to wire brush the loose paint, then spray or brush it on. A few seconds later, the rust starts turning black. I leave it on overnight. Then you can wash it, prime and paint. It only takes a few minutes of work and the results are amazing. No More Rust! I also found most of my tools had started to rust, so I treated them. They turned black and have stayed that way for years without any additional protection.

I even did the face on my hammer, fully expecting the iron phosphate to chip or fall off. After a year, the hammer has been used a great deal but the coating is still there, and it has stopped rusting.

I applied various chemicals to the coating, including sodium hypochlorite (bleach). Nothing had any effect. I use pressure cookers to sterilize fabrics each day. An inner liner was starting to rust even though it is stainless steel. A quick coat of rust converter, and it turned black in all the places that were rusting. The rust stopped and no new rust has appeared. This stuff is amazing.

I found the chemical reactions which I list here. There are two reactions, a fast one and a slow one.

The fast reaction works on rust and takes 15 minutes to an hour:

Fe2O3 + 2H3PO4 --> 2FePO4 + 3H2O

The slow reaction works on bare steel and takes overnight to run:

Both reactions produce iron phosphate, FePO4. This is a hard coating that bonds to the surface and prevents rust from forming.

The Rust Converter from Canadian Tire also contains Tannic Acid. This apparently produces ferric tannate, which is another hard coating. There have been numerous papers published on the protection from rust, and a good example is "Performance of rust converter based in phosphoric and tannic acids", by L. M. Ocampo, I. C. P. Margarit, O. R. Mattos, S. I., available at

If you want to make your own rust converter, I believe Jeff Lieberman posted a link to phosphoric acid in 1 gal jugs available at Home Depot for $12.97. The link is

It is not available in Canada.

All you need is then some Tannic acid powder and isopropyl alcohol, and you should be able to duplicate the rust converter that sells for $250 for 5 gallons. But you can try just phosphoric acid, since many of the rust converters only contain it and none of the tannic acid or isopropyl alcohol.

One caution - if you leave parts immersed in phosphoric acid overnight, it will eat them down to nothing. Obvious, but I learned the hard way.

Sorry, it was James Arthur. I take the liberty to repost his information since it confirms the effectiveness of H3PO4 on rust:

Date: Tue, 30 Sep 2014 18:03:32 -0700 (PDT)

Yes. Home Depot sells it, pretty concentrated too. It was for etching concrete IIRC, or some such. Liquid, not gel. It was a lot cheaper than the Naval Jelly, which they had too.

Oh, here it is:

Phosphoric-acid containing products are magic as rust-converters. I used a bit on ye olde car as a stop-gap to temporarily keep an exposed rusty spot from propagating. It stopped the rust cold, and it's been years.

I never did get around to "fixing it."

Cheers, James Arthur