I'm opening this topic because I'm tired of seeing that many products for rust treatment out there on the market, and thousands of videos in youtube talking about which product is the best to remove rust. I think finding the best product is too chaotic and you have to invest a lot of time in it.
I would like to offer the best solution on this thread to help people and readers to easily find the best solution for their needs in a quick and clear way based on chemistry: Rust is a chemical reaction. And for that I would need the help of all of you.
I found some methods like vinegar, cocacola, citric acid... but honestly... let's get serious, we need something effective.
The quickest and most aggressive compound i found to literally dissolve rust is:
Muriatic acid or Hydrochloric Acid.
Please watch an example:
However I found a better option which is Phosphoric acid and it's less toxic than Hydrochloric acid and less dangerous, and I heard it dissolves the rust and at the same time it protects the surface.
If someone could tell me some information about it like:
What is the ideal ratio of water/acid to not to eat into the metal?
Any ideal ratio to not to cause burns on the skin but still remove rust?
What to do after applying it? Wait until it dries?
There was a guy complaining that he washed the phosphoric acid with water and this happened:
Therefore, I'm guessing that after applying phosphoric acid and letting it dry, then you can apply some rust inhibitor on top of the metal, what would be the best chemical compound to inhibit rust?
Please let me know,
Basically, most acids will dissolve rust, because most acids will react with iron and create Fe+
compound where FE is iron and A is the negative ion from the acid.
Some of the iron salts are water soluble and some are not. The problem with using HCl, muriatic acid,is that it leave chlorine on the surface and chlorine accelerates rusting by removing a hydrogen from water leaving OH- ion, which in turn combines with iron and eventually converts to iron oxide, rust.
Phosphoric acid will create iron phosphate coating on the surface, which is not water soluble and would appear to offer some rust protection. In reality the iron phosphate is not a rust protecting layer, but one can rub oil on iron phosphate to impregnate that layer with oil, which in turn becomes a rust inhibitor by blocking water access to the metal.
Commercially available rust removers based on phosphoric acid have up to 25% concentration of H3PO4. I usually buy the 85% concentration and dilute as needed.
The tricky part in making rust remover is knowing what you want it to do. If you have stubborn rust and you want it gone fast, then a very aggressive acid like HCl or HF can be used, but if you have high concentration then it becomes dangerous and may leave deposits on the surface. HCl or HF are usually reserved for rust stains on concrete, since you do not have to worry about Cl meing deposited onto the surface, but you would worry about it if you were cleaning a metal part. If you have very low concentration of strong acid then there may simply not be enough acid to dissolve the large quantity of rust. For this reason acid based cleaners have more than one acid in them. A low concentration of strong acid like HF or HCl and higher concentration of weak acid, like Oxalic to keep dissolved iron dissolved. When designing a rust remover one must know the metal that is being cleaned to select the best acids for the remover. If you want the surface to be clean then you do not want acids that form insoluble salts with metal being cleaned. So in this example phosphoric acid may not be an ideal choice, since it will deposit iron phosphate onto the iron surface. If you have to use Cl based acid for cleaning rust off metals, you should wash the surface with NaOH or baking soda immediately after the HCl treatment.
Another way of getting iron dissolving ions to the rust is to use salts instead of acids. Sodium phosphate or sodium sulphate can be used, but the same cautions about the formation of iron salts should be accounted for as is with use of sulphuric or phosphoric acid.
The protective coatings like iron phosphate will not form with too high concentration of phosphoric acid but other coatings from other acids can form specifically at high concentrations of acid. For example nitric acid will burn the surface and create a protective layer.
For some applications like passivation of stainless steel you want to select acid that will dissolve iron but not chrolium, leaving a high concentration of chromium on the surface of the part, allowing chromium oxide layer to form in effect protecting the part.
This should get you started...