Muriatic acid is another name for dilute hydrochloric acid and has the same chemical formula: HCl. Before the French chemist Joseph Louis Gay-Lussac coined the term hydrochloric acid in the early 19th century, all HCl solutions were known as muriatic acid, which is a reference to the brine solution used to produce hydrogen chloride gas.
You can treat rusting metal with muriatic acid, and it will dissolve the rust, which is why steel pickling, a process that removes tarnish from steel prior to marketing, makes use of it.
You're right to have reservations about using such a strong acid for cleaning rust around the house, though. It not only causes severe skin burns, it dissolves iron as well as rust. It's safer to use phosphoric acid for rust removal, but if muriatic acid is all you have, it does have the advantage of working quickly.
How to Treat Rusting Metal with Muriatic Acid?
Two chemical reactions occur when you wash rusting metal with muriatic acid. The first occurs between the rust, which is iron (III)oxide (Fe2O3), and the acid:
6 HCl + Fe2O3 → 2 FeCl3 + 3 H2O
FeCl3, or iron (III) chloride is a yellowish compound when dissolved in water, and it washes away easily, but washing it exposes the underlying iron to the acid, and the following reaction occurs:
Fe + 2 HCl → FeCl2 + H2
FeCl2, or iron (II) chloride is a light bluish compound without the structural integrity of iron and stays behind as a type of corrosion while the hydrogen gas (H2) bubbles up to the surface and escapes.
If you're going to use muriatic acid to remove rust, you want to apply just enough to make the first reaction happen without starting the second one, and you also want to rinse and dry the steel quickly. If too much of the second reaction occurs, the acid could easily end up doing more harm than good.
How to Remove Rust With Muriatic Acid
Muriatic acid has drawbacks, but it works quickly, and people use muriatic acid to clean car parts and other objects that can sustain a certain amount of corrosion without being damaged. Around the house, you might use it to clean rust off a wrought-iron fence, a heavy-duty chain or old outdoor metal furniture. Keep in mind, though, that home improvement specialists like Bob Vila recommend using it only on stainless steel, not cast iron.
First, make sure you put on gloves, goggles and protective clothing. Dilute the muriatic acid solution you bought at the store with 10 parts water, always pouring acid into water and never the other way around. Brush the acid on the rust and scrub with a scrub brush until the rust is gone. Finish up by neutralizing the acid with a paste made from baking soda and water, then rinse with clean water.
A Safer Method to Clean Rust
Muriatic acid is great for efflorescence and brick acid rust removal because it won't damage masonry, but it's a drastic treatment for metal. Phosphoric acid is much less drastic and is the main ingredient in many commercial rust removers. The phosphorous and rust chemical reaction is:
2 𝐻3𝑃𝑂4 + 𝐹𝑒2𝑂3 → 2 𝐹𝑒𝑃𝑂4 + 3𝐻2O
The products are iron (III) phosphate and water, and since iron (III) phosphate is soluble in water, it just dissolves. The reaction happens more slowly, so the secondary reaction between phosphoric acid and iron is easy to prevent. Phosphoric acid is also safer to handle, and since it comes as a commercial product, all you have to do is follow the instructions for using it.