Hello there, welcome back to beyond ballistics! Today we’re talking about rust removal:
I’m sharing with you over two years of rust removing experiments, and the final method that they led to, that I’m quite satisfied with. If you’re only here for the formula, you can find it at this timestamp, but I suggest you watch the whole thing to understand how to se it at the best of its potential, and how it compares to other methods.
What we’re covering today is chemical rust removal, which is the best approach for restoring complex and stuck steel items. You could either go with a pre-made commercial solution, or mix your own. The problem with commercial products is that there’s a lot of variability between them, and you won’t know what the results will be until you try them out. Some will lead to an excessive base metal damage, others will leave residues on the surface meant to improve paint adhesion, but that we don’t want if we’re planning to use a different finish. Also, availability varies greatly from one country to the other, so what is available on the market for me, probably will not be for most of you.
What we want to do then, is prepare our own mix, from worldwide available chemicals. If you look online for a tutorial on how to remove rust, you’ll be met with a plethora of remedies, from groundless kitchen made concoctions all the way to industrial strength acids. They all suck, for one reason or another.
The most common way of dissolving rust is to use an acid substance. Acids react with the oxides and hydroxides of iron that rust is composed of, transforming them into water soluble salts, that then simply dissolve in water, leaving behind solid metal. I tried most of them, either on or off camera, and none works satisfactorily for restoration; The most commonly used ones are hydrochloric, phosphoric, citric and oxalic acid. There’s also vinegar, which is a 5% solution of acetic acid.
Even though some are definitely better than others, all acids have one big issue in common: they cause harm to the base metal, and that is due to the very low ph they work at. At the end of the day, that’s what an acid does, lowers pH. One way of dealing with this issue, is adding a corrosion inhibitor to the acid, that will shield the metal, letting the acid work almost exclusively on the rust. This is the way most commercial rust removers are made, usually consisting of inhibited phosphoric acid, a famous example being naval jelly.
However, no inhibitor is 100% effective, they’re often uncommon chemicals, and making a homemade mix that way didn’t seem the best option to me. A different approach is to remove rust through chelation instead of acid attack. This is what a very famous commercial product claims to do: EvapoRust. I say claims to do, because the actual composition is kept as a trade secret, and nobody has a clue what’s actually in it.
I’m a wannabe chemist so I’m not the right person to go in too much detail about the process, but in a nutshell, certain molecules have a high affinity for certain metals, so they envelop them forming a large, water-soluble complex, that is then carried away by water.
I experimented with EvapoRust extensively: it works at a good pace, does negligible damage to the base metal, and at the time, it was better than anything I’d ever tried; I was very happy with it.
But my honeymoon with Evaporust didn’t last long. The product is incredibly expensive, and doesn’t last nearly as long as advertised on heavily rusted parts. I had to find an affordable alternative to Evaporust, that worked on a similar principle.
I searched far and wide, and then settled on citrate ions: they also dissolve rust through chelation, while being cheap and widely available.
After some testing, this is the mixture that I settled upon:
To each liter of water, I’m adding 100grams of citric acid and 40 grams of sodium carbonate, also known as washing soda.
These two immediately react with each other, giving off carbon dioxide gas, and forming mostly monosodium citrate, and some disodium citrate salts.
Free citric acid is instead entirely used up in the reaction, but it turns out that the salts formed are so efficient at chelating iron that they work even better than the acid itself. But at a much higher ph, which is exactly what we want. The pH rises, from just over 1 of the citric acid alone, to about 4. The pH scale being a logarithmic one, this means that the new solution is about 300 times less acidic than if I’d used citric acid alone. Just for comparison, my solution has now about the same acidity as tomato juice.
The same result can be obtained using different amounts of sodium bicarbonate, also known as baking soda, or sodium hydroxide, also known as caustic soda.
For 1 liter of water and 100 grams of citric acid, you’ll need either 40 grams of washing soda, or 63 grams of baking soda, or 30 grams of caustic soda.
If you use the caustic one, you won’t see any fizzing, but the end result will be the same.
In any case, once the reaction is complete, the active ingredient of our rust removing solution is already there.
We just want to add one more ingredient, to act as surfactant. I’m using regular dish soap, in somewhat arbitrary amounts, but I’m sure there’s something better suited for the application, so if you’re into the chemistry of detergents, feel free to experiment. If you haven’t got a clue, better stick with dish soap.
