The Best Acid for Rust Removal (Which Acids Are Used to Remove Rust)

Rust is the bane of everything made of metal. It only takes some exposure to water and open air for the rusting process to start. And the longer you leave rust untreated, the longer it will keep eating away at the metal underneath. I think I don’t have to tell you that there are hazards if you cut yourself on a rusty piece of steel or copper, right?

Now, you could remove the accumulating iron(III) oxide with a decent, commercial rust remover. Brands such as Krud Kutter, Evapo-Rust, and Loctite Naval Jelly have been dominating the top 10 lists all over the web. And with good reason, too — they get the job done. But they tend to be expensive, which is why lots of people go after the DIY option. More specifically, many people opt for acids. But which acid works best against rust?

To answer that question, I’ve decided to compile a list of acids that can remove rust efficiently. We’ll go over all of the popular options, discussing their benefits and the potential setbacks. However, I’ll also cover a few other, non-acid solutions for removing rust.

Acid Strength: Strong vs. Weak Acids

Acid strength refers to the tendency of a specific acid to dissociate into a proton and an anion. Strong acids completely dissociate, and the chemical formula for this process is HA → H+ + A−. On the other hand, weak acids only partially dissociate, with the formula being HA ⇌ H+ + A−.

In terms of rust removal, these two types of acid react differently to iron(III) oxide. For instance, a strong acid in its pure form (e.g., sulphuric or hydrochloric acid) can react quickly and remove rust within hours or even minutes. However, these acids are quite caustic and eat away at the metal underneath the rust. More importantly, they are really dangerous to work with and you have to take lots of safety measures. 

On the other hand, weak acids (such as phosphoric, oxalic, acetic, and citric acids) react slowly with rust. They might not be the best option if you need quick results, but they’re incredibly safe to handle. More importantly, most of them are biodegradable and therefore easy to dispose of after use.

List of Acids Known to Remove Rust Effectively

Oxalic Acid

Oxalic acid or oxalate is known as the simplest of dicarboxylic acids. This antinutrient occurs naturally in plants that we eat, including sweet potatoes, beets, kale, rhubarb, and peanuts. While consuming oxalate-rich food is not particularly harmful, too much oxalate can contribute to kidney stones. 

Of course, the food we consume contains small amounts of oxalic acid, so it’s not harmful to us. However, in its purest concentrated form, it can be extremely toxic. That’s why I have to stress that you need to be extremely cautious when handling oxalic acid during rust removal; always wear a protective mask, rubber gloves, and a pair of safety glasses. If you can, do the whole process outside, in the open and away from children.

Oxalic acid is usually sold as a powder, and you can get it in pharmacies, hardware stores, janitorial shops, and even online retail. Normally, you just need to add a tablespoon of oxalate to a cup of warm water, mix it up, and apply the solution on the rust-ridden surface. Next, you let it dry up for a few hours and then rinse with water. If you don’t manage to remove all of the rust, simply repeat the process.

For a more detailed step-by-step guide, check this article.

Phosphoric Acid

Phosphoric acid is odorless and colorless, and it either appears as a transparent liquid (with 85% water) or as solid, colorless crystals. Have you ever wondered why Coca-Cola or Gold Peak Unsweetened Tea have that sharp, tart taste? Well, it’s because they contain some trace amounts of phosphoric acid. That might be one of the reasons that leads so many people to think that Coca-Cola is a good rust remover.

Just like oxalate, phosphoric acid is harmless to us in really small doses, but extremely harmful as a pure, concentrated substance. Since this acid can work well with light levels of corrosion, it is often used to perform light passivation, i.e. anti-rust coating of steel equipment.

Now, phosphoric acid isn’t necessarily a “rust remover”. It doesn’t really get rid of the rust, but rather converts it into a black outer coat called ferric phosphate. More importantly, it doesn’t react with the iron or the iron alloy underneath.

Depending on the form of the phosphoric acid product you chose, the process of rust removal will be different. If the acid you have is liquid, dilute the recommended amount in water and soak the rusted items in. After a few hours, you can easily remove the outer ferric phosphate coat with rinsing. However, if you manage to find phosphoric acid in gel form, simply apply it with a brush onto the rusty areas and, after some time has passed, scrub the coating off with some steel wool.

For more information on how to properly use phosphoric acid, check this article.

Acetic Acid

Almost every household has a readily available source of acetic acid, considering it’s one of the main ingredients found in vinegar. The acid itself has a strong, pungent smell and a sour taste. If concentrated, it has strong corrosive abilities and it can harm your skin, but it’s harmless in small quantities.

When you use acetic acid as a rust remover, you don’t have to dilute it. It’s both an effective remover and a form of protection against further prevention of oxidation. However, it does work slower than phosphoric acid, so you’ll have to wait a while before you see the iron(III) oxide flaking off.

Naturally, you can use pure, concentrated acetic acid, but white vinegar costs less per liter, and is just as effective. In fact, you can even use food-grade apple cider vinegar and it’ll do the trick. The process is roughly the same as with most acids — pour, submerge, let it sit, rinse, and if needed, repeat.

Of course, white vinegar on its own is a weak acid, so if you want to speed up the process, add salt to the mix. However, keep in mind that you then risk the danger of acetic acid eating through the healthy metal underneath. In order to neutralize the acid, soak the item in water and add some baking soda (a few tablespoons ought to do the trick).

Citric Acid

As its name suggests, citric acid is found in citrus fruits, such as lemons, grapefruits, etc. It is the most budget-friendly option, since you can quite literally buy it at your local grocery store. A few healthy lemons are all you’ll need for the process. Of course, you can get the acid itself easily, as it is usually sold in the form of a powder.

One benefit that citric acid has over others mentioned here is that it’s biodegradable. So, when you’re done removing rust from an object, you can just pour the contents down the sink. With other acids, you’d need a proper method of disposal in order not to harm the environment.

Like other acids listed here, you use citric acid by submerging the rusty items in a liquid solution. If you have a lemon, slice it in half and leave it in the water, then submerge the object. On the other hand, you can add one tablespoon of pure acid powder to the water. If the rusted object cannot be submerged, e.g., a doorknob or a pipe, simply slice a lemon in half and use the wedge to scrub or dab at the rusty spot.

One setback of citric acid is that you have to actively scrub the item with a scouring pad or a lemon wedge instead of just letting it soak (or sit, with the non-submersible metal items). It might take a few hours to get all of the rust out, so it’s a bit time-consuming. People with a busy schedule might want to try a different acid option.

Sulfuric Acid

Sulfuric acid is an extremely strong, potent mineral acid that has highly corrosive capabilities. Even at low concentrations, it can cause severe burns and damage various materials. However, it’s an important component present in many different facets of our life. People use it in the manufacture of fertilizers, drugs, dyes, pigments, detergents, explosives, as well as in oil refining and metallurgy.

With that potency, it’s no wonder that sulfuric acid is an excellent rust remover. And despite how dangerous it can be to handle, you can still get it online at reasonable prices. However, make sure that you dilute it properly depending on what type of metal you want to treat and how big the item is. Because of its strength, it can eat through metal if you don’t take proper steps to dilute it.

In order to get the best result, you will have to let the item sit soaked in sulfuric acid for several hours. A few people I know even had to let the items sit overnight. While soaking for 24+ hours might not be a problem for you, you can reach the same result with other, less harmful acids.

Hydrochloric Acid

Also known as muriatic acid, this substance has quite a few everyday uses. Concrete etching is the most common use for it, as is pool treatment (or rather, lowering the pH levels in pools). Pure hydrochloric acid is also a key component of gastric acid, which is responsible for food digestion and is produced naturally in the digestive systems of both animals and humans.

Because of how strong it is, hydrochloric acid can be quite dangerous if you don’t dilute it properly or don’t take proper precautions when removing rust with it. Luckily, commercially sold muriatic acid concrete-etching products are usually safe to humans, but I’d still advise wearing rubber gloves and a mask. 

Soaking an item in hydrochloric acid is not a good idea. Moreover, I suggest that you only use it to treat stainless steel. With other metals, the acid will start oxidizing too quickly and start to eat away at the healthy layer of metal as well. Instead of soaking, apply some hydrochloric acid with a brush onto the rusted surface and scrub gently with a stainless steel scouring pad; when the rust is off, apply baking soda quickly to neutralize the acid.

Non-Acid Rust Removal Methods

I understand that some people don’t want to handle acids. After all, even at low concentrations, some of them can still cause harm to our skin upon contact, or our lungs if we inhale the exuding fumes. So, I think it’s instructive to cover a few other methods of removing rust.

Electrolysis

In general, electrolysis is a process where the electric current passes through an object and initiates a chemical change. Removing rust using this process is quite common nowadays and you can easily find instructions on how to do it yourself.

While electrolysis is generally safer than using some of the acids listed before, it’s still not entirely free of risk. You have to be extra careful when dealing with electricity, and there’s still a danger of toxic fumes exuding from the container during the process.

Water-Based Solutions

Popular products like Evapo-Rust and Metal Rescue sell extremely well because of how safe they are to use. When going over the ingredients, you’ll notice that most brands emphasize that their rust removers contain no acids. In fact, you will see the term “water-based” both on the packaging and the adverts for the product.

Most of these popular removers don’t react to rust the same way acids do. While an acid might eat away at the rust AND the metal underneath, a water-based solution will undergo a different process called chelation. During said process, the solution’s molecules bond with those of rust and separate it from the metal. That makes the outer coat easy to remove with next to no damage to the item underneath.

Mechanical or Manual Removal

More often than not, you can remove light layers of rust with simple sanding or scrubbing with a rust remover tools. While this method might be effective, it has its setbacks. Namely, you can easily scrub off important bits of metal and the item will have visible scratch marks. Moreover, it will lose volume and look uneven.

Final Thoughts

So, which rust is the best for treating a rusty object? Well, it all depends on what type of removal you’re looking for.

If you want something done fast, a strong acid will be your best bet. However, it’ll require lots of preparation. On the other hand, weak acids will take longer to get the job done. More importantly, your own body won’t suffer in the meantime and you can easily dispose of the acid later.

Whichever acid you might be using, always remember to pay close attention to the instructions and to wear protective gear while handling them.

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