How to Remove Rust with Oxalic Acid

Sometimes, you can’t get your hands on a popular water-based rust remover. The retailer might be out of stock, or you might not be able to afford enough of the product due to its price. Of course, the next best thing to do is to use an acid. Acids are easy to buy and, more often than not, they get the job done quickly. All you have to do is pick the right acid to get the job done.

In this article, I’ll cover oxalic acid and its rust removal properties. And while it is far from being the only acid that can remove iron(III) oxide, it’s definitely one of the more budget-friendly solutions out there.

What Is Oxalic Acid?

In short, oxalic acid is a compound that’s odorless and, in its purest form, appears as a white crystalline solid. When you mix it with water, you’ll get a colorless, odorless solution.

While it is classified as a weak acid in terms of strength, it’s still not a compound you should take lightly. For instance, it can cause serious damage when it comes in contact with your skin, in both its solid and liquid form. So, I strongly advise using rubber gloves, a mask, and some safety glasses.

Oxalic acid appears naturally in various different plants and their fruits, such as rhubarb, spinach, buckwheat, almonds, and sweet potatoes. When we ingest it in small quantities, it doesn’t harm our bodies. However, some related salts, like calcium oxalate, are key components in kidney stones if they accumulate over time.

The Many Uses of Oxalic Acid

Because of its properties, people often use oxalic acid for both cleaning and bleaching of various surfaces. For example, the quickest way to find oxalic acid is to search for wood bleach. Due to its potency, oxalic acid can brighten any wooden surface, so people use it to bleach their front porches, railings, wooden stairs, and so on.

In addition to rust removal and wood bleaching, you can also use oxalic acid to clean minerals or anodize aluminum. Interestingly, beekeepers would sometimes coat their bees with oxalate crystals. Some mites, like the varroa mite, attack bees, and the powdery oxalates react as miticides, protecting your winged honey maker against the parasite.

Where Can I Buy Oxalic Acid?

As I stated earlier, you can buy oxalic acid in the form of wood bleach. Naturally, you also have the option of buying pure, concentrated acid crystals, but they are a little harder to come across. Of course, any pure acid will cost a pretty penny, so I honestly suggest going with wood bleach; it’s cheaper, not as dangerous as the pure compound, and you can use it for many different purposes other than rust removal.

When shopping offline for oxalic acid, make sure to visit all your local hardware stores and janitorial equipment shops. Some people I know even managed to find pure oxalic acid in pharmacies.

Removing Rust with Oxalic Acid

The effects of oxalic acid as a remover of iron(III) oxide are well known. In fact, entire studies were done to see just how effective it can be in rust removal. So, how exactly do we use it?

The Preparation Step

Before getting your oxalic acid and the piece of rusty metal that you want to treat, you need to think about your safety. I reiterate, just because this acid is classified as weak, you still require protection. So, rubber gloves, safety goggles, and a mask are an absolute must, since even inhaling the acid can cause you harm. In addition, make sure you read the manual carefully before even opening the oxalic rust container.

The next thing you’ll need is a good container. I suggest transparent containers with lids, since you can literally observe the process in real time. More importantly, the fumes will stay contained within the vat thanks to the lid. However, if you have a lidless container, place it somewhere outside, away from children and pets.

Step 1: Clean the Metal Item

Usually, there is a lot more on a worn piece of metal than iron(III) oxide. Your old nuts and bolts might also have some grime, dirt, or oil stains on them— anything, really. So, before you soak the metal item(s) in oxalic acid, you need to clean them up well. Using any commercial soap and rinsing with water will do.

Some items happen to have lots of rust that flakes in big chunks while others merely have a thin layer of rust on top. If you can, remove as much rust as possible with a stainless steel scrubbing pad or a brush with metal bristles. However, make sure to do it slowly, since you can damage the metal underneath.

Step 2: Prepare the Acid Solution

After cleaning the metal item, you’ll need to prepare the oxalic acid solution. Pour some water in the plastic vat and add the appropriate amount of the acid crystals. However, make sure that you don’t add more (or less) than the recommended amount listed in the package instructions. More often than not, the ratio of water to oxalic acid will be 10:1.

Once you’ve prepared the solution, soak the metal item inside. Do it slowly so you don’t cause a splash.

Step 3: Waiting

Depending on how large your metal item is, you’ll have to wait a certain amount of time before you see the results. The cool thing about oxalic acid is that it works fast. After roughly 20 minutes, you’ll already see major rust areas disappear and make way for the pure steel that lurks below. Leave it on for as long as you deem necessary (24 hours would normally do).

Step 4: Rinse and Dry the Metal

After a certain amount of time has passed, remove the metal from the vat. You should be able to see a clear change, with the rust gone and the metal staying almost completely intact. If you’re satisfied with the result, take the piece of metal to a tap or a hose and start rinsing the acid off. This shouldn’t take you more than 5–10 minutes.

When you’ve finished rinsing, dry the metal quickly with an old rag. The oxalic acid will get the job done, but it’s not a rust prevention substance. So, unless you dry the metal object, it will simply start to rust again.

Step 5: The Finishing Touches

This step really depends on where you are with the metal object you wanted to treat. If, for instance, you still spot any amount of rust, repeat everything from step 1 onwards. Oxalic acid may be a strong remover, but it can also “miss” a few spots if there is too much oxidation.

If, on the other hand, you’re satisfied with the rust removal, you’ll need to coat the object with something. Coating metal allows it to stay clean and avoid the effects of future oxidation.

Now, what should you do with the remaining oxalic acid in the vat? The best answer is to dispose of it by pouring it down your kitchen sink drain. As potent as it is, oxalic acid is biodegradable, so it doesn’t pose any threat to the environment.

Oxalic Acid vs. Other Acids

As I mentioned earlier, there are plenty of acids that people use to remove rust or even rust stains. The ones that are weaker than oxalic acid include the citric, phosphoric, and acetic acids. All three of them can remove rust effectively, which is why people often recommend them over other products. However, they are slow-acting and need more time to remove everything.

On the other hand, there are strong solutions, such as sulfuric and hydrochloric acids, which remove rust much faster than oxalic acid and other weak alternatives. But these powerful acids have one setback in common. Namely, if you leave the metal item in contact with them for too long, the acids will eat through its surface.

A Few Final Words on Oxalic Acid as a Rust Remover

Overall, oxalic acid is not a bad choice if you want to save money on rust removal equipment. It’s not that difficult to find and it gets the job done relatively quickly. Moreover, it’s biodegradable, so you can dispose of it anywhere. However, it is a safety hazard, so make sure you take every safety precaution you can before you decide to use it.

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