Clean Rust Off Concrete

By Rob Robillard





This article will help you save time, labor and money by showing you the best way to clean rust off concrete or bluestone. Many homes have concrete and bluestone walkways, steps and patios. The problem is that rust stains can occur from patio furniture, umbrella stands, fire pits or toys. These stains can be very difficult to remove.

Following are the necessary functions needed to easily remove a rust stain from your concrete using muriatic acid.

The metal stand of a patio umbrella created unsightly stains on the patio surface.


Muriatic acid should be a measure of last resort. Use it only after you have already tried less caustic cleaning agents.

Safety Information

As a safety precaution, do-it-yourselfer’s should avoid muriatic acid where possible. Only use muriatic acid after exhausting other cleaning methods like TSP (tri-sodium phosphate), or less caustic concrete stain removers.

In other words, muriatic acid is not the first choice for masonry cleaning but the last resort.

Muriatic acid is a highly reactive liquid acid, and one of the most dangerous chemicals you can buy for home use. It is an industrial-strength solution of hydrogen chloride gas dissolved in water, also known as hydrochloric acid. With the exception of some plastics, muriatic acid can damage most anything it touches, including clothing, metal and skin. It emits a suffocating odor that can quickly burn the lining of the nose, throat and even the lungs.

Typical home applications include heavy-duty masonry cleaning, preparation of masonry for painting or sealing, removal of efflorescence or mineral deposits and pH reduction in swimming pools. If you consider using muriatic acid, please heed all safety recommendations both here and on the product’s label.

Contact with the eyes, for example, can cause irreversible damage and permanent blindness. Contact with the skin can cause severe burns. Dress appropriately; wear safety glasses, acid-resistant gloves, long sleeves and pants, and use a NIOSH-approved respirator equipped with the appropriate acid-grade filter.

Additionally, have a neutralizing agent and a reliable, steady source of water available. Baking soda or garden lime can quickly neutralize the acid if spilled. Water should be freely available in case you accidentally get acid on your skin.

Keep baking soda or lime on hand to neutralize the acid.

Since muriatic acid can damage or kill foliage, cover or wet all nearby foliage with water before application of the acid.

Work in an area with adequate ventilation. Use a fan to bring fresh air to the work area if necessary. Muriatic acid is nonflammable, but the vapors are highly corrosive and irritating. Using muriatic acid indoors is not recommended. The corrosive vapors can begin chemical reactions in metals, leading to long-term permanent damage.


Cleanup & Disposal

In some cases you may need to clean up an accidental spill. Spreading a generous quantity of baking soda or lime (the powdered or crushed type used for lawn or gardens) and adding water will cause a distinctive “fizz” as the chemicals reacts with the acid, releasing carbon dioxide and producing harmless salt and water. Garden lime is less expensive than baking soda and is sold in larger bags.

Muriatic acid should never be poured down a sink or storm drain, or flushed down a toilet. Doing so can cause extreme damage to pipes, dissolve solder and damage the biological balance of your septic system. Throwing away even a closed container of muriatic acid with the trash can be dangerous for trash handlers, their trucks and possibly cause unexpected chemical reactions in landfills. Neutralize your container prior to discarding. (Call your local recycling center for more information.)


How to Use Muriatic Acid

Muriatic acid should be diluted to at least 1 part acid to 10 parts water. For small areas, a quart container of acid is usually sufficient. Mix the acid and water in a plastic container. Always pour acid into water, never water into acid. Mixing the two causes a reaction that gives off heat. This reaction is much more sudden and violent when water is poured into the acid.

Let the acid sit for no more than a few minutes, less if you can see the rust lifting.
Always pour acid into water—never water into acid.

The diluted acid can be applied with a long-handled masonry scrub brush or sprayer, depending on the circumstances. If you’re going to spray, it’s best to buy a cheap sprayer and throw it away when finished. If using a sprayer to apply the acid, take precautions to cover nearby objects and vegetation with plastic. Do not use a sprayer on a windy day.

Neutralize the acid with baking soda or lime and rinse.
Let the acid sit for no more than a few minutes, less if you can see the rust lifting.


Steps to Cleaning Masonry

Begin by wetting the concrete stain and all of the surrounding area.

Mix the acid with water. Although, one part acid to 10 parts water (by volume) is typical, dilutions as light as one part acid to 16 parts water also work well. (Note: A 1/16 ratio is 1 cup acid to 1 gallon of water.) Read the label on your product and follow the recommendations.

Brush or spray the acid onto the affected area. Do not use a metal sprayer. A plastic sprayer will work for a while, but will eventually be destroyed by the acid. For large jobs, have a few extras nearby and throw away used sprayers when finished.

For stubborn stains, reapply the acid to the stain only and repeat the cleaning process.

Let the acid sit for no more than a few minutes, and even less if you can see the rust lifting.

Scrub off any remaining residue with a stiff brush while rinsing thoroughly with water. Long-handled brushes are ideal for this job.

To minimize the difference in concrete appearance after cleaning, work only on the stain and rinse quickly. The longer the acid is left on the concrete, the more it will clean and possibly etch the surface.
Rinse the area thoroughly with a high-pressure garden hose.

Neutralize the acid with lime or baking soda and rinse thoroughly. Use a garden hose with a high-pressure nozzle.

For stubborn stains, Rinse the area again, wetting the stain thoroughly. Apply a small amount of muriatic acid directly to the stain and repeat the cleaning process.

You need to be aware that this process may leave your concrete or blue stone looking cleaner than the surrounding area. To minimize this difference in appearance, work only on the stain and rinse quickly, trying to keep the acid mixture on the surface of the stain. The longer it is left on the concrete, the more it will clean, and possibly etch the surface creating a noticeable contrast.

You can also use lime to neutralize leftover muriatic acid. Get a large bucket. I prefer the 5-gallon size since the chance of dangerous spattering is minimized in a large bucket. Put three or four cups of lime in the bottom of a |gallon of water. Give it a stir. Slowly add the acid to the bucket, keeping your face away while pouring (and wearing your respirator). Stir, adding more acid and more lime until all the chemical “fizzing” has stopped.

The fully neutralized acid can then be safely disposed down a sink or storm drain without fear of damage to your septic system or the environment.

Note: Consider recycling your leftover muriatic acid. If you know someone who owns a pool you can give them the leftover acid. It’s a great pH reducer.


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