cleaning rust stains off concrete

Cleaning Rust Stains Off Concrete

By Brad Caldwell – Owner of Roof, Rinse & Run – March 8, 2014
I recently performed a test of cleaning rust stains off concrete, using various touted chemicals.
Here’s the ranking of the results:
  1. HCl (Hydrochloric acid). Also known as muriatic acid, this one did the best (as in, it was the strongest chemical).
  2. CLR (Calcium, Lime & Rust Remover). This one performed well, but didn’t seem to be economical. You only get a small amount of chemical in the bottle.
  3. Vinegar. In a sense, this one was the best. That is, if you have a small area to remedy. Scrubbing may be required on this one, so if you’ve got lots of rust, you may want to go with the hydrochloric acid.
  4. B.K.F. (Bar Keeper’s Friend). Albeit a great chemical for cleaning about anything else, it didn’t do well on removing rust, at least not for our test. Oxalic acid is what powers B.K.F.
  5. Lemon Juice. This one didn’t really do anything. Though highly touted as being great for removing rust stains, we didn’t find it to be so.
  6. T.S.P. (Trisodium Phosphate). We weren’t able to follow the instructions on this precisely, because it asked for warm water, which we didn’t have access to, but if our test was any indicator of what it does when applied properly, it was a dud.

But enough talk, take a look at the pictures and see what you think.

Test Results: What Works Best in Cleaning Rust Stains Off Concrete?

Rust stain after HCl
A Rust Stain After Coming in Contact with Hydrochloric Acid – Not Much Left
Rust stain after HCl and scrubbing
After Hydrochloric Acid and Scrubbing, the Rust Stain Comes Mostly Clean
after pouring CLR on a rust stain
After Pouring CLR on a Rust Stain – Works Fairly Well
After CLR and scrubbing on rust stain
After Scrubbing Area of Rust Stain Where CLR Had Been Poured – Pretty Clean
Vinegar does a good job in removing rust
Vinegar Alone with Not Scrubbing – A Decent Improvement in the Rust Stain
Vinegar and a little scrubbing
The Same Spot After Vinegar and Some Scrubbing – Coming Very Close to Clean
Vinegar plus lots of scrubbing
Vinegar Plus a Lot of Scrubbing (With Wire Brush) Gets the Concrete Brand-New!
Bar Keepers Friend doesn't help rust stains
Bar Keeper’s Friend – Great for Everything Else, Not So Much for Rust
Lemon juice a poor performer for removing rust stains
Lemon Juice Just Didn’t Do Anything to Remove Rust
Trisodium phosphate loses the contest
Trisodium Phosphate Came In at Last Place – The Rust Stain Was Nothing Helped (By the Way, This Rust Stain Was Less Pronounced to Begin With)

Conclusion: Which Worked Best in Cleaning Rust Stains Off Concrete?

My conclusion was that, while hydrochloric acid was the strongest and most powerful option, it may or may not be the most desirable. In fact, if you’re not careful with it, you can end up creating acid stains in place of your rust stains!

don't replace rust stains with acid stains
As You’re Cleaning Rust Stains Off Concrete, Don’t Create Acid Stains!
Don't create acid stains
While These Two Stains Are Mild, Carelessness with HCL Would Be Disastrous

My recommendation is, trying using regular old store-bought vinegar first. If you’re able to scrub and remove the rust stain, leave it at that. If your stains are really stubborn, or quite extensive in area (such that scrubbing is prohibitive), then you may want to try to HCl. Just make sure to dilute it somewhat and rinse it off the concrete quickly.

Safety Considerations When Using HCl in Cleaning Rust Stains Off Concrete

Okay, we brought up a dangerous subject, so here’s a bit of safety advice. As always, we’re not telling you how to properly do it, we’re just giving you some common sense tips. HCl is a dangerous chemical, and you use it at your own risk. We are not providing you with all the safety information you need here in this article. But here are some pointers.

Muriatic acid is the most potent choice by far. However, be prepared to do your whole concrete surface, as the acid changes the color of the concrete from having a hint of yellow to having a hint of blue (see the yellow dotted line in the HCl pictures). Also, you want to stick to one chemical, and, as much as possible, one chemical strength. Otherwise, you will get “rivers” of different color running down your driveway at the end of the project!

Also, try to dilute the acid down from 45% (which is how it came in the bottle for me) to maybe 4 or 5 parts water, 1 part “bottle solution” so that the end product is something like 10% acid), unless you need it full strength. Remember, diluting acid is a funny thing – you want to start with a bucket of water, and add acid to that bucket of water (don’t add water to a bucket of acid, unless you want an exothermic reaction).

If you have a really stubborn rust stain, and you’re not concerned about seeing a little localized acid-discoloration, pour some acid straight out of the bottle (45% acid) and carefully pressure-wash it off within 1 minute (if you wait 3 minutes, you’ll have a nasty acid stain left behind). Make sure to neutralize the acid runoff with baking soda.

In the pictures, “scrubbing” means scrubbing at the spot with water, after you’ve rinsed the acid off. You don’t want to be scrubbing and have particles of acid flying toward you!
Hydrochloric (muriatic) acid is very dangerous – it requires protective clothing, as goggles, rubber boots, vinyl gloves, long sleeves, long pants, etc., as well as being very careful not to breath the fumes as you are diluting the acid or the few seconds after you have poured it onto the concrete. Nothing has fumes like hydrochloric acid does – you won’t even be able to breathe it, even if you didn’t care – it’s that strong. But it’s relatively easy to just run a couple of steps over and get out of the way of the fumes until they blow away. Be careful not to allow the acid to splash on you. If you’re using a 5-gallon bucket to dilute it, the solution has a tendency to splash back at you if you dump a little acid here and tilt back up (when you tilt back up with some solution still in the bucket, that’s when it wants to splash at you); therefore, I almost think it’s safer to get a got shot with your bucket, and dump the whole things carefully all at once, making sure not to let the flooding solution approach to your feet (unless you have rubber boots on).

Also, as you dilute the acid, be sure to get water in the bucket first, THEN add the acid (and not vice-verso – otherwise it could “explode” and get on you).

Also, it’s good to have some baking soda on hand – (1) in case you get any acid on you, you can neutralize it, and (2) you can neutralize the acid solution you pour on the concrete before it goes into runoff. If you’re using a few gallons of acid (diluted, many gallons of solution), you need a whole lot of baking soda – like ten or twenty small grocery-size boxes.

Also, hydrochloric acid goes crazy with metal, but doesn’t seem to react at all with plastic, rubber, etc. So, mix your acid in a plastic bucket, use plastic tools, and don’t let your acid hit anything metal at all. And should anything metal come in its path, be prepared for it to eat through it and create toxic fumes!

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