My concrete slab got rust stains on it because the lawn sprinklers were fed from a well that was full of iron. It took about eight years for it to turn a deep rusty color. So I worked out a system that was cheap, easy and effective although one of the steps did take a bit of elbow grease. The whole process only took an hour or so for an 8-ft. x 20-ft. slab. You’ll need to buy a gallon or two of Acid Magic for about $15, a pump up sprayer and a good stiff brush that you can screw into a sturdy handle.
The first thing I did was power wash the slab to get rid of any surface dirt. I’m not sure power washing is important, but I figured any surface dirt might insulate the rust deposits preventing them from being dissolved by the next step-the acid.
In a pump up sprayer, I mixed a 50/50 solution of water and a product called Acid Magic. Acid Magic is a brand name for hydrochloric acid, with a stronger pH than that very same acid you have in your stomach, Buffering makes the acid a bit less caustic to bare skin. But you should still wear rubber gloves and eye protection.
The next day I began the process by dampening the concrete with water from the garden hose.
Then I began spraying the acid on the concrete making sure everything had a good coat. I left the acid on for 30-minutes or so, and resprayed any areas that began to dry out.
About halfway through, I scrubbed all of the concrete with a stiff brush. Then I rinsed everything off and let it dry. I saw that there were a few spots that needed retreating so I gave them another shot with the sprayer, let them soak, scrubbed again and rinsed it all off.
I was afraid that all that acid would kill the grass growing against the concrete so I gave it a soaking for about an hour with a little sprinkler to dilute all that acid runoff.
The grass did die a few inches away from the concrete but seems to be coming back two weeks later. In hindsight, a longer soaking would be a good idea. By the way, my rust stain issues are now over because I added a water softener, which removes the iron from the well water.