The reddish-brown rust stains on your driveway or patio may have come from a radiator leak, a metal tool that was left out in the rain or from lawn fertilizer that contains iron as one of its ingredients. The strategy for removal is simple -- dissolve the rust with an acid -- but that doesn't always work because iron oxide -- rust -- can react with ingredients in the concrete to create permanent discoloration. Moreover, the stains may not even be caused by rust; tannins from leaves or conifer needles have the same color. You remove by scrubbing with detergent.
Acids that Can Remove Rust Stains
Acids can dissolve rust, but strong acids, such as muriatic acid, can also etch and damage the concrete and should be used only as a last resort. You can make a list of household cleaners and condiments that can do the job because of the acids they contain:
- Vinegar -- acetic acid
- Lemon juice -- citric acid
- Tart soft drink -- phosphoric acid
- Milk -- lactic acid
In addition, some drain cleaners contain sulfuric or hydrochloric acid, but, like muriatic acid -- which is a form of hydrochloric acid -- they are strong and may damage the concrete.
You can also opt for a commercial rust remover. One such product contains phosphoric acid as its main ingredient.
Rust Removal Procedure
Start by choosing an acid. The best choices are vinegar, lemon juice, certain soft drinks and commercial rust removers. Although the lactic acid in milk has a pH comparable to that of citric, acetic and phosphoric acid, it doesn't occur in a high concentration. Consequently, milk may take a prohibitively long time to make a difference.
Pour the acid liberally on the rust stains. Cover the concrete with plastic to prevent evaporation and leave it for several hours or overnight.
Remove the plastic, pour more acid and scrub the spot with a stiff-bristle scrub brush.
Rinse with water and check your progress. If significant discoloration remains, repeat the procedure.
Mix a solution of 1/2 cup of baking soda per gallon of water and wash the spot to neutralize the acid. If you don't do this, the acid may continue reacting with the concrete to produce a brand new stain.
The Detergent Option
Acids don't always work, especially if the stain isn't actually rust but tannins from leaves. If you aren't having any luck with vinegar or a comparable acid, try removing it with laundry detergent.
Mix a cup of laundry detergent with 1 or 2 tablespoons of baking soda, or purchase a detergent that already contains baking soda. Make a paste with water.
Wet the concrete and spread the paste generously over the stains. Scrub it in with a hard-bristle scrub brush, then spread more and leave it for an hour, wetting it occasionally to keep it from drying out.
Scrub again, then rinse the spot with clear water. Besides removing the stains, this procedure may lighten the concrete somewhat.