Removing rust stains from a concrete patio is easier than you might think -- all you need is an acid and, no, it doesn't have to be a strong one that will melt your fingernails if you happen to spill some. In fact, you can use an acid that's safe enough to drink, and there are many candidates, including vinegar, lemon juice and -- yes -- cola. Acid won't make a difference if the stain isn't rust, though, and tannin stains from leaves look like rust. That isn't much of a complication; tannin stains come out with detergent and water.
Just Acidic Enough
Rust is iron oxide -- it may have been deposited on your patio by metal lawn furniture, a garden tool you left out in the rain or a fertilizer that contains iron as one of its ingredients. Acids dissolve rust because they contain a large percentage of ionic hydrogen; the hydrogen ions combine with oxygen to make water while the other elements in the acid combine with iron to form soluble compounds. The stronger the acid, the faster the rust dissolves, but strong acids can also damage the concrete. Vinegar, lemon juice and tart soft drinks are all acidic enough to dissolve rust without damaging the concrete. You can also use a commercial rust-dissolving product.
Keep It Wet, Then Scrub
The weaker the acid you use to dissolve rust, the longer it must stay in contact with it, and it must remain wet, or the iron compounds can't dissolve.
Rust Removal Procedure
Mix a 1-to-1 solution of white distilled or apple cider vinegar or lemon juice and water to remove moderate rust stains. If the stains are deep and intense, use the vinegar or lemon juice full strength. If you choose to try to dissolve the rust with cola, don't dilute it -- just pour it straight from the bottle.
Soak the rusted area with a generous amount of solution, then cover the area with plastic sheeting to prevent the water from evaporating.
Let the solution work for an hour or two, checking back regularly and applying more solution to areas that have dried out.
Scrub out the rust with a fiber-bristle scrub brush. Don't use a wire brush -- you might scratch the concrete. Rinse with clear water as you scrub and wash the solution off the patio. The concentration of acetic acid in household vinegar isn't very high, but it's enough to stress and possibly damage your plants, so cover nearby plants or catch the runoff instead of allowing it to flow into the garden. Similarly, prevent citric acid and phosphoric acid from soaking into the garden soil or the lawn.
A Stronger Rust Cleaner
You can buy oxalic acid crystals at a hardware store and mix them with water to make a stronger rust cleaner -- an oxygen bleach powder that contains oxalic acid might work just as well.
Mix a saturated solution -- which is one in which no more powder can dissolve -- and apply it carefully with a paintbrush. Wear goggles and gloves to protect yourself when using this acid.
Let the solution sit for a few minutes, then scrub, rinse and repeat if necessary. Do not allow the runoff to go into your garden.
Neutralize the concrete with a solution of 1/4 cup of baking soda per gallon of water.