Removing Rust on Concrete 2012

In May 2012 (or so), I bought a bag of "Scotts Turf Builder Green Max" Lawn Fertilizer 26-0-2 with 5.17% Iron.

It rained the night I spread it, and the next day my concrete front porch was 60% covered in rust streaks.

To be fair, the instructions on the back of the bag say: "Step 3 -- Clean up. The ingredients in this product may stain. Sweep product from hard surfaces back onto the lawn and remove particles from clothing or shoes. Do not clean spreader over hard surfaces or near storm drains."

However, to me stain is something that happens to clothes, or occasionally wood when I paint 'wood stain' on wood. It says hard surfaces, but it never occurred to me that this mess would happen to concrete.

Stuff that didn't work

  1. blasting it with a water hose.
  2. Palmolive Dish Soap
  3. White Vinegar
  4. CLR. (Calcium Lime Rust cleaner). Everyone I talked to expected this one to work.
  5. TSP (Tri-Sodium Phosphate powder)
  6. A neighbour graciously loaned me a product, I think it was Stone Cleaner, and while it removed some dirt it did not remove any rust. It was meant to clean concrete, but didn't mention rust in the list of things it expected to be successful with.
  7. blasting it with a presure washer (Simoniz S1800 Electric Presure Washer)
  8. Evapo-Rust (Canadian Tire Product #47-7902-2)
  9. Zep Calcium, Lime and Rust Remover (Home Depot model CACAL128, sku 1000137888)
  10. Technoseal Rust Remover for Pavers (Rona article #7924095 model #141-521) -- almost worked
  11. I looked for, but didn't find: Singerman Laboratories Concrete Rust Remover
  12. I looked for, but didn't find: Super Iron Out 30-oz (Canadian Tire Product #53-2033-4)
I spent a couple hours with the above products, with little success. The dirt came off, but not the rust.
Technoseal Rust Remover for Pavers was the first time that I had hope, as perhaps a third of rust was removed. I poured it on, and used a deck brush (2" x 8" on a broom handle) to scrub it into a white lather, then a bit more elbow grease for a few minutes, then rinsed it off with a garden hose.

I didn't try CocaCola (aka phosphoric acid), nor Sulphuric Acid, oxalic acid, or lemon juice.

What worked

  1. Muriatic Acid (4L, Rona article #20094095 model #50-AM001)
Muriatic Acid is Hydrochloric acid. A decade ago I was wearing blue jeans and got a little bit of acid on them, and three days later the fibers of the blue jeans fell apart in spots and I ended up with holes. I've seen acid melt human hair in shower drains, so I wouldn't want any on my skin, and it can't be good for the grass/lawn. I considered the effects of acid on a scrub brush, but ultimately I didn't use a brush so that was moot. I really really wanted to try the acid last -- or to put it another way I really really didn't want to pour 4L of acid near my lawn.

June 18 2012, I wore gloves, and a face shield. I had a painter's outfit, that covers everything but face/hands/feet, but I didn't put it on. I wet down everything with water first (obeying the rule of adding acid to water, don't add water to acid) I poured close to the ground to avoid splashes. My front porch is 25 square feet, which I did in 4 square foot chunks at a time. I'd pour to cover 4 square feet, and stand back. It smelled like rotten eggs, let off steam, turned yellow, and bubbled. After about two minutes, the steam stopped, and another minute the bubbling stopped. At which point I rinsed it off with the power washer -- and the rust was gone. Totally. I moved onto the next 4 square foot section (2'x2'), and did the same. I figure that when the acid stops bubbling, its effects on the lawn/grass are minimal, nonetheless I wanted to dilute/deactivate it with lots of water. When I was done, I did a thorough rinse with the power washer. Then I disconnected the power washer and just used a hose to add more water, flooding the area. Then I ran a sprinkler for an hour, to douse both the porch as well as the grass, further diluting any remaining acid. I also washed my clothes immediately afterwords, in the pointless hope that any acid that splashed on them would rinse off. In the end, for this 25 square feet, I used 3L of Muriatic Acid.

The concrete is awfully clean. Some might call it etched. I hope I haven't removed any surface that's required to keep the concrete from falling apart over the winter.