The authoritative public forum
  for Metal Finishing since 1989
-----

Acid that converts rust to inert black surface

I am in the industrial water treatment business. I am looking for an inexpensive acid (It may be phosphoric) that can be used to turn underwater iron surfaces in cooling towers and on water covered chiller parts to a paintable black material. I ran across this in the past but do not remember what acid is used. Ii am looking to use the acid while the systems are down and the surfaces dry.

Brad Ramsey
- Taylorsville, Kentucky
2005

---

I recall coming across a formulation suggesting: 15% phosphoric acid + 4% n-butanol + <0.1% sulfuric acid, but if the solution can be washed away, then a faster acting 33% phosphoric acid + 2% n-butanol mixture is preferred.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2006

---

2006

Years ago I was told of a product by folks that worked on automobiles/motorcycles. They told me this stuff worked wonders & saved them from having to completely rebuild/replace body parts but I have never used it myself. The product is called Ospho [adv: Ospho on eBay or Amazon [affil links] , some type of phosphoric acid compound. You remove any large flakes/peeling of rust and brush/apply this over the remaining surface. It is suppose to return the rust to a firm ferrous base structure and at the same times leaves some sort of seal that prevents further oxidization. Final appearance is a black surface. When I have looked for the product online I recall usually finding it easiest in marine supply stores.

Have no idea if it is safe to use for fresh water piping but don't think the same concerns apply to waste water.

Tony Vaickus
- Chicago, Illinois

---

Maybe mixture of phosphoric and tannic acid ⇦this on eBay or Amazon [affil links] is thing you need. You can use 20 gm tannic/15 gm phosphoric/1 lit water solution-that is simple and cheap rust converter ⇦this on eBay or Amazon [affil links] (instead of water you can use 850 ml water 150 ml alcohol mixture) -- mainly used in conservation/restoration of iron objects.

Good luck!

Goran Budija
- Cerovski vrh Croatia
2006

I worked for 2 years during high school at an automotive paint store and we sold Rustmort [affil links]. The local customers would brush it on and said it worked and sealed the area.

Robert McComas
- Starkville, Mississippi
2006