“OSPHO” is a good rust converter, comprised of (75%) Phosphoric Acid, Sodium Dichromate, Wetting Agents (Surfactants) and Extenders (pH <2).
Rust converters are solutions or primers designed to be applied directly to a rusty surface to convert residual rust on steel
surfaces to harmless and adherent chemical compounds. The most frequently used formulations are based on Phosphoric Acid, which causes the rust to be converted to a firmly adhering, and protective, Iron Phosphate film.
Rust converters can be simply brushed or sprayed on the surface of the metal, after all loose scale, oil
, and waxes have been removed. The temperature of the metal should be between 50 & 90oF (10 - 30oC), and (if used outdoors) there should be no risk of rain for 24 hours. Within 20 minutes after application, the converter will turn any rust it touches dark grey or black. The reaction is usually cured after 24 hours, but longer if the ambient humidity exceeds 75% to 80%. The acid will eventually cause hydrogen embrittlement*, so use the minimum etch time that removes all rust.
The iron phosphate coating is not very thick, and not very durable, so some additional protection is still required. Rust converters are formulated to be used as primers, which should always be followed with a compatible topcoat. Unlike traditional coatings, the surface must not be sanded.
As with all acids, use rubber gloves and splash goggles, no matter how weak the solution. When thinning acid, add acid to water
. Never add water
To avoid contaminating the stock solution, never pour used rust converter back into the bottle.
Skyco “OSPHO” is one commercial
solution, as are DuPont “QUICK-PREP”, and Sherwin Williams “METAL PREP”. Check the individual product MSDS, noting the constituents, and acidity. The higher the pH, the quicker the acid will etch the metal.
* All acids contribute some hydrogen to the metal structure. This weakens steel by a process called hydrogen embrittlement. I’ve heard it claimed (uncomfirmed) that if you bake the metal at 400F for an hour after acid cleaning, this will drive out the hydrogen and prevent embrittlement.