[TheForge] Phosphoric Acid

[James Binnion]

On Jun 29, 2008, at 7:45 AM, Peter Hirst wrote:

> Repeated the procedure and got a different result?
>
> You sure the products you used were exactly the same?  Phosphoric  
> acid is a rust remover, and should have left the I beams bright, not  
> black,  after cleaning off the acid.  A product called Ospho, which  
> is reputedly phosphoric acid and some secret sauce, although I have  
> heard it described as phosphoric acid alone,  is a rust converter  
> which turns rust to a black polymer and makes a great sealer and  
> undercoat.

If you paint phosphoric acid on rust it converts it to iron phosphate  
a black, water soluble, adherent coating on the surface of the iron  
that has some limited ability to keep further rusting from occurring.  
You can paint directly over it  when dry but I like to use a primer  
first then the paint. The when dry part is the key, it sounds like the  
railing may have pockets or crevices that might have retained liquid  
that was painted over causing the problem. If you rinse it off your  
steel will immediately rust right before your eyes unless you take  
some other measures because that clean, oxide free steel is just  
itching to combine with oxygen from the air and water as it dries.

   According to the manufacturer OSPHO is " a balanced formula of  
Phosphoric, Dichromate, Wetting Agents and Extenders" the MSDS lists  
its active ingredients as being 75% phosphoric acid at a 45% by weight  
concentration. The manufacturer suggests painting it on, then when  
dry, priming and painting.


James Binnion
jbin at well.com