Using phosphoric acid on sheet steel

Any of these work. Kitchen roll is good if you have a local area that is particularly corroded.

The rust eater products used to come as a gel with a brush. Fine for small to medium spot treatment.

You don't say how much rust there is to treat.

Worth considering using some strong plastic sheeting to rest the BBQ bits on whilst you are treating them. Acid tends to rot most things it comes in contact with, especially clothing.

Cheers

Dave R

... and teeth. Many fizzy drinks contain a significant amount of phosphoric acid. Dentists use concentrated phosphoric acid to etch the surface of cavities so as to improve the adhesion of composite filling resins. It also makes an excellent flux for soldering to stainless steel.

John

It's not the rotting which would concern me - it's the splashing of the ferric phosphate (formed when the rust reacts with the phosphoric acid). Any porous surface, including clothing, wood, stone, etc will get stained and such stains will never be able to be removed completely.

Hmmm...ferric phosphate is practically insoluble. That's not to say that you wouldn't get bits of ferric phosphate flaking off the treated sheet, but I doubt it would stain. And to do it properly, you should wire-brush or otherwise abrade the steel first to remove loose bits. The whole idea of treating rust with phosphoric acid is that the acid reacts with the rust to give an insoluble, fairly impermeable skin on the steel.

Save your money next time, use HCl. HCl leaves a surface that rusts very quickly, it must be dried & coated right away when rinsed off.

NT

Indeed. There is little doubt about the "black" ferric phosphate being completely insoluble. However, the Wikipedia article for ferric phosphate says that the dihydrate has a solubility of about 0.67g/100ml, which would be enough to make a stain mark. But how and under what circumstances it is formed I haven't been able to establish. I might have a play with some phosphoric acid rust remover and rusty nails to see if the solution left on the surface of the blackened nails can stain porous surfaces.

Get it done by a company? Buy a new one. Brian

Brush on, cover in clingfilm?

Thomas Prufer

?? As suppled phosphoric acid was clear. I can't recollect having my clothing or cleaning materials stained.

It turns your fingers black after a while, but it's not permanent.

Not sure how it would work with a barbecue though. Phosphoric acid is not used to remove rust. It is a rust "converter". The stuff turns the oxide coating into a phosphate, preventing further corrosion. Whether this would be effective on a sheet mild steel barbercue, I wouldnt like to predict.

The adverts in the 70's told fibs incidentally, it does nor "turn rust into good metal" :-)

AB

I just tried it with Loctite 7503 Rust Remedy. The stuff is a greyish solution and stains by itself - never mind any rust! I did it by touching the surface of cotton material and a piece of 15x38 pine with the solution 15 seconds after applying it to the rusty surface. The solution remaining on the converted "rust" left stains which were a lot darker. BUT it took me some time to find that Loctite 7503 contains tannic acid. There is no phosphoric acid in it at all!

So if anyone would like to repeat this with phosphoric acid and report back, it would be interesting to see the result.

No stain is permanent on the surface of skin as the cells are shed after time, but the fact there was a stain at all is interesting. Have a look at my reply to myself (to Chris Hogg's comment). If you've got something rusty and phosphoric acid, it would be of interest to repeat what I had tried3.

I would assume the ferric phosphate formed is heatproof to some extent. I doubt it would stand up to any scraping though!

Why am I not surprised?...