On 23/08/19 21:02, Jeff Layman wrote:
On 23/08/19 18:58, Chris Hogg wrote:
On Fri, 23 Aug 2019 18:38:47 +0100, Jeff Layman
wrote:

On 23/08/19 14:39, David wrote:

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.

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.

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.

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.

--

Jeff