Thanks Lee and Mark for your very informative posts regarding rust
removers. You've confirmed my suspicion that those expensive
"environmentally friendly" alternatives work pretty much the same way as
phosphoric acid. That is to say, they mostly likely could cause
hydrogen embrittlement.
I've read four or five articles on rust removers over the years and none
of them included information on how they actually work. In my opinion
this is important information.
DANIELS@LMSBVX.TAMU.EDU (Lee M. Daniels, Texas A&M) writes:
>The phosphoric acid is the good stuff; more on the chemistry involved
>below. I'm pretty sure that Metalprep is mostly phosphoric acid, too.
>The best part is the nice phosphate coating it leaves on the surface,
>which protects it (in the short term) from further rusting.
Yes, Metalprep does contain phosphoric acid. So does the equivalent product
for aluminum.
>"Muriatic acid" is just a solution of HCl; i.e. hydrochloric acid.
>This is *not* the good stuff, because it keeps on dissolving the metal
>after the rust is gone, and leaves bare metal (no coating) which will
>again oxidize very quickly as soon as it hits the air. Use with
Mark_Banaszak-Holl@brown.edu responded:
>
>I hope that helps you. One thing about HCl I didn't mention
>before and I didn't see that he mentioned:
>
>the Cl- ion catalyzes rust formation. (ack)
>
>That is why road salt is so bad.
>
>This is one big advantage of a naval jelly over muriatic acid
>
>Make sure you rinse very well to get all the residual
>chloride ion off.
I have noticed that parts rust again almost immediately after treatment.
I have also noticed that bare steel surfaces (such as tools, nuts and
bolts, et cetera) in the immediate vicinity also rust as well. I had
never associated this with the effect road salt has on auto bodies
though.
If I plan to leave a part for awhile after using muriatic acid, I rinse
it off with water and re-treat with phosphoric acid for the coating.
Guess I've been lucky and have used this stuff in way way that minimized
metal loss. I've painted directly on rusty areas rather than soaking
entire pieces. Strictly out of lazy, not because I thought this was
preferable.
Mark_Banaszak-Holl@brown.edu continues:
>discretion. HCl also dissolves concrete very efficiently ;-) --- I
So does phosphoric acid. Where I've spilled it on the garage floor it
has left a white powdery residue behind. I suppose the acid reacts
with the alkaline concrete to form some kind of salt.
DANIELS@LMSBVX.TAMU.EDU continues (re: muriatic acid):
>it. It comes in plastic bottles, which are *not* impervious to HCl
>gas---don't leave the bottle on concrete, or on metal shelves, or
>you'll find big holes in said concrete or metal.
Hmmm, the cardboard box the bottles came in is starting to disintegrate.
Up 'til now I'd assumed the fumes were somehow getting past the cap.
Guess I better find a better way to store this stuff or get rid of it.
>which performs (in the long run) any better. The "environmental"
>issue is moot here; e-mail me if you want to know why.
I'd be interested in this!
>
>(2) Fe + H_3PO_4 -----> FePO_4 + H_2 (gas)
> slow
>
>You're not going to lose any worthwhile amount of metal to this
>reaction, but this is also the reason you don't leave the acid on the
>part for more than about 15 minutes. The layer of FePO4 that is left
One more good example of useful information. I'd always assumed that
the only reaction taking place was the one involving rust. I've left
stuff soaking in Metalprep for hours, even days. Now I know better!
>Hydrochloric ("muriatic") acid gives the same two reactions with Cl in
>place of PO4, but in this case *both* of the reactions are relatively
>fast, so you lose the actual metal at too fast of a rate. Also, the
Another good example. Use this stuff sparingly, if at all.
>So what about hydrogen embrittlement? OK, this is outside of my
>direct training, so here's my OPINION:
>a structural problem. If your springs are rusted that much, you need
>new ones anyway. I can't see that a few minutes of H3PO4 on the
>surface is going to affect them structurally AT ALL. I may be way
>off base here; if so, someone may want to set me straight.
I agree; badly rusted springs should be replaced. I'm thinking more
in terms of small, impossible to replace, but not structurally critical,
pieces, that might be made of spring steel or case hardened steel. Sounds
like short exposure might be okay.
Good stuff; thanks a lot.
Roland
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