**WARNING - >100 LINES OF MARGINAL BRIT-CAR CONTENT** (unless yours
is as rusty as mine.)
Roland Dudley <cobra@cdc.hp.com> wrote:
>Mostly I've used Naval Jelly and similar phosphoric acid based gels.
>I've also used Metalprep, and for really bad rust cases, muriatic
>acid.
Jaguar
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.
Lotus
"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
discretion. HCl also dissolves concrete very efficiently ;-) --- I
used a 10% solution of it to "etch" my garage floor before painting
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.
MG
>"organic", environmentally safe de-rusters [...]
>Do these organic de-rusters work the same way as the acid based ones do?
Pretty much the same chemical strategy as phosphoric acid: dissolve
the rust, coordinate the surface Fe atoms with something that will
keep them from grabbing oxygen and turning into rust too quickly. I
can't recommend them over phosphoric acid at all; I haven't seen one
which performs (in the long run) any better. The "environmental"
issue is moot here; e-mail me if you want to know why.
Healey
>Are there any de-rusting chemicals (for home use) that do not cause
>hydrogen embrittlement in such alloys? How about these so called
>environmentally safe de-rusters.
Triumph
Hang on, I'll get to this in the chemistry section below.
>Also something I've mentioned before- trisodium phosphate crystals
>... Drano ... bare metal ... Sure wished I could use this trick on
>my snake body.
You can - that's just what places like "Redi-Strip" use. The "dip
your frame/body" string was recently discussed here.
Lucas
OK, the rest of this is a little more detail on the chemistry and
practice of the derusting question. If you haven't flushed this
message already and don't really care about the details, bail out now.
=====================================================================
There are two main reasons for using phosphoric acid for rust removal:
It dissolves rust at a much faster rate than it dissolves iron, and it
leaves a nice iron phosphate coating on the clean metal surface. The
reactions are: (a "_" before a number means to subscript the number.)
(1) Fe_2O_3 + 2 H_3PO_4 -----> 2 FePO_4 + 3 H_2O
fast
(2) Fe + H_3PO_4 -----> FePO_4 + H_2 (gas)
slow
In reaction 1, the rust (Fe2O3) gets turned into iron phosphate and
water; this mostly gets washed away when you rinse the part. The
phosphate part of phosphoric acid is responsible for this reaction.
Reaction 2 is the reason you may see some bubbles. The iron itself is
actually dissolving, but this is a relatively slow reaction. The H+
ions from the phosphoric acid are responsible for this reaction.
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
on the surface adheres strongly enough due to surface effects that it
does not wash away with the rinse, hence the good protection from
further rusting. (Note: reaction 1 is a simple exchange, 2 is
oxidation-reduction; i.e. Fe+++ ---> Fe+++ and Fe(0) --> Fe+++.)
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
FeCl3 produced does *not* stick to the metal surface, so you get no
protection of the clean metal surface. For those of you took any
chemistry, HCl is a "strong" acid, while H3PO4 is a relatively "weak"
acid. (These terms relate to the percent ionization in solution.)
The reason that the iron oxide does not provide surface protection
like the phosphate does is that the oxide flakes off, continually
exposing new metal to continue the oxidation process. On aluminum
parts, the aluminum oxide *does* stick and is the reason why "bare"
aluminum is so stable (pure aluminum metal is actually more reactive
that pure iron.) This is also the reason that antifreeze solutions
that contain phosphates should not be used in engines with aluminum
heads; i.e. the phosphates break down the protective aluminum oxide
coating on the parts. Surface oxide on iron is the bad stuff, but
surface oxide on aluminum is the good stuff, see?
Now, as for the "organic" rust-removers; I don't know nearly as much
about them, but they operate on the same principle, AND there is still
an acid involved. It just happens to be an organic acid in this case.
As far as being "oxygen-scavengers", this is exactly what the
phosphoric acid is doing, too, i.e. the "oxide" part of the rust is
turned into water --- see equation 1 above.
So what about hydrogen embrittlement? OK, this is outside of my
direct training, so here's my OPINION:
This would be a serious concern if you were to submerge parts in acid
long enough to allow substantial reaction and substantial generation
of hydrogen. When phosphoric acid is used: 1. You generally don't
want to leave it on long enough to even get much evolution of hydrogen
anyway, 2. We're talking about *surface* rust here. Embrittlement is
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've spent waayy too much time on this today - I better get back to
work.
Lee M. Daniels - Laboratory for Molecular Structure and Bonding - Texas A&M
daniels@lmsbvx.tamu.edu | DANIELS@TAMLMSB.BITNET | (409) 845-3726
"Naughty, you can reform, but stupid is forever" -Andy Capp
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