This page was last updated on $Date: 2000/03/06 00:01:08 $
Lee M. Daniels - Laboratory for Molecular Structure and Bonding - Texas A&MCompilation of notes regarding derusters:
firstname.lastname@example.org | DANIELS@TAMLMSB.BITNET | (409) 845-3726
"Naughty, you can reform, but stupid is forever" -Andy Capp
>Mostly I've used Naval Jelly and similar phosphoric acid based gels.
>I've also used Metalprep, and for really bad rust cases, muriatic
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.
"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.
>"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.
>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.
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.
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) slowIn 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.
Now, what's the current number one household use for phosphates today? Fertilizer! And especially Iron Phosphate, the product of the reaction that we know and love. I put iron phosphate around my trees annually.
The point is this: I don't wash my used Naval Jelly down the drain, it usually ends up on my driveway or my yard. What a great place for it! It's a fertilizer! And even if somebody did send the runoff from their de-rusted parts to the sewer, it's certainly not enough to ever be noticed like the laundry soap was.
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
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
>"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
>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.
>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
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.
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