Sciencemadness Discussion Board - Best Acid that will get rid of rust quickest without harming iron?

Best Acid that will get rid of rust quickest without harming iron?

bolbol - 12-1-2015 at 08:51

I've tried coke, vinegar, nitric acid, NaOH

None worked the way I wanted it. They failed at getting rid of thick rust sediments, except nitric acid but then again nitric acid got rid of the metal too

.

What I haven't tried so far is Conc HCl, Conc Acetic acid, Conc Sulfuric acid.

I've heard Phosphoric acid and HF are good too but they create a passivating layer on the iron which is not what im looking for.

So anyone knows of any fast acting acids that will get rid of rust leaving behind shiny iron?

gdflp - 12-1-2015 at 08:58

Hydrochloric acid will dissolve the iron as well as the sulfuric acid. Glacial acetic acid would most likely work, though you might need to dilute it slightly. Phosphoric acid and hydrofluoric acid will also both work, but be careful, HF is really nasty.

Bert - 12-1-2015 at 09:08

Old fashioned wrought Iron? Cast Iron? Or mild steel?

Bases may be a better choice than acids, depending on what you want to do with the metal after-

Materials for Iron conservation

CHRIS25 - 12-1-2015 at 09:17

I have used HCl on steel and it worked beautifully, about a 6M solution on a heavily rusted replica medieval sword, clean as a whistle.

Hellafunt - 12-1-2015 at 09:39

have you considered using electrolysis to remove rust? i have done it a number of times, and it is super easy, very safe. you only need sodium carbonate as an electrolyte, and you can use a "wall wart" for a power supply. saves a lot of elbow grease. there is mucho info on line.

careysub - 12-1-2015 at 09:41

Phosphoric acid is the acid of choice in most commercial rust removing products (Naval Jelly, etc.).

HCl is probably not popular due to its volatility.

DraconicAcid - 12-1-2015 at 09:46

I've read that oxalic acid can be used, but I've never tried it. You could also try EDTA.

WGTR - 12-1-2015 at 10:33

I've soaked rusty bumper brackets in dilute oxalic acid solution. It worked very well, with periodic brushing and rinsing. In a 5 gallon pail, I mixed in probably 100 grams or so. It took a few days overall for it to work.

alive&kickin - 12-1-2015 at 10:35

I've used oxalic acid numerous times, being here on the beach (seems like everything rusts), and like DraconicAcid says, I can attest to the fact that it works like a champ. I let the metal soak in a saturated solution for as long as it takes (depending on the amount of rust), and it seems to help prevent any new rust from starting back. Haven't done any actual testing but that's just what I've observed over the years.

bolbol - 12-1-2015 at 10:38

Im actually trying to de rust a differential housing.
I can also get my hands on dilute perchloric acid but would it work?

and NaOH so far worked the best of all the ones I listed. It actually softened the rust pretty nicely but then again still lots of work to carve it all off.
I might try glacial acetic acid later, never worked with it tho.

gdflp - 12-1-2015 at 10:45

GAA isn't too bad, just wear eye protection and gloves(any should be fine) and work outside. You can work with it inside, but it will make a room smell like vinegar within a minute of having an solution exposed to air. I would assume perchloric acid would be overkill and attack the iron as well, similar to the nitric acid.

WGTR - 12-1-2015 at 10:52

The thing is that you don't want something that the bare iron will react with. Oxalic acid won't attack the iron without the presence of dissolved oxygen, and it's a pretty slow process. It will dissolve rust quickly by comparison.

I also noticed, like alive&kickin, that it doesn't seem to activate the metal the same way that other acids do. If you use hydrochloric acid, after rinsing the acid off the metal, you will immediately see a thin film of rust flash on the metal as it dries. I have the same problem with phosphoric acid to a lesser extent. With oxalic acid, the bare metal part can sit in a dry garage for a while without seeming to rust. It should still be prepared with a standard metal prep before painting, though.

I would recommend staying away from anything that contains chlorine. Chloride is bad news as far as metal corrosion is concerned.

You can get oxalic acid from a marine or boating store. It is used extensively for rust removal in these applications. I get mine at West Marine.

Fulmen - 12-1-2015 at 13:26

Inhibited hydrochloric acid (Clarke's solution) is another option that eats rust like a champ without affecting steel. It's basically hydrochloric acid with antimony trioxide and stannous chloride added:
http://www.ecfr.gov/cgi-bin/text-idx?c=ecfr&sid=f1491021...

feacetech - 12-1-2015 at 14:07

Boiling in citric acid may work it is slow but will work.

Metacelsus - 12-1-2015 at 17:30

Quote: Originally posted by CHRIS25

I have used HCl on steel and it worked beautifully, about a 6M solution on a heavily rusted replica medieval sword, clean as a whistle.

When I tried this a few years ago (on some pliers), it got the rust off just fine, and the metal was nice and shiny. However, after a week or so, the rust was back, and it was even worse! I think the residual chloride ions (and yes, I did wash it in deionized water) served as an electrolyte to speed up rusting.

cyanureeves - 12-1-2015 at 17:41

sodium hydrogen sulfate or maybe just sodium sulfate. i use sodium hydrogen sulfate on rusted tools and it works great except it leaves the steel jet black.black as ink and it also stains my hands black but it gets rid of rust for sure.sodium hydrogen sulfate is what the label says on our sewer drain cleaner and it's also on Rust Out brand label.

CHRIS25 - 13-1-2015 at 01:21

Quote: Originally posted by WGTR

If you use hydrochloric acid, after rinsing the acid off the metal, you will immediately see a thin film of rust flash on the metal as it dries. I have the same problem with phosphoric acid to a lesser extent.
I would recommend staying away from anything that contains chlorine. Chloride is bad news as far as metal corrosion is concerned.

You can get oxalic acid from a marine or boating store. It is used extensively for rust removal in these applications. I get mine at West Marine.

What kind of Iron/steel composition are you referring to here? I use HCl due to its speed. And I have used it all the time and have never had returning rust. The only thing you need to do is to Thoroughly rinse with tap water, a good rinse. then I always dry with a hairdryer to get the metal hot enough to expel any possible unwanted residues that might be in in any microscopic pores.

[Edited on 13-1-2015 by CHRIS25]

bolbol - 13-1-2015 at 06:51

I used a solution that apparently sucks the iron from rust forming a complex polymer as explained by the label but that thing is pure pure garbage.

WGTR - 13-1-2015 at 17:50

I ran a test with some mild steel coupons.

They were cleaned with degreaser, scrubbing them with a plastic bristle brush. They were thoroughly rinsed in DI water, and then blown off with nitrogen until dry.

5g. of oxalic acid (Fisher) was mixed into 120mL of DI water, and stirred on a hot plate. The solution was heated mildly to about 35°C to speed things up. 40mL of 37% HCl (Fisher) was added to a different beaker.

One coupon was suspended in the stirred oxalic acid solution for about 6 hours. During this time the coupon was periodically rinsed and scrubbed with a plastic brush to remove loose rust. After rinsing again, the part was re-suspended in the solution.

The other coupon was submerged in the HCl solution for a few minutes, towards the end of the other sample's cycle in the oxalic acid. The reaction was very fast.

Both parts, at the same time, were removed from their solutions, and rinsed thoroughly with DI water. For the picture below, after rinsing they were immediately blown off with dry nitrogen. The oxalic acid sample is on the left, the HCl sample on the right. The oxalic acid sample was taking a while, and due to my lack of patience I did not give it enough time to clean up completely. I probably should have left it overnight. The HCl sample is very bright and clean, and only took a few minutes to clean up.

The same samples were then dipped back into their respective acids, and then re-rinsed in DI water. They were then placed wet into an oven pre-heated to 100°C. After drying, a thin film of DI water was re-added about half a dozen times, allowing the samples to dry before reapplication. The complete set of heating cycles took about 30 minutes.

The results are open to interpretation. Personally, I think that the rust on the oxalic acid sample was limited primarily to the areas that weren't cleaned off all the way. The overall sample seemed to darken slightly. The oxidation on the HCl sample appeared to spread and be quick to turn into "red" rust.

Zombie - 13-1-2015 at 23:05

I cant help but wonder why you simply don't take a grinder to it, and progressively polish it back out.
I build boats, and restore cars/bikes for a living, and acid as a rust remover will gain you nothing but added cost/trouble.

I understand it's a "science" question but a "manual" answer.

bolbol - 14-1-2015 at 08:11

Honestly, I like the HCl since its quick. I can sand it off really fast after it has dried to paint it and that will take care of the problem.
Thanks for taking your time and actually doing an experiment of such sort.

etoxiran - 14-6-2015 at 02:51

I suggest use of citric acid with some non-ionic / anionic surfactant (even washing-up liquid will work) eventually with addition of benzotriazole (as corrosion inhibitor). I used it on mild steel, tool steel, cast iron and similar material with good efect. Need only removing carbon black with brush and protecting with oil.
Another option, working with almost every metal (steel, aluminium, copper, monel, SS) is hydrofluoric acid, but is very toxic and hard to get.

AJKOER - 14-6-2015 at 05:26

If you are seeking a method based on cheap and accessible ingredients (like local household items), may I suggest mixing vinegar, salt and chlorine bleach (NaOCl). I have not tried this reaction particularly for rust removal, but I suspect it will work, albeit slowly.

The underlying chemistry is probably on the complex side (not discussed much by pundits, at least, fear of not getting it precisely right, I suspect) as it likely involves an electrochemical reaction mixed in with your standard chemical reactions involving ligand formation and such (via the presence of the acetate). If you are acquainted with my half cell reactions (previously presented on SM, see http://www.sciencemadness.org/talk/viewthread.php?tid=33247#...) for the so called bleach battery consisting of NaOCl + NaCl + vinegar acting on Aluminum metal in the presence of another metal (Copper), just envision replacing the Al with your Iron source which likely is an alloy. Both galvanic cells produce some Chlorine, but in the case of Iron, the Cl2 does not appear to make it of solution.

Caution, your iron source will slowly and unceasingly dissolve forming a caustic mix, so do not let sit for an excess period of time. You can heat the mixture to jump start the reaction. Good luck.

[Edited on 14-6-2015 by AJKOER]

gatosgr - 14-6-2015 at 07:31

What would work to remove sea stains? The reduction potential of Na+ is 2.7V so I don't think any galvanic reaction will work right?

blogfast25 - 14-6-2015 at 08:18

Quote: Originally posted by gatosgr

What would work to remove sea stains?

Sea stains? You mean corrosion by sea water?

gatosgr - 14-6-2015 at 10:36

I mean that some polymers get corroded from salts(from sea spray). Also the polymer adsorbs? the salts and it turns whitish I don't know how to describe it better.

I'm talking about polyurethane polymers I guess there are many unreacted OH groups left that the salt can attack.

[Edited on 14-6-2015 by gatosgr]

blogfast25 - 14-6-2015 at 11:15

Maybe. Not sure what the SRP of Na<sup>+</sup> has to do with anything though...

gatosgr - 14-6-2015 at 13:12

It has something to do with how do you remove the salt that gets into the polyurethane due to open pores from salt corrosion? I'm going slight oftopic but what the heck.

[Edited on 14-6-2015 by gatosgr]