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best acid to dissolve rust but not steel


pippo
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Special situation, car rust on door, thought of hydrochloric acid, brush on , wait, then rinse with bicarb, then water, then dry asap. But, HCl attacks the "good clean" steel next to rust, so is there a better alternative? Oxalic? isnt oxalic slow reacting?

 

Phosphoric just converts the rust to iron phosphate, and does not "remove" it. Id rather remove it, dissolve it, rinse it away, but afraid of further good/intact steel digestion in the process. Tips appreciated.

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You want a selective chelating rust remover which will just bind with and loosen the rust. There's some stuff called Evapo Rust but Black Strap Molasses is supposed to work as well - I part molasses to 9 parts water. This is soaking stuff in a bath. Maybe you could thin some down with water just enough to work into the rust with a brush and keep wetting the area with it periodically until you have the desired result. It has to be Black Strap and no other type. This type of molasses apparently has a high mineral content which does the chelating.

Edited by StringJunky
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big thanks, people!! So cool to get this advice here!! Now, Moon, phos will "convert" the rust, but wont remove/dissolve it. It will convert it to Fe Phosphate (or phosphite??). I was hoping to REMOVE the rust, not just convert it. But phos may be my only practical solution, I fear. In auto body circles, converting is frowned upon.

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big thanks, people!! So cool to get this advice here!! Now, Moon, phos will "convert" the rust, but wont remove/dissolve it. It will convert it to Fe Phosphate (or phosphite??). I was hoping to REMOVE the rust, not just convert it. But phos may be my only practical solution, I fear. In auto body circles, converting is frowned upon.

 

 

Well you do have to apply a little elbow grease, you remove the loose rust, swab it liberally with jelly wait then rinse off smooth out with a sander or sand paper and paint over it. works quite well and stop rust in it's tracks. 40 years of driving jeeps on the beach...

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Steels are often pickled in a solution that contains both hydrochloric and nitric acids. These solutions usually contain some inhibitors as well to protect the clean from attack by the acids. Numerous commercial pickling solutions are available but the recipes for them are guarded secrets.

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Well you do have to apply a little elbow grease, you remove the loose rust, swab it liberally with jelly wait then rinse off smooth out with a sander or sand paper and paint over it. works quite well and stop rust in it's tracks. 40 years of driving jeeps on the beach...

good tip- I will try that

The iron phosphates are somewhat soluble in the excess acid.

Thanks, John, but not sure what you mean- asssuming you apply the phos acid, it converts to iron phosphate (excess phos acid is at this point "gone"), why would the phosphate product still be in an acid environment? I mean, its not like the phos acid keeps continually dripping down on the rust area.......

 

I would expect he phosphate product forms in a dry environment, no more phos acid around.....

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"it converts to iron phosphate (excess phos acid is at this point "gone")"

That's not what excess means is it?

Unless you plan to carefully measure the acid to get exactly enough to react with the rust, but not more, you are going to have leftover acid.

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Youre not goinf to have left over/excess acid, as the extra phos will drain off. If not enough phos was doused on the rust, repeat of the phos application (say, with a brush) will react with the unreacted rust. Again, after a brushing/dousing, the 2nd application of phos wil allow the excess to again drain off. Repeat until theres no more rust to react with another application of brushed on phos. Why would there be excess of phos under these conditions? Fe phosphates then ultimately form , rust is completely reacted, and no excess of acid.

 

Thats why I was unclear when you said Fe phosphates are soluble in excess phos acid. I took that as a caveat- react the rust to a phosphate, but "beware" as they will redissolve type of thing. I hope they will not redissolve. I actualy understand they are very insoluble (could check the Merck index on that, I guess.....)

 

Thanks, John.

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