removing rust with citric acid

dian · Jul 17, 2020

i thought i would share my latest findings.

if you have light (surface) rust, a saturated citric acid solution will work well and not discolor steel like other methods. let it soak overnight, rust will be gone and a polished surface still shiny. its especially interesting if the part is large, because citric acid costs nearly nothing. i got 11 lbs for $15 and that is food grade. this gives you 10 gallons of solution for around $2.

i left a polished piece of mild steel in there for 3 days and it got a very light coating (i wonder what it is) that was removed with a kitchen sponge quickly. nothing like the black phosphate you get with phosphoric acid. unlike vinegar (the other inexpensive rust remover) citric is odorless.

Bdog507 · Jul 17, 2020

Good afternoon.

That's interesting. I work on motorcycles, & often find rust in the petrol tank, especially on bikes that have been standing.
I use a product called Bilt Hamber Deox-c which is available here in the UK. It leaves the steel bright & shiny with no residue whatsoever. As it's considerably more expensive that Citric acid I'm going to give Citric acid a bash & see how it performs.

Cheers.

Stew.

janvanruth · Jul 17, 2020

beware of hydrogen embrittlement

Phil in Montana · Jul 17, 2020

hydrogen embrittlement would not a problem, citric acid works so slow it would take years, the water would rerust the part before that would happen. I use citric acid to clean water passages in old engines...Phil

crrmeyer · Jul 17, 2020

Interesting. What dilution ratio are you using? Have you tried it with heavily rusted steel?

I currently have been using animal grade molasses diluted 10:1 with water for rust removal. In the attached photo, I have two fairly rusted 1/2" nuts from the same project where one was soaked in molasses for 6 days and the other one is in the as-found rusted state. Post molasses bath, I wash with water and air dry. The I use red scotch bright and solvent to polish off any left over film from the molasses or sometimes a thin layer of black oxide? It took about 30 seconds to clean 8 surfaces on the nut as shown.

I'll have to try citric acid and see how that compares to molasses.

dian · Jul 18, 2020

actually i have a number of pipes outside sitting in 15% molasses. it works and is also inexpensive, but it usually takes a week or more as you say. insects get attracted and in winter it takes even longer indoors. it was not easy to get molasses, btw.

the citric acid is saturated and and only for light rust, because its slow. i find its less of a mess than molasses, would be interesting to see which is faster.

for heavy rust i use phosphoric but its expensive. i think for big, really rusted parts electrolysis would be the way to go.

swarfless · Jul 18, 2020

I use molasses 1:9 of water to de-rust but also to descale hot-rolled steel prior to machining. In summer it takes 2 days to perform the latter function. It CAN look like not much has happened but when hit with a jet of water from the garden hose the surface is left clean light grey almost like cold-rolled. My 'vat' is a 200litre black, poly pickle barrel with screw-on lid. Outside, it only gets the sun from noon on but the heat helps. Must try citric acid for small items.

dian · Jul 18, 2020

i have noticed molasses are more popular down under than elsewhere. probably due to temperature.

Newman109 · Jul 18, 2020

dian said:
actually i have a number of pipes outside sitting in 15% molasses. it works and is also inexpensive, but it usually takes a week or more as you say. insects get attracted and in winter it takes even longer indoors. it was not easy to get molasses, btw.

the citric acid is saturated and and only for light rust, because its slow. i find its less of a mess than molasses, would be interesting to see which is faster.

for heavy rust i use phosphoric but its expensive. i think for big, really rusted parts electrolysis would be the way to go.

Locally near me at a chemical outlet, phosphoric is sole in 70% solution for $20-25 a gallon. It can be cut with water to make up to five gallons. It works well when diluted only takes longer to remove rust. It does darken metal, however but that's a good thing for me, since it won't rust again very soon.

Heated phosphoric acid is the basis for the Parkerizing process that is sometimes used on gun and automotive parts.

Parkerizing - Wikipedia

richard newman · Jul 18, 2020

I've citric acid to derust parts. It worked fine, but I forgot about a batch for and it ate into the steel. Don't remember how long they were in the bath, sorry. Now I prefer Evaporust, safer if stuff is in too long.

EmGo · Jul 18, 2020

dian said:
it was not easy to get molasses, btw.

Consider yoruself lucky, used to get it on my pancakes. Reminds me of ...

I hate Bosco !
It tastes like chocolate pee.
Mommy puts it in my milk to try to poison me !

I fooled mommy
I put it in her tea
Now there is no mommy, to try to poison me !

Used to be common at Safeway but I haven't seen it there in years. Now they just have that phony Aunt Jemima crap that isn't maple syrup and isn't molasses. Bain Capital musta bought them.

crrmeyer · Jul 18, 2020

You can buy 5 Gallon buckets of molasses pretty cheap at the animal feed store.

Homebrewblob · Jul 18, 2020

I second the Phosphoric acid, I get it at autozone but I’m sure it’s pricy compared to other stuff in my situation. I REALLY suggest and prefer over anything the loctite naval jelly rust dissolver, I didn’t notice any darkening of my mill when using it on the ways (index 55 rusted to hell) and I left it on a few times way too long and it hardened and turned white, I then put a new layer on it and that took all the white stuff off and worked great, after cleaning the jelly off you must oil the parts or they’ll rust almost immediately. I also used the rustoleum rust dissolver and it lists phosphoric acid but it leaves a film of something behind to protect the metal and I dislike that.

Edit: this is for “small” parts in my situation At or smaller then a bp guideway

dian · Jul 19, 2020

crrmeyer said:
You can buy 5 Gallon buckets of molasses pretty cheap at the animal feed store.

they didnt even know what it was. doesnt get used anymore. had to special order it. i live in a "high tech" country, unfortunately.

Bill D · Jul 19, 2020

I use EDTA in water as a cheap substitute for evaporust. One pound of EDTA will neutralize one pound of rust. It is safe enough that some people eat it. Keep everything fully submerged to avoid lines at the air water interface.
Bil lD

challenger · Jul 19, 2020

Citric acid works great. It's cheap and fast. My favorite is electrolytic but citric can be walked away from.

metalmagpie · Jul 20, 2020

crrmeyer said:
Interesting. What dilution ratio are you using? Have you tried it with heavily rusted steel?

I currently have been using animal grade molasses diluted 10:1 with water for rust removal. In the attached photo, I have two fairly rusted 1/2" nuts from the same project where one was soaked in molasses for 6 days and the other one is in the as-found rusted state. Post molasses bath, I wash with water and air dry. The I use red scotch bright and solvent to polish off any left over film from the molasses or sometimes a thin layer of black oxide? It took about 30 seconds to clean 8 surfaces on the nut as shown.

I'll have to try citric acid and see how that compares to molasses.

View attachment 294352

I drove all the way across the country once, stopping at feed stores and farm supply stores looking for the mythical cheap molasses. All I found were the pellets.

Do you have a link where such molasses can be purchased?

metalmagpie

plumberpieco · Jul 20, 2020

I get both molasses and phosphoric acid at Tractor Supply. The phosphoric acid there is sold as Dairyland Sterosol Milkstone Remover 53% though.

crrmeyer · Jul 20, 2020

metalmagpie said:
I drove all the way across the country once, stopping at feed stores and farm supply stores looking for the mythical cheap molasses. All I found were the pellets.

Do you have a link where such molasses can be purchased?

metalmagpie

This is where I get mine:

Molasses, 5 Gal

CalG · Jul 20, 2020

vinegar and salt?

removing rust with citric acid

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