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topic 32106

Using HCl to remove rust

A discussion started in 2004 but continuing through 2018


Q. I have been trying to remove rust from various vehicle parts using 36% HCl diluted 7 parts water to 1 part HCL. However the process is extremely slow (little to no visible change after 4-5 hours). I suspect the ambient temperature is possible slowing the process down, (14 ° C) or perhaps the concentration is incorrect. Any advise would be greatly appreciated.

James [last name deleted for privacy by Editor]
- Cork, Ireland


A. Hi James. It's not too cold, it's too dilute. 10 to 20 percent by weight would be better. Dilute it 2 or 3:1 rather than 7:1.

Ted Mooney, amadeya42.com Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


A. Personally I would not use hydrochloric acid because chloride ions readily promote rusting of iron and steels. To ensure you have removed all the acid will require extensive post treatment processing. I would recommend either sulphuric or possibly sulphamic acid, depending on how much rust is present. You can then treat the steel with either strong nitric acid or phosphoric acid to give it a passive layer.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


Q. Thanks for the advise, Ted. I have tried your suggestion but am still disappointed with the results. After 3 hours even light rust has not been removed. However rubbing with a wire brush at this stage does remove some of the rust. I had hoped to be able to remove all rust in 2-3 hours, is there possibly a better acid for the job?

James [last name deleted for privacy by Editor]
- Ireland


A. Although HCl is widely used for pickling and as prep for plating, Trevor Crichton is correct that it is corrosive and may not be ideal for older parts or complex pieces. I don't think a hobbyist should use the nitric acid post treatment, but the phosphoric acid sounds good. Alkaline electrolytic de-rusting is yet another possibility.

But what I don't understand is why strong HCl is not dissolving the rust; the reaction should be quite quick.

Ted Mooney, amadeya42.com
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

simultaneous (2004)

A. For hobbyist citric acid based solution is safest. You can use 5 % ammonium citrate solution(pH 3,5),temp. up to 8o C!

Good luck!

Goran Budija
- Zagreb, Croatia


Q. Thanks for the replies, guys. I have been using phosphoric acid and although the results are quite good the process is painfully slow ... hence my usage of HCl. Can I post-treat HCl dipped parts in phosphoric acid?

James [returning]
- Ireland


A. Dear James,

You can use sulphuric acid 10 percent at about 65 to 88 °C, time 5 to 10 min. With good exhaust system sulphuric acid gives off no noxious fumes.


Aly Gomaa
- Egypt

February 12, 2018

Q. Good morning, I used HCl as metal rust removal liquid, but excess fumes come out from acid. How to reduce fumes? Otherwise any other alternative clean liquid for metal rust?

- hosur, tamilnadu, india

February 2018

A. Hi A.LEO,
It's not clear if you are an industrialist regularly processing parts in acid treatment vats, or a hobbyist doing a one-off treatment in a plastic bucket. If you are a hobbyist, you could wire brush the parts, and then treat with phosphoric acid, or use the HCl outdoors. If you are an industrialist you should be using inhibited HCl, not commodity HCl, and your tanks should be equipped with exhaust ventilation systems. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

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