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topic 32106

Using HCl to remove rust

A discussion started in 2004 but continuing through 2020


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, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha


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 [returning]
- 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, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

simultaneous 2004

A. For hobbyist citric acid based solution is safest. You can use 5 % ammonium citrate solution(pH 3,5),temp. up to 80 °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
Aloha -- an idea worth spreading

May 21, 2018

Q. Hi, I want to remove rust from newly made window grills and gate. It's made of pipes; please advise what I should do. Grill size is 6 feet x 8 feet and gate is 14 feet x 6 feet

Ahsan Rasheed
- Islamabad, Pakistan

May 2018

affil. link
Ospho Rust Converter

A. Hi Ahsan. If these are plain steel pipes, they are not rust resistant and must be painted. You can remove heavy rust with a wire brush, then apply phosphoric acid based "rust converter", then you must paint these items. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

May 23, 2018

Q. Hi, please advise for rust removal should I use HCl in concentrated form or in diluted form. If diluted what will be the ratio of HCl and water?

Ahsan Rasheed [returning]
- Islamabad, Pakistan

May 2018

A. Hi cousin Ahsan. I suggest 10 to 20 percent by weight, which means if you had pure HCl you would add 1 part of HCl to 4 parts of water to get 20%. But if the concentration of your acid is 36%, like James had, 1 part acid to 1 part water would give you 18% acid, which would probably be fine.

But please tell us who you are and what your situation is; your question is abstract, as we've told earlier posters. When we don't know if you are a hobbyist de-rusting one old glass cutting wheel in a shotglass of acid, or a plant manager running a high production pickling line for automotive parts, no one can talk about safety, environmental issues, hydrogen embrittlement, ongoing corrosion from chloride ions, the Kleingarn curve, and the 101 other "ifs, ands & buts" of the subject. Good luck!


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

August 2, 2018

Q. Hi Everyone.
I have 310 fairly heavy rusted 11 foot pipes for cleaning.
I would say I am a little advanced over average as I have been doing DIY projects for some time.

I got HCl 31-33% for the job, but new to HCl.

My question is,
01. Looking at plastic rain gutters for the bath. Will it hold the HCl?

02. If I don't dilute it, the process will be faster. Any cons?

I intend to paint the pipes after the acid treatment.

Here is a pic of a sample pipe which was treated using Acetic Acid, but the process is a little too slow for me.

32106-1b   32106-1a  

Thank you.

Shehan Daulagala
- Sri Lanka

August 2018

A. Hi. I hope these pipes will be used strictly for a decorative application because they look like they have no integrity with such extensive pitting. Put a little piece of the rain gutter into the acid bottle for a week or two and see what you learn about it's resistance; are you sure you can cap it water-tight?

You'll probably want to cap the pipes before immersing them. I think you'll have to re-fill the gutter with fresh acid a dozen or more times to do 310 pipes. A plating shop with 12 foot wide tanks could probably do this in an hour or so, but it will take you a week, with countless opportunities for accidents or pollution incidents doing them one at a time in a rain gutter -- but good luck :-)


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

August 8, 2018

thumbs up sign Hi Ted, Thank you for your response. Yes they are decorative, the pits are what attracts them to me, the very rustic look ;)

the plating shop was a good idea, will look for one.

The HCl comes in high molecular weight polyethylene(HDPE) containers, and gutters are made of Polyvinyl chloride (PVC) or uPVC, PVC is much more stronger than HDPE. But like you recommended I will just submerge a piece of gutter to HCI and see.

The danger and accident ratio using gutters are true :(

Thank you

Shehan Daulagala [returning]
- Colombo, Sri Lanka

How to neutralize hydrochloric acid in an engine

June 4, 2019

Q. Hi guys. I have a classic car engine, and after 15 years sitting in a garage I decided to restore it. I have done the normal process, opened it, cleaned everything, etc., but when I assembled it the coolant didn't circulate. After I opened it about three times I decided to fill it up with hydrochloric acid and left it for a night. In the morning I rinsed it out many times until the water came out clean. I assembled everything and the coolant circulated properly, the problem is that every about 3 months the coolant is coming rusty every time -- I think the acid is still alive! Can I dissolve baking soda with water and leave it for a while so it neutralizes the hydrochloric acid please? I appreciate if someone can help please, thanks regards, Chris

Christian Gialanze
Hobbyist - Zurrieq, Malta

June 2019

A. Hi Christian. Yes, I think you can do that under your conditions, although I would hate for other readers to misread things and extrapolate (the reaction of baking soda with HCl creates huge amounts of CO2 gas). Use only a pinch or two of baking soda, because there surely can't be much acidity left.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

June 4, 2019

thumbs up sign Thanks very much for your immediate reply Ted, I'm going to try it and will see if the problem is solved; thanks again Ted.
Kindly regards, Chris

Christian gialanze [returning]
- Zurrieq malta

June 6, 2019

A. After your baking soda treatment, fill it with any modern anti-freeze and future rust will be prevented.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina

February 20, 2020

Q. G'day crew, my name's Mark from Australia ... I'm artistic in about 5 different mediums and at present, I'm working with knives -- making handles, sheaths, shoulder holsters ... unique unusual. And being a knife lover, I recently acquired two Raindrop Damascus steel knives but they had a little bit of rust starting on them which now is getting outta hand... I was told to use sulfuric acid, I think 70/30 mix but it was too casually said for me and thought I'd try my luck on line ... I've read through the earlier Q and A's and thought the answers came from a knowledgeable place, so if anyone has any ideas, I'd been keen to try them ... thanks for your time.

Mark Erlandsen
- Ashmore, Gold Coast, Queensland, Australia

February 28, 2020

affil. link

A. Hello Mark,
Have you tried CLR? It would be much safer for you than HCl.

Mark Baker
Retired - Winston Salem, North Carolina, USA

March 1, 2020

Q. Hey Mark, CLR ???
I have got some around here somewhere. Have you had Damascus steel with rust and tried this yourself mate ?
I've been doing a heap of reading and YouTube watching and tried WD40, Bi Carb, lemon, etc ... they've taken the loose stuff off, no problems but the knife still has the discolouring or scars, and ultimately what I'm looking for is the stark difference of steel and carbon, the black and silver of a clean blade to use as props or include in art pieces. It was mentioned the best way to achieve what I'm looking for was with some sorta acid wash ... lol, but I haven't tried the CLR yet so I'll give it a go. Much appreciated for your input.

Mark Erlandsen [returning]
- Gold Coast Queensland

April 19, 2020

A. You should look at ultrasonic cleaning; it can work here.

vedant aggarwal
- delhi,india

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