Remove rust with muriatic acid (HCl / hydrochloric acid)

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, P.E.
Striving to live Aloha
finishing.com - 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 sulfuric or possibly sulfamic acid [on eBay or Amazon], 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
R&D practical scientist
Chesham, Bucks, UK

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 completely fail to understand is strong HCl not dissolving rust; the reaction should be nearly instantaneous -- a few minutes at the absolute most, not a few hours. Are you sure the parts were free of oil, grease, lacquer and dirt? -- proper cleaning must precede acid treatments.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. For hobbyist citric acid [on eBay or Amazon] 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 sulfuric acid 10 percent at about 65 to 88 °C, time 5 to 10 min. With good exhaust system sulfuric acid gives off no noxious fumes.

Thanks,

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.
Regards,

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 [on eBay or Amazon] , then you must paint these items. Good luck.
Regards,

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% [on eBay or Amazon] , like James had and is commonly available, 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 because your question remains abstract, as we've said. We don't know if you are a homeowner trying to dangerously de-rust your own grills & gates with a sponge, using HCl although we've told you it isn't appropriate, or whether you are a plant manager running a high production pickling line for grills. No one can talk about safety, environmental issues, hydrogen embrittlement, ongoing corrosion from chloride ions, inhibitors, the Kleingarn curve, and other "ifs, ands & buts" of the subject when we don't know who you are and what your situation is. Sorry, and good luck, or please try to understand what we need from you.
Regards,

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. Pure PVC is very acid resistant, so are polyethylene and polypropylene, but other plastics like nylon are not. So put a little piece of the rain gutter into the acid bottle for a week or two and see what you learn about its resistance; are you sure you can plug it water-tight?
You'll probably want to plug the pipes before immersing them. I think you'll have to re-fill the gutter with fresh acid at least a dozen or more times to do 310 pipes, with countless opportunities for safety accidents or pollution incidents doing them one at a time in a rain gutter -- but good luck. A plating shop with 12 foot wide tanks could probably do this in an hour or so, but it will take you a week, maybe more :-)
Regards,

 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 HCl and see.

The danger and accident ratio using gutters are true :(

Thank you

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 (because the reaction of baking soda with HCl creates huge amounts of CO2 gas). Use only a pinch or two of baking soda [on eBay or Amazon] , because there surely can't be much acidity left after all that rinsing.

Regards,

  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,

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

Jeffrey Holmes, CEF
Spartanburg, South Carolina

A. Let me emphasize Jeffrey's use of the word "modern". I have read in several places now that people often mistakenly believe that ethylene glycol is a corrosion-inhibitor -- but it isn't. Modern antifreeze works as a corrosion inhibitor because modern antifreeze contains corrosion inhibitors.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

CLR

on eBay or

Amazon

(affil links)

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

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

A. Try ammonium citrate solution ( 5 %, pH 3,5) - better, safer and more environment friendly solution.
Hope it helps and good luck!

A. Hi Thanes. If you were involved in major ongoing de-rusting of pipes where economics was a driving factor, the answer would not be easy, because there is actually a fairly well developed science just on this subject -- see Barlow Campano's article "The Kleingarn Regenerated Spent Acid" in our on-line library as a starting point, or use the search term 'Kleingarn Curve' with a search engine.

But for a "one off" project like this I'd probably just fill the tank halfway with water, then slowly fill the remaining half with acid because a little over 15% ought to work fine.

There are a couple of other things to consider here, though. Your acid really ought to have an 'inhibitor' in it so the acid is consumed only (or mostly) in dissolving rust, not in attacking the steel. Also, in case you don't know, the pipes will rust again very quickly if you just acid dip them and rinse them. You must neutralize the acid with a dip in some sort of alkali, and then dip in some sort of rust preservative.
Luck & Regards,