What is inhibited HCl? Which inhibitor to use?

A. Hi Sanghani.

Straight HCl will attack steel as well as scale and rust. Inhibited HCl has organic additives that tend to coat the cleaned steel to inhibit the attack on the steel, directing more of the power of the acid into dissolving rust. Good luck.

A. The word "inhibited HCl" is usually used to indicate hydrochloric acid with acid corrosion inhibitor.

Hydrochloric acid is usually used as diluted in water, particularly 3 to 10% which varies by the fouling characteristics and material in the system.

Acid corrosion inhibitor means organic compound which adsorbs metal surface to protect metal from acid atmosphere. It is commercially available and typically composed of several mixture of organic amines, imines, and/or Thiourea [on eBay or Amazon], and so on.

There are a lot of products from chemical companies in the market and several types depend on the applications such as hydrochloric acid, sulfuric acid, organic acids, and so on.

The inhibited acid is at times heated usually up to 60 °C to facilitate the dissolving reaction between scale and the acids.

You should be careful to apply these acids, because hydrochloric acid, even inhibited, could attack some materials, like highly mechanically arranged devices and could lead to serious damage like cracking or partially detrimental corrosion. Especially for austenitic stainless steel, when it is finely modified.

A. To make an inhibited HCl solution according to ASTM, add 500 ml HCl to 500 ml water, and dissolve in 5 g of antimony trioxide, and 5 g of stannous chloride. Ta-da!

Q. I too would like to know what the answer will be regarding the question on what solvent to use when you do have a scale in hydrocarbons pipe lines .

A. Dear Mr. Mohammad;
We recommend that you can use sulfamic acid for descaling your boiler tubes.

A. Hi Nassar. Ned's advice was posted 8 years ago, so I doubt that he is monitoring the site and will see your question.

There are dozens or hundreds of proprietary formulas for inhibitors depending on the basis metal and what you wish to strip from it, such as rust alone, or zinc plating, etc. Most of them include organic compounds as well as inorganic salts, but the solution Ned refers to is called "Clarke's Solution", developed by S.G. Clarke in 1936, and the ASTM spec he refers to seems to be ASTM G1-03.

Although much of the important technical literature, like that seminal paper from 1936, has been moved behind "pay walls" offering only expensive short-term access (and so has been largely lost to the world), there are a few helpful papers like this one you can download it as a .pdf ...

Good luck.

Regards,

A. finishing.com's primary emphasis is surface finishing. If further surface treatment such as plating, phosphating or painting is contemplated, the inhibitor should not only inhibit the reaction of hydrochloric acid as Clarke's solution does, but it should also be rinseable. Clarke's solution is not rinseable and there are many proprietary formulations that inhibit acid attack and are rinseable as well.