Metal pickling by inorganic acids 1 – hydrochloric acid ” HCl “

I would like to give a brief overview of those inorganic acids, which are traditionally used in pickling applications.

Hydrochloric acid HCl:

Pure HCl is gaseous at atmospheric pressure because its boiling point is -85.1 ° C. If we speak about hydrochloric acid, we normally have HCl gas dissolved in water in our mind . At room temperature, the saturation point is about 38 % (m/m) and commercially available hydrochloric acid is in the concentration range of 30 – 38 % (m/m) (~ 346- 452g / l). The azeotropic point at 20.24% (m/m) has a significant impact to the handling and use of HCl solutions.

At room temperature is HCl gas is already evaporatig from solution. The result is the characteristic smell. Even at small concentration the HCl has negative impact to our body. There could also be seen significant corrosion at unprotected metal which is located next to HCl tanks.

HCl dissolves base metals according to the following formula.

Me + 2HCl -> MeCl2 + H2

When HCl is used as a pickling agent, the HCl dissolves the metal oxides as follows. This leads to the desired surface cleaning effect.

MeO + 2HCl -> MeCl2 + H2O

Me2O3 + 6 HCl -> 2 MeCl3 + 3H2O

Hydrochloric acid as pickling agent is mainly used in carbon steel pickling lines at steel mills. The advantages compared with sulfuric acid are less pickling time, lower temperatures as well as a better surface quality in combination with less pickling loss. Finally it should be mentioned that recovery rate at HCl Acid regeneration plants is almost 100% and a closed utility circuit is possible.

Data source: Beizen von Metallen, Dr. Ralf Rituper , Eugen G. Leuze Verlag

See also the blog posts on other inorganic acids: sulfuric acid; nitric acid; phosphoric acid; hydrofluoric acid