**2.2 Picking inhibitor in hydrochloric acid**

Most of the corrosion inhibitors effective for carbon steel hydrochloric acid pickling are organic heterocyclic compounds containing N, O, S, and P atoms, and most are used as nitrogen-containing compounds. There are three common hydrochloric acid pickling inhibitors:


Behpour et al. [2] studied 2-({-1-methyl-3-[(2-sulfoaminophenyl)imin] butylene}amino)-1-thiophenol and 2-({-1,2-diphenyl-2-[(2-phospheophenyl) imine]acetal}ammonia)-1-thiophenol for the corrosion inhibition properties of copper in 15% hydrochloric acid. The results show that both of them are mixed corrosion inhibitors. When the mass fraction of two Schiff bases is 500 μg/g, the inhibition efficiency is 95.94 and 96.75%, respectively. Bentiss et al. [3] found that the three 4-hydrogen-1,2,3-triazole derivatives have better corrosion inhibition performance on low carbon steel in hydrochloric acid solution. They are all mixed corrosion inhibitors, in which the corrosion inhibition efficiency of 3,5-bis(4 methylthiophenyl)-4-hydrogen-1,2,4-triazole can be as high as 99.6%.
