Acid pickling inhibitors used in industrial plants are generally organic compounds consisting of a hydrocarbon part coupled to a ionizable or polar group. A new group of inorganic inhibitors whose industrial performance is not proven, because of its cost, consists of halogens and halides.
Investigations were conducted in order to observe the mechanism of retardation and acceleration of acid pickling by the addition of compounds to determine the effect of halogens and halides on the acid attack against ferrous metals. Inhibition of acid attack on base metal was found to increase with the increase in atomic weight of the halogen in the molecules. The rate of oxide removal on rusted ferrous metal surfaces was not noticeably affected regardless of the halide used. Iodine and iodides were found to be effective inhibitors in concentrations of 0.1 percent and over. Bromine and bromides were next in effectiveness, and chlorine and chlorides were also inhibitors, but fluorine and fluorides were accelerators of acid attack on base metal. The halide part of the salt (the anion) was found to be the active component because, regardless of the cation used less noble than iron, results within limits of experimental error were of the same magnitude.
The theory of halide inhibition of iron dissolution may be in the formation of a passivating monomolecular film of the iron halide. Mild steel (SAE. 1010) pieces were used as material for test specimens, so that acid attack on clean, polished, unoxidized steel could be determined. Tests were made in pickling solutions of 1 and 5 normal sulfuric acid and 5 percent nitric acid using varying concentrations of inhibitors and several different temperatures for a number of test durations. These tests show that further research and study on the use of halides as pickling inhibitors is warranted. Iodides, having excellent inhibitor properties, may not be adequately efficient in the concentrations that commercial inhibitors are employed but may be satisfactory in combinations with various compounds as acid pickling inhibitors. The study of the mechanism of halide passivation disputes the belief they are accelerators of acid attack.
