The role of metal cations in improving the inhibitive performance of hexamine on the corrosion of steel in hydrochloric acid solution

Abstract

Hexamethylenetetramine (HA) or hexamine (or urotropin) have a moderate inhibitive effect on the corrosion of mild steel in concentrated acid solution (3N) but have a negligible effect in very dilute solutions (N/200) of the acid. Incorporation of Cu2+, As3+, Sb3+ and Sn2+ with HA improves its performance, which is synergistic in nature. These additives (except As3+), however, exhibit an antagonistic effect when tested in dilute acid solutions. Cu2+ and As3+ have the most pronounced effect in 3N acid solution. In N/200 HCl solution, the antagonistic effect is a maximum in the case of Sb3+ followed by Sn2+ and Cu2+ cations. Weight-loss, electrochemical polarization and zeta potential measurements are performed to understand the mechanism of action of these inhibitors. The positive role played by the cations on the inhibitive performance of HA is due to the formation of anionic complexes with the chloride ions of the acid solution. These anions replace the adsorbed chloride ion from the metal-electrolyte interface owing to their higher affinity toward the interface and help the protonized molecule of HA to be adsorbed more strongly at the interface. Accumulation of FeCl3 in concentrated acid solution lowers the performance of HA to a greater extent (about 100 times) than HA blended with Cu2+. The latter composition also has a substantially stronger inhibitive role on hydrogen absorption by the steel than the former one.