THE ROLE OF METAL-CATIONS IN IMPROVING THE INHIBITIVE PERFORMANCE OF HEXAMINE ON THE CORROSION OF STEEL IN HYDROCHLORIC-ACID SOLUTION

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 addit ives (except As3+), however, exhibit an antagonistic effect when teste d in dilute acid solutions. Cu2+ and As3+ have the most pronounced eff ect in 3N acid solution. In N/200 HCl solution, the antagonistic effec t is a maximum in the case of Sb3+ followed by Sn2+ and Cu2+ cations. Weight-loss, electrochemical polarization and zeta potential measureme nts are performed to understand the mechanism of action of these inhib itors. The positive role played by the cations on the inhibitive perfo rmance of HA is due to the formation of anionic complexes with the chl oride ions of the acid solution. These anions replace the adsorbed chl oride ion from the metal-electrolyte interface owing to their higher a ffinity 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 e xtent (about 100 times) than HA blended with Cu2+. The latter composit ion also has a substantially stronger inhibitive role on hydrogen abso rption by the steel than the former one.