Durability and synergistic effects of KI on the acid corrosion inhibition of mild steel by hydroxypropyl methylcellulose.

The performance of hydroxypropyl methylcellulose (HPMC) as safe corrosion inhibitor for mild steel in aerated 0.5M H2SO4 solution was appraised by weight loss, impedance and polarization measurements. Results indicate that HPMC functions as a good inhibitor in the studied environment and inhibition efficiency increased with increasing concentration of inhibitor and temperature. Time-dependent effect of the inhibition efficiency reveals that inhibition efficiency increased with time up to the fourth day after which it waned, but improved on addition of KI. The synergism parameter evaluated confirmed the synergistic effect of KI and HPMC. Impedance results clearly show that HPMC inhibited the corrosion reaction via adsorption onto the metal/solution interface following Freundlich adsorption isotherm. Polarization results indicate that HPMC acts as a mixed-type inhibitor with predominant cathodic effect. Theoretical study using density functional theory was employed to establish the correlation between the structure (molecular and electronic) and the inhibition efficiency.