Homopiperazine as a Corrosion Inhibitor^*

Corrosion rates of iron in 6N HCl measured by the polarization technique show a sharp decrease at a homopiperazine concentration of 0.016M. Below this concentration, corrosion rates decrease slowly from 2200 μA/cm² for uninhibited iron to 1400 μA/cm² at a homopiperazine concentration of 0.016M. At slightly greater inhibitor concentrations, the corrosion rate drops suddenly to 220 μA/cm², and then shows little further change with increased homopiperazine concentration. These two ranges of corrosion rates are attributed to two different modes of adsorption of the inhibitor. At low concentration, homopiperazine adsorbs vertical to the surface via one imine group, but at higher concentrations, the molecule adsorbs in the flat position via both imine groups. This latter configuration gives improved inhibition by virtue of multiple bonding and by enhanced exposure of a hydrogen layer to the environment. Differential capacitance measurements support this mechanism.