Influence of C16 Quaternary Amine on Surface Films and Polarization Resistance of Mild Steel in Carbon Dioxide-Saturated 5% Sodium Chloride

Experiments were performed to understand the behavior of a C16 quaternary amine on initially clean and precorroded surfaces in carbon dioxide (CO2)-saturated brine solutions. Although the inhibitor was slow to work at pH 6.5, results indicated high efficiency could be obtained regardless of precorrosion. On initially clean surfaces, greater polarization resistances were measured at the start of the experiments with increasing inhibitor concentration. Once the surface was covered by visible iron carbonate (FeCO3), the lower concentrations tended to perform better than higher concentations. This point was confirmed in experiments under conditions of increasing precorrosion. Scanning electron micrographs revealed that coinciding with improved inhibition was a transformation in the surface structure of FeCO3, from that of voids and grain boundaries under blank conditions to a fine dispersion-type particulate structure, as a consequence of inhibitor action. To develop this change, local FeCO3 supersaturation was believed to be required. This was achieved by the alkyl chains of the inhibitor leading to ferrous iron (Fe2+) entrapment at the metal-solution interface. The rate of FeCO3 deposition was thought to have become dependent on the passage of carbonate (CO32−) past the alkyl chains, which in turn was affected by inhibitor concentration, to regions locally saturated in Fe2+.