A SYNERGISTIC EFFECT BETWEEN ZINC SALT AND PHOSPHONIC ACID FOR CORROSION INHIBITION OF A CARBON-STEEL

This work is devoted to the corrosion inhibition of a carbon steel by a zinc salt/phosphonic acid association. Steady-state current-voltage curves and electrochemical impedance measurements carried out in the p resence of each compound and for the mixture show a synergistic effect between the two molecules. The concentrations of the compounds in the mixture were lower than the concentrations used for each compound sep arately. Phosphonic acid was observed to act as an anodic inhibitor wh ereas cathodic action was shown for the zinc chloride. Electrochemical measurements and surface analysis (XPS and reflection-adsorption spec troscopy at grazing incidence) showed that the synergistic effect affo rded by the mixture was attributable to the reaction of the phosphonic acid with the zinc salt. The inhibitor film acts as a protective laye r impermeable to ionic or molecular diffusion. The film is very thin a nd homogeneous in composition. A chemical structure of the film is pro posed.