A CORROSION INHIBITION STUDY OF A CARBON-STEEL IN NEUTRAL CHLORIDE SOLUTIONS BY ZINC SALT PHOSPHONIC ACID ASSOCIATION

Electrochemical measurements (steady-state current-voltage curves and AC impedance) were coupled with analytical techniques (reflection-adsorption spectroscopy at grazing incidence and XPS) to investigate the inhibition of corrosion of a carbon steel by a mixture of a zinc salt and a phosphonic acid. From the polarization curves and impedance measurements it was shown that the electrochemical reactions occurring at the metal interface were not controlled by mass transport. The inhibitor him acts as a protective layer impermeable to ionic or molecular diffusion. Corrosion proceeds only through small defects of the inhibitor film. XPS analysis shows that the him is very thin and homogeneous in composition. It is essentially composed of Zn, P and O. The formation of an iron oxide under the inhibitor layer was observed. The FTIR spectrum of the film formed on steel indicates a reaction of the phosphonic acid with the zinc hydroxide and the iron oxide to produce metal salts.