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-adso rption spectroscopy at grazing incidence and XPS) to investigate the i nhibition of corrosion of a carbon steel by a mixture of a zinc salt a nd a phosphonic acid. From the polarization curves and impedance measu rements it was shown that the electrochemical reactions occurring at t he metal interface were not controlled by mass transport. The inhibito r him acts as a protective layer impermeable to ionic or molecular dif fusion. 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 spe ctrum of the film formed on steel indicates a reaction of the phosphon ic acid with the zinc hydroxide and the iron oxide to produce metal sa lts.