The role of phosphonates as transporters of Zn2+ ions in the inhibition of carbon steel in neutral solutions containing chlorides

Phosphonates have shown synergistic effects in combination with Zn2+, ions in controlling the corrosion of carbon steel immersed in neutral aqueous environments containing 60-ppm Cl-. The role of phosphonates in the presence of Zn2+ ions has been investigated by polarisation tests and weight loss results, It was observed that the phosphonates functioned as transporters of Zn2+ ions from the bulk of the solution towards the metal surface. The phosphonate-Zn2+ bond is sufficiently strong to carry Zn2+ ions from the bulk solution towards the metal surface, but is weak enough for them to break to form a phosphonate-Fe2+ bond at anodic sites on the metal surface. Inhibition efficiency increases when the phosphonate-Zn2+ complex remains in solution in soluble form. Inhibition efficiency decreased when the phosphonate-Zn2+ complex was precipitated in the bulk of the solution.