Effect of Coating Thickness on the Corrosion Behavior of Galvanized Steel in 3.5 NaCl Solution

摘要

Corrosion behavior of three carbon steels with increasing galvanized coating thickness of 5.6, 8.4 and 19.2 urn named as T1, T2 and T3, respectively, was studied by immersion test, electrochemical impedance spectroscopy and potentiodynamic polarization in freely aerated 3.5% NaCl solution. The major phase in the product of corrosion of all the samples after immersion test was found to be zincite, as confirmed by Fourier Transform Infrared Spectroscopy and X-Ray Diffraction techniques. The product of corrosion on sample T1was well adhered and was compact in most regions. Samples T2 and T3 showed porous and non-adherent growth of product of corrosion. Rate of corrosion were found to increase with the increase in the thickness of coating. The impedance provided by the coating as well as the substrate was the highest for the sample with thinnest coating (T1). The early exposure of the underlying steel in sample T1 resulted in higher resistance to corrosion, probably because of the combined effect of zinc products of corrosion and Fe-Zn alloy layer. Higher amount of protective y-FeOOH as well as compact product of corrosion could have also enhanced the resistance offered by sample T1 to corrosion. Although the average uniform resistance to corrosion was higher for T1, the localized pitting corrosion was also observed, probably because of the thin galvanized layer.