Effects of Cr(III) and Cr(VI) passivating treatments on the corrosion resistance of galvanized steel

J.V. FERRARI; C.R. TOMACHUK; C.I. ELSNER; A.R. DI SARLI; M.F. MONTEMOR; I. COSTA; H.GOMEZ DE MELO

Nice

Congreso; EUROCORR 2009. The European Corrosion Congress; 2009

Unión Europea de Corrosión

The increasing concern with environmental protection and sustainable development has led industry and researches to look for substitutes for the efficient Cr(VI) based passivating treatments usually applied on galvanized steel. In the last few years Cr(III) based treatments has raised as a potential replacement. In this work the effects of Cr(III) and Cr(VI) passivating treatments on the corrosion resistance of electrochemically galvanised steel was investigated by means electrochemical impedance spectroscopy (EIS) and polarization curves in 50 mM NaCl solution, and by surface characterization methods: SEM and XPS, prior and after corrosion tests. The results of the EIS tests have shown that the overall impedance responses associated with the Cr(III) treated samples were inferior to those exhibited by the Cr(VI) treated ones. However, the corrosion responses associated with the former samples were more regular, and failures were more frequently observed on the Cr(VI) treated samples. The higher impedances associated with the Cr(VI) samples could be explained by self-healing tendency identified in the EIS diagrams. On the other hand, failures exhibited by these samples could be associated to microstructural features of the substrate that are not adequately healed during the Cr(VI) passivating step.