No abstract

No abstract

The corrosion resistance of pure zinc coatings can be improved through the application of suitable chemical passivation treatments. Hexavalent chromium (Cr 6+) compounds have widely been used to formulate conversion layers providing better anticorrosive protection as well as anchorage properties to painting systems. However, taking into account that they are produced using hazardous chemical compounds, the development of alternative and "green" technologies with equivalent protective performance is a paramount purpose of many R&D laboratories working around the world. In the present paper, the corrosion behavior of industrially electrogalvanized steel subjected to a Cr 3+ + Co 2+-based passivation treatment was studied. The experimental work involved electrochemical impedance spectroscopy (EIS) measurements and polarization curves in a 0.1 mol/L Na 2 SO 4 solution, surface microstructural and morphological characterization by electronic microscopy as well as chemical analysis by EDXS and XPS. The most commonly observed failures on the Cr 6+ treated samples were attributed to microstructural features of the substrate that were not adequately healed by the Cr 6+ passivation treatment.

Hot-dip galvanized steel sheets with different spangle sizes were deformed by means of rolling and tension. The change of preferential crystallographic orientation and of superficial characteristics due to the deformation was analyzed by means of both X-rays diffraction and optical and scanning electronic microscopy. A correlation between such changes and the involving deformation modes was intended to be done and the spangle size influence on these modes was studied. Coating reactivity change due to the deformation was investigated by means of quasi-steady DC electrochemical tests. The results allow to infer that, in great spangle samples, the main deformation mechanism is twinning whereas in small spangle ones, pyramidal slip systems happen as well. The increase of the reactivity with the deformation is greater in tension than in rolling and it is more important in small than in great spangle samples.

No abstract