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51312-01493-Influence of the pH Value on the Corrosion of Zn-Al-Mg Hot-dip Galvanized Steel Sheets in Chloride C

Product Number: 51312-01493-SG
ISBN: 01493 2012 CP
Author: Timothy A. Keppert
Publication Date: 2012
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Previous studies of our group showed that Zn-Al-Mg hot-dip galvanized steel sheets have a superior corrosion resistance compared to conventional hot-dip galvanized steel sheets. In recent works the corrosion resistance of Zn-Al-Mg coatings is ascribed to the formation of a Magnesium containing Zink-Aluminium-Hydrotalcite-like compound. This work especially focuses on the stability of this compound at different pH values. Therefore experiments based on the normal salt spray test according to DIN EN ISO 9227 were conducted. During those tests the solution was adjusted to different pH values from acidic to alkaline. All other parameters were left untouched. The samples were compared to conventional hot-dip galvanized steel sheets of the same coating thickness. The results showed that the corrosion resistance is severely influenced by the pH value of the testing medium. Also the formation and protection capability of the Zn-Al-Hydrotalcite-like layer formed on the samples is related to the different media. Corrosion products were analyzed by SEM EDX XRD IR and RAMAN spectroscopy. A cross section analysis in SEM showed that an Al-rich layer is formed on the surface of the steel substrate. XRD RAMAN and IR analysis confirm that this layer is a Magnesium containing Zink-Aluminium-Hydrotalcite-like compound.
Previous studies of our group showed that Zn-Al-Mg hot-dip galvanized steel sheets have a superior corrosion resistance compared to conventional hot-dip galvanized steel sheets. In recent works the corrosion resistance of Zn-Al-Mg coatings is ascribed to the formation of a Magnesium containing Zink-Aluminium-Hydrotalcite-like compound. This work especially focuses on the stability of this compound at different pH values. Therefore experiments based on the normal salt spray test according to DIN EN ISO 9227 were conducted. During those tests the solution was adjusted to different pH values from acidic to alkaline. All other parameters were left untouched. The samples were compared to conventional hot-dip galvanized steel sheets of the same coating thickness. The results showed that the corrosion resistance is severely influenced by the pH value of the testing medium. Also the formation and protection capability of the Zn-Al-Hydrotalcite-like layer formed on the samples is related to the different media. Corrosion products were analyzed by SEM EDX XRD IR and RAMAN spectroscopy. A cross section analysis in SEM showed that an Al-rich layer is formed on the surface of the steel substrate. XRD RAMAN and IR analysis confirm that this layer is a Magnesium containing Zink-Aluminium-Hydrotalcite-like compound.
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