Influence of the pH Value on the Corrosion of Zn-Al-Mg Hot-dip Galvanized Steel Sheets in Chloride Containing Environments

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.
This work focuses especially on the stability of this material at different pH values and the formation of corrosion products. 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 amount and the type of corrosion products formed in the course of the tests varies greatly. While in severely acidic conditions mainly Simonkolleite is formed. In mildly acidic to neutral conditions, Simonkolleite and Hydrozincite are formed, while, in strongly basic conditions, Simonkolleite and ZnO are formed.
Corrosion products were analyzed by SEM, EDX, XRD, IR, and RAMAN spectroscopy. A cross section analysis in SEM showed that the severity of the attack of the coating is very much dependent on the pH value of the testing solution. The corrosion products could be identified as Simonkolleite and Hydrozincite. In alkaline conditions the formation of Zincoxide and Zn-Al-Hydrotalcite was also observed.