Corrosion Behavior of Zn-Al-Mg Coated Steel in Different Environments

Recent studies already showed the excellent corrosion resistance of zinc alloy coatings for steel sheets with additional aluminum and magnesium. The influence of the alloying elements on the corrosion mechanism is an important topic for many research groups world wide.

This work especially focuses on the corrosion resistance of a specific Zn-Al-Mg alloy coating (ZM) in comparison to a conventional zinc (HDG) coating for steel sheet, both produced via hot-dip galvanizing and comprising a coating thickness of 7 μm, in standardized laboratory corrosion test.

ZM shows a decisively enhanced corrosion protection in salt spray test according to DIN EN ISO 9227 compared to Z. The time to the first appearance of red rust is for ZM more than ten times higher than for Z. The reason for the enhanced corrosion resistance of ZM in salt spray test is the formation of a well adherent aluminum-rich passive layer next to the steel substrate within a short period of time, which protects the steel substrate against corrosive attack. By the means of X-ray diffraction (XRD) and IR spectroscopy this aluminum-rich protecting layer can be identified as zinc aluminum carbonate hydroxide, Zn₆Al₂(CO₃)(OH)₁₆×4H₂O.

The corrosion behavior of the ZM is also investigated in chloride-free corrosion tests. Two testing methods are used: A condensation atmosphere with constant humidity (CH) according to DIN EN ISO 6270-2 and a condensation climate with alternating humidity and air temperature (AHT) according to DIN EN ISO 6270-2. Also in a chloride-free atmosphere ZM shows improved corrosion protection properties in comparison to conventional hot-dip galvanized steel (Z). Cross-section analysis shows that similarly to the results from the salt spray test investigation in chloride-free environments, too, an aluminum-rich passive layer may form on the steel substrate. According to XRD the chemical composition of this layer is similar to that found on samples exposed to salt spray test. The reason for the enhanced corrosion resistance of zinc-aluminum-magnesium coated steel (ZM) in different corrosive environments is the formation of an aluminum-rich passive layer identified as Zn₆Al₂(CO₃)(OH)₁₆×4H₂O.