Increased Corrosion Resistance of Zinc Magnesium Aluminum Galvanised Coating through Germanium Additions

Improvements in the corrosion resistance of hot dipped galvanised steel coatings have been brought about over the last 20 years through binary alloy additions such as Al and more recently ternary Mg additions [1], [2], to further improve corrosion resistance quaternary additions were made to a commercially produced Zn-Mg-Al (ZMA) alloy.

Corrosion performance of the ZMA alloy coating was shown to improve through the addition of a quaternary element, Germanium. This was added in the molten state and the alloy mixture was rapidly cooled to mimic the cooling rate of a galvanised steel production line.

The increased corrosion resistance can be attributed to microstructural changes in the alloy composition due to Ge addition while in the molten state. The proportion of the most active MgZn₂ phase which has been shown to initiate the corrosion reaction in a ZMA alloy [3] was reduced thorough the formation of Mg₂Ge crystals. The formation of crystal structures within the experimental alloy also increased the heterogeneous nucleation of the primary zinc phase.  

The Scanning Vibrating Electrode Technique (SVET) was used to measure the rate of corrosion, anode life and zinc loss of the alloy samples. Phases present in experimental alloy were identified using SEM in conjunction with EDX analysis and a relationship was determined between the observed microstructural phases and the corrosion rates for varying Ge addition.

[1]      D. A. Worsley, H. N. McMurray, J. H. Sullivan, and I. P. Williams, “Quantitative Assessment of Localized Corrosion Occurring on Galvanized Steel Samples Using the Scanning Vibrating Electrode Technique,” Corrosion, vol. 60, no. 5, pp. 437–447, May 2004.

[2]      A. R. Marder, “The metallurgy of zinc-coated steel,” Prog. Mater. Sci., vol. 45, no. 3, pp. 191–271, Jun. 2000.

[3]      J. Sullivan, S. Mehraban, and J. Elvins, “In situ monitoring of the microstructural corrosion mechanisms of zinc–magnesium–aluminium alloys using time lapse microscopy,” Corrosion Science, vol. 53, no. 6, pp. 2208- 2215, Jun. 2011.