Examination and Analysis of 2 Pct Mg-55 Pct Al-1.6 Pct Si-Zn Coating on Steel
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AFTER the commercial production of the 55 pct Al-1.6 pct Si-Zn (wt pct, weight percent, which will be used throughout this paper, unless otherwise indicated) coated products (trademark Galvalume* or Zincalume** and various other brand names, such as COLORBOND** for prepainted Zincalume steel) in 1972,[1] a new generation of Zincalume and COLORBOND products was launched in 2013 by Bluescope, Australia.[2,3] These new products are manufactured based on the original Al-Si-Zn composition, but with 2 pct Mg added, having a nominal composition of 2 pct Mg-55 pct Al-1.6 pct Si-Zn, designated as ‘‘AM’’ according to the Australian Standard AS1397-2011.[4] * Galvalume is a trademark of BIEC International Inc. ** Zincalume and COLORBOND are trademarks of BlueScope Limited, Australia There was a relatively short history of the development of the AM coating. In 1993, Nolan conducted a study,[5] evaluating the effects of separate additions of
YISHENG R. CHEN is with the Steel Research Australia, 20 The Crescent, Springvale, VIC 3171, Australia. Contact e-mail: [email protected] Manuscript submitted March 21, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
magnesium at a range of 0.1 to 2.5 pct, and other elements (calcium at 0.1 to 0.5 pct, gallium at 0.05 to 1.0 pct and copper at 0.05 to 1.0 pct) to the traditional Zincalume coating on the microstructure development, mechanical properties, and corrosion resistance of the coating. The main contribution of this work was the discovery of the much-improved corrosion property of the Mg-added coatings. The 2 pct Mg addition was finally selected after 20 years of further research and development.[3,4] One of the main reasons for the performance improvement was identified to be the formation of Mg-containing phases, namely Mg2Si and MgZn2, in the interdendritic region of the coating microstructure.[2,3,6] The optimized composition at 2 pct Mg was later confirmed by the studies of Fujii and Shimoda[7] and Shimoda et al.[8] After a comparison of the Mg addition levels of 0.5, 1.5, and 2.5 pct, Li et al.[9] also found that the 1.5 pct Mg-55 pct Al-1.6 pct Si-Zn coating had the best corrosion resistance in a neutral salt spray environment. Another study carried out by Liu et al.[10] assessed the microstructures and corrosion resistance of several Mg-added 50 pct Al-1.5 pct Si-Zn coatings. The results of this latter study indicated that out of the four levels of Mg additions examined (0, 1.25, 3, and 4 pct), the 3 pct Mg-added coating had the best performance.
Despite the various studies conducted, little information is available to reveal the coating microstructure in detail. It appears that a consistent understanding of the solidification sequence and microstructural development in the coating, and even a consistent interpretation of the coating microstructure, has not been achieved. Figure 1 is the only micrograph published thus far[4,6] to reveal the cross-sectional microstructure of the 2 pct Mg-55 pct Al-1.6 pct Si-Zn coating. The microstructure shown in Figure 1 was
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