ZnAlMg: AN INNOVATIVE METALLIC COATING THAT OFFERS PROTECTION IN THE HARSHEST ENVIRONMENTS(95)

摘要

ArcelorMittal started to produce in 2010 a new ZnAlMg coating offering superior corrosionresistance, especially in chloride and ammonia environments. This new metallic coating is producedon an industrial hot dip galvanising line, in a molten bath containing zinc, aluminium andmagnesium. It is dedicated to construction market and civil engineering.The outstanding corrosion resistance of this new coating is linked to the strong inhibiting propertiesof corrosion products that precipitate on the surface. These corrosion products have been identifiedby infra red, X ray diffraction (XRD) and SEM- EDS on samples after exposure to salt spray testor cyclic corrosion tests. When analysing pure zinc (standard hot dip galvanised), mainly ZnO isdetected while simonkolleite (Zn5(OH)8Cl2) and AlMg-Layered Double Hydroxide (LDH) and/orAlZn-LDH are observed for ZnAl3.5Mg3.The barrier properties of simonkolleite and LDH have the same nature: plate morphology, compactlayered structure with difficult ion-transfer. Owing to these properties, oxygen transport towards themetallic surface is inhibited and this mechanism explains the very good behaviour of ZnAl3.5Mg3in sea coast area, both on full panels and cut edges.