Quantitative Microstructure of Multiphase Al-Zn-Si-(Mg) Coatings and Their Effects on Sacrificial Protection for Steel

A new combined analysis of SEM-BSE and EDX images using AphelionTM software was proposed to describe the quantitative microstructure (quantity and neighborhood of sacrificial phases) of Al-Zn-Si-(Mg) coatings on steel. Three materials with different Al/Zn ratios and Mg content were analyzed. The quantitative microstructure allowed us to describe their corrosion behaviors in a chloride environment and understand their ranking for sacrificial protection of steel in accelerated corrosion tests. For the analyses, interdendritic Zn-rich or Mg-rich phases were expected to be more sacrificial to steel than Al-rich dendrites. Without Mg (AZ coating), Al-rich dendrites created a percolating network, but interdendritic phases did not, suggesting their sacrificial protection to steel to be very limited. Additionally, significant Zn gradients inside dendrites led to a premature coating consumption on the surface, creating new zones of naked steel. In the coatings with Mg (AZM), sacrificial interdendritic phases created a percolating network, which is expected to improve long-time sacrificial protection and contribute to a more uniform formation of Zn corrosion products. For Al content between 30 wt.% and 45 wt.%, a lowering of the Al/Zn ratio (L-AZM) increased the connectivity of the sacrificial interdendritic phases, which is expected to improve the long-term sacrificial effect. Accelerated corrosion tests of scratches in the steel coatings validated the hypotheses.