Corrosion protection with Al/Zn-based coatings

A method for forming a corrosion resistant Al—Zn—Si—Mg alloy coating on a metal strip, the method comprising:

• (a) passing the metal strip through a molten bath of Al—Zn—Si—Mg alloy and forming a coating of the alloy on one or both surfaces of the metal strip, wherein the alloy coating comprises 40-65 wt. % Al, 35-50 wt. % Zn, 1-3 wt. % Si, 1.5-2.5 wt. % Mg;
• and
• (b) controlling a cooling rate of the coating on the metal strip to form a solidified coating having a microstructure that comprises dendrites of Al-rich alpha phase and interdendritic channels of Zn-rich eutectic phase mixture extending from the metal strip, wherein particles of Mg2Si phase are positioned in the interdendritic channels in the solidified coating and block corrosion along the interdendritic channels, wherein greater than 60% of the interdendritic channels are blocked by the particles of Mg2Si phase;
• wherein the solidified coating has an OT:
• SDAS ratio greater than 0.5:1, where OT is an overlay thickness and SDAS is a secondary dendrite arm spacing for the Al-rich alpha phase dendrites of the coating;
• and
• wherein the solidified coating has an overlay thickness greater than 5 μm and less than 30 μm.