Influence of Microstructure on Corrosion Resistance in Zn-Al Alloy Coating

The changes in microstructure of Zn-Al alloy coating according to the alloy composition and the cooling rate after coating were investigated, and the influence of microstructure on corrosion properties was discussed.1) When the specimens were slowly cooled at 5°C/s after coating, zinc-rich primary phase precipitated in the coatings which contained less than 5%Al. On the other hand, the coating became eutectic structure without primary phase when it contained 5.25.4%Al.2) Zn-4.5%Al alloy coatings were cooled at various rates after remelting. When the specimen was cooled at 5°C/s100°C/s, the primary phase precipitated in the coating. The microstructure of the coating became uniform when the specimen was quenched.3) The primary phase in the coating deteriorated the corrosion resistance in the salt spray test. The uniform microstructure without primary phase showed the best corrosion resistance.4) It is effective to adjust Al content precisely at eutectic composition or to cool coated sheets as rapidly as posible for getting the uniform microstructure and the good corrosion resistance.