Effect of the microstructure of Zn-Al and Zn-Al-Mg model alloys on corrosion stability

• Microstructure affected atmospheric corrosion of Al- and Mg- alloyed zinc.
• Materials with finer microstructures were usually more stable.
• Mg-rich phases dissolved preferentially.
• Al-rich phases activated at cathodic sites at alkali pH.
• Al and Mg reduced zinc run-off by stabilisation of protective corrosion products.

Zn-5Al and Zn-3Al-2Mg model alloys were cast and heat treated in order to obtain specimens with distinct microstructures and identical chemical compositions. The microstructure was characterised in detail to identify composition, size and distribution of present phases. Mass losses of samples with different microstructures and identical chemical compositions that were subjected to a cyclic corrosion test and a test under non-rinsing conditions differed by a factor of up to two. The mechanism is discussed based on measurements of corrosion stability of individual phases and chemical and phase compositions of corrosion products.