Relationship between corrosion behavior and microstructure in an AZ91 magnesium alloy

Abstract

Magnesium alloys are remarkably corroded in chloride environments. The corrosion behavior of the AZ91 alloy is mainly dependent on Fe/Mn ratio and size and distribution of the intermetallic Al-Mn-Fe phase and Mg 17Al 12 (β-phases) particles. Grain size and heat-treatment condition also influence both the microstructure and corrosion behavior. The AZ91E was melted and casted into various permanent molds in order to obtained several cast ingots with different grain sizes (110 to 250 μm). Some ingots were heat-treated to control the distribution of β-phase particles. Corrosion characteristics were evaluated by a salt-water immersion testing and the open circuit corrosion potential measurement. Metallographic characteristics were evaluated by the average distance between intermetallic phase particles. The corrosion rate was increased with increasing the grain size in the as-cast (F) specimens. But it was independent of Fe/Mn ratio. Both of grain size and Fe/Mn ratio affected the corrosion behavior in the homogenized (T4) specimens. Independent of grain size and Fe/Mn ratio. Artificially aged (T6) specimens showed a good corrosion resistance. It was found that the β-phase act as a good barrier against salt-water corrosion. Corrosion potential of the Al-Mn-Fe phases depending on the composition.