The improvement of corrosion resistance of AZ91 magnesium alloy through development of dense and tight network structure of Al-rich α phase by addition of a trace amount of Ti

The effect of the addition of a trace amount of Ti on the microstructure and corrosion behavior of the as-cast Mg-9Al-1Zn (AZ91) magnesium alloy with a high volume fraction of β-Mg17Al12 phase was studied, and the results were compared with those for the trace Ti-added as-cast Mg-3Al-1Zn (AZ31) alloy with a low volume fraction of β phase studied in the previous work. The addition of Ti reduced the size and amount of the divorced eutectic β phase and lamellar structure β phase in the AZ91 alloy. The Ti-addition also reduced the interdendritic spacing, resulting in the formation of a more dense and tight Al-rich divorced α phase network structure along dendritic interfaces. This Al-rich α phase network structure acted as an effective barrier to corrosion attack and because of this, the Ti-added AZ91 alloy exhibited superior corrosion resistance to the AZ91 alloy in the 3.5 wt.% NaCl solution. The Ti-added AZ91 showed comparable (slightly higher) corrosion resistance to the Ti-added AZ31 alloy. Compared with the Ti-added AZ31 alloy, the Ti-added AZ91 alloy showed a more densely packed divorced eutectic α phase, and the average Al content in the divorced eutectic α phase was higher. The two alloys showed different types of corrosion mechanisms. Pitting corrosion and uniform corrosion were the corrosion mechanisms operative in the Ti-added AZ91 and Ti-added AZ31 alloys, respectively.