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 beta-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 beta phase studied in the previous work. The addition of Ti reduced the size and amount of the divorced eutectic beta phase and lamellar structure beta 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 alpha phase network structure along dendritic interfaces. This Al-rich alpha 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 a phase, and the average Al content in the divorced eutectic alpha 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. (C) 2016 Elsevier B.V. All rights reserved.