AZ91 alloy phases

The effect of heat treatment is to alter the size, amount and the distribution of the precipitated /(-phase, Mg17Al12, which in turn affects the corrosion rates of AZ91 alloys, as shown in Table 4.72. 

Figure 17.11 Traditional representation of impedance data obtained for the AZ91 alloy at the corrosion potential after different immersion times in 0.1 M NaCI a) complex-impedance-plane or Nyquist representation (the lines represent the measurement model fit to the complex data sets) b) Bode representation of the magnitude of the impedance as a function of frequency and c) Bode representation of the phase angle as a function of frequency. (Taken from Orazem et al. ° and reproduced with permission of The Electrochemical Society.)...

In summary, short rod-like Al4Ce phases were formed in wronght AZ91 alloy with 1.5 mass% Ce, and DRX had been retarded. The passive cnrrent density inereased and the passive film stability decreased after Ce addition into wrought AZ91 alloy. 

A high aluminum content in alloys like AZ91 is associated with an appreciable amount of p, which is Mgi7Ali2 (Linder et al., 1989). No Si, Fe, Mn, or Ni was detected in this phase at levels greater than the noise level in the X-ray spectra (Lunder et al., 1987). 

The corrosion behavior of the skin of diecast AZ91 Mg alloy has been examined as a function of the thickness of the cast alloy in 3.5% NaCl solution saturated with Mg(OH)2 at room temperature. It was found that the corrosion resistance of cast specimens with relatively more important thicknesses was higher than that of the less thick ones. This was explained in terms of the increasing amount of A1 and P phase (Mgx7 Alij) in their skins (Aghion and Lulu, 2009). It has been also stated that the morphology, the level of porosity and the composition of the passive layer especially in this passive alkaline medium were showing a better corrosion resistance. 

The microstructure of as-cast AZ91 (a typical two-phase alloy) consists of primary oc-Mg and the eutectic micro-consituent which contains eutectic-a plus the p phase (the intermetallic Mgi7Ali2) [2,8]. 

The possible types of behaviour are summarised by the corrosion rate data in Table 3.3 for HP Mg and Mg alloys corroding at their free corrosion potentials in 1M NaCl at pH 11. HP Mg, taken as standard for comparison, showed a corrosion rate of 1.1 mm/yr. A higher corrosion rate was shown by the interior of diecast AZ91D and by the HP sand-cast AZ91. The 3-phase accelerated the corrosion. In contrast, the surface of diecast AZ91D had a corrosion rate lower than that of HP Mg. The 3-phase provided protection as is clear from the still lower corrosion rate shown by pure p. 

Zhao et al. [12,24] studied the influence of the microstructure, particularly the morphology of the 3-phase, on the corrosion of Mg alloys using AZ91 as a model Mg alloy. Corrosion was characterised for five different types of microstructure produced by heat treatment of as-cast AZ91 as illustrated in Fig. 3.13. The influence of microstructure can be understood from the interaction of the following three factors ... 

MC Zhao, M Liu, G Song, A Atrens, Influence of the P-phase morphology on the Corrosion of the Mg Alloy AZ91, Corrosion Science, 2008, 50, 1939. 

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