AZ91D Mg alloy

SEM back-scattered image of AZ91D Mg alloy showing a-phase grains. In the grain boundaries the p-Mgi7Ali2 phase can be found (1). Between the a-phase and the a-phase a eutectic area of p-phase and a-phase precipitates. AlsMns inclusions are also present in the alloy (2). 

SEM image of the microstructure of AZ91D Mg alloy. Arrows 1 and 2 point at intermetallic particles of the AlsMns type. 

Abstract To improve the corrosion resistance of magnesium (Mg) alloys, surface modification is applied in an attempt to produce a corrosion resistant barrier. In this chapter, a new method to deposit aluminum (Al) film on an Mg alloy surface is demonstrated. Electrodeposition of Al on AZ91D Mg alloy, using an acidic aluminum chloride-l-ethyl-3-methylimidazolium chloride ionic liquid (AICI3-EMIC), has been shown to be feasible. The existence of Al coating can cause a substantial increase in corrosion resistance, reducing the susceptibility of Mg alloy to aqueous corrosion. 

The deposition of A1 from various AICI3-EMIC ionie liquids on the Mg alloy surfaee is eonhrmed by EDS and X-ray diffraetion analysis (XRD). The EDS result for eaeh of the above Al-eoated AZ91D Mg alloy only reveals the speetrum for A1 element. The absenee of the alloying elements of the substrates indieates that the eoated layer ean be quite thick. Figure... 

The corrosion resistance of various Al- and Al/Zn-coated AZ91D Mg alloys has been evaluated by salt spray and electrochemical methods. In the following, the results from salt spray test, polarization curve and electrochemical impedance measurements manifesting the benehcial effects of Al and/or Al-Zn coating on AZ91D Mg alloy are demonstrated. 

X-ray diffraction patterns of the various samples. Curve a presents the bare AZ91D Mg alloy. Curves b, c and d present the Al-coated Mg samples deposited in the ionic liquids containing 53, 57 and 60 m/o AICI3, respectively. 

Digital micrographs of AZ91D Mg alloy (a) as-polished, (b) after salt spray for 1 h and Al-coated AZ91D Mg alloy (c) as-coated, (d) after salt spray for 8h. 

The polarization behavior of the bare and Al-coated AZ91D Mg alloy has been compared in 3.5 wt% NaCl solution, as can be seen in Fig. 14.8. As illustrated in curve a of this figure, the anodic curve of the bare Mg alloy exhibits an active dissolution behavior. The passive region can hardly be... 

The potentiodynamic polarization curve of an Al/Zn-coated AZ91D Mg alloy in 3.5 wt% NaCl solution is demonstrated in Fig. 14.9, in comparison with those of the bare and the Al-coated Mg alloys. The AFZn coating was formed under the same condition as shown in Fig. 14.6. The results show that Al/Zn film can also be passivated in 3.5 wt% NaCl solution. However, the passive region becomes narrower and the passive current density is lower. The presence of Zn in the coating causes a slightly deterioration of the passivity as compared with that of a pure Al film. 

The Electrochemical impedance spectroscopy (EIS) results for the Mg alloy without and with surface Al coated from the 53 m/o and the 60 m/o ionic liquid, respectively, are depicted in Fig. 14.10. For bare Mg alloy, the polarization resistance was about 470 Qcm. A substantial increase in the polarization resistance, as evidenced by an enlarged diameter of the semicircle of the Nyquist plot, can be obtained for Mg alloy if it is electroplated with Al. For those with surface Al electrodeposited at -0.2 V from the 53 m/o and the 60 m/o ionic liquid, the polarization resistance in 3.5 wt% NaCl solution are 3000 and 5200 Qcm, respectively. The results were consistent with those revealed in the polarization curves demonstrated in Fig. 14.8. The improved polarization resistance of AZ91D Mg alloy with Al coating from ionic liquid is clearly demonstrated. However, the passivity or the polarization resistance of the Al-coated Mg alloy depends on the deposition conditions. The Al film formed in more acidic AICI3-EMIC and at a lower deposition rate renders a better passivation behavior. 

The matrix a-phase in Mg alloys is normally anodic to the second phases and usually preferentially corroded. Fig. 4-24 shows the microstructure of die-cast AZ91D (Song et al., 1999b). 

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. 

Some corrosion rates of Mg alloys at different field exposure sites given in the literature (25-27) are displayed in Table 7.2. The highest corrosion rate, 8.8 (J,m/year, is shown for the AM50 alloy exposed in marine environment for one year, i.e. 2005-05-31 to 2006-05-23. The corrosion rate of AZ91D, measured in J,m/year, is after 12 months of exposure 4.2, 2.2 and 1.8 for the marine, rural and nrban exposures, respectively. The weight loss of the Mg alloys is linear with time (25). This was also seen in the laboratory (22) and has been reported in the literature (26). The weight of the field-exposed... 

Table 7.2 Corrosion rate, given in im/year, of Mg alloy AZ91D obtained from three different field stations (25). Also included in the table are corrosion rates, found in the literature (26, 27), of field-exposed Mg alloys...

Mg alloys incorporate rare earth (RE) elements [128] to improve (i) creep resistance, which is primarily achieved by RE-containing phases along grain boundaries [129,130], (ii) castability, (iii) age hardening [131], and corrosion resistance [132]. Chang et al. [132] reported that Mg-3Nd-0.2Zn-0.4Zr had a corrosion rate lower than AZ91D. Nordlien et al. [133] reported that RE elements improved passivation. Krishnamurthy et al. [134] suggested that pseudo-passivation in rapidly solidified Mg-Nd was due to Nd enrichment at the surface. 

JW Chang, XW Guo, PH Fu, A Atrens, LM Peng, WJ Ding, XS Wang, A comparison of the corrosion behaviour in 5% NaCl solution of Mg alloys NZ30K and AZ91D, Journal of Applied Electrochemistry, 2008, 38, 207. 

Time-dependent creep rate of pure magnesium (a) and diecast Mg alloys (b) AZ91D (1), AM50 (2) and AS21 (3) in 3.5% NaCI (a) and in 0.1 N Na2B407 + Mg(OH)2 (a, b) Abbreviation TB + MH means sodium tetraborate and magnesium hydroxide (buffer solution) [42]. 

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