AZ91D alloy rates

Potentiodynamie polarization curves of untreated and H-coated AZ91D alloy in 5% NaCl at pH 10.5 solution from CCP to anodic and cathodic directions at scan rate of 1 mVs (A) H-coated, (O) untreated (Nakatsugawa eta/., 1999). 

The influence of mechanochemical effects (MCEs) and creep on corrosion parameters were studied by Bonora et al. (2002). They stated that deformation increases the anodic current densities and shifting the potentials to more active values. It was also shown that the AZ91D alloy has a high corrosion rate in the deformed state than AM50 alloy under stress, while in the nonloading state the corrosion rate was found to be higher for the AM50 alloy. This behavior confirms the MCE theory and the behavior of these alloys at creep. 

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. 

Dubd et al. (2001) reported that the LSM of AZ91D and AM60 alloys using a pulsed Nd-YAG laser did not improve the corrosion resistance and instead it resulted in an enhanced corrosion rate. A non-uniform dendritic... 

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. 

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]. 

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. 

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