Tolerance limits, magnesium alloys

Figure 22.1. Corrosion of magnesium in 3% NaCI, alternate immersion, 16 weeks, showing tolerance limit for iron and beneficial effect of alloyed zinc and manganese [Figure 4 from J. Hanawalt, C. Nelson, and J. Peloubet, Corrosion studies of magnesium and its alloys, Trans. AIME, Inst Metals Div. 147, 281 (1942)].

Table 2 summarizes the known contaminant tolerance limits for the common commercially die cast alloys of magnesium plus the recently published tolerance limits for the developmental alloy AE42 [11], As may be noted in Table 2, while the tolerance limits for nickel and copper are essentially independent of the alloy composition, within its normal... 

Table 4-2. Tolerance limits for magnesium and magnesium alloys.

The mechanism which fixes the tolerance limit could be related to the solubility of the impurities in the magnesium alloy matrix. When the concentrations of Fe, Ni, and Cu exceed their tolerance limits, they could segregate and serve as active catalysts for electrochemical attack (Mercer and Hillis, 1992). However, in their study of tolerance limits, Hanawalt et al. (Hillis, 1983) failed to find any correspondence between the magnitude of the tolerance limit and the solubility of the added element in solid or liquid magnesium. They found only that corrosion began at discrete centers, and they supposed that the elements showing tolerance-limit phenomena were dispersed in the alloy as fine particles (Hanawalt et al.. 

Iron, nickel, copper, and cobalt are the four main elements so far found to have significant detrimental influence on the corrosion-resistance of magnesium alloys. They have different tolerance limit values. It is... 

Figure 4-18. Tolerance limits for nickel in pure magnesium, and in Mg-Mn and Mg-Zn alloys (Makar and Kruger, 1993).

Cobalt has strongly adverse effects on the corrosion resistance of magnesium alloys (Chapter IX in Emley, 1966), but cobalt is not a common impurity of magnesium, and its tolerance limits have not been well documented (Hillis, 1983). 

The presence of zinc in binary or ternary alloys has a decisive effect on film characteristics. Zinc can increase the tolerance limits and reduce the effect of impurities when the tolerance limit has been exceeded (Hillis, 1983). Addition of 1% Zn to pure magnesium increases the tolerance limit for nickel, but to lesser extent than addition of 1% Mn (Loose, 1946). Zinc is believed to improve the tolerance of Mg-Al alloys for all three contaminants (Fe, Cu, Ni), but its amount is limited to 1-3% (Froats et al.,... 

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