Impurities magnesium alloys

Magnesium anodes usually consist of alloys with additions of Al, Zn and Mn. The content of Ni, Fe and Cu must be kept very low because they favor selfcorrosion. Ni contents of >0.001% impair properties and should not be exceeded. The influence of Cu is not clear. Cu certainly increases self-corrosion but amounts up to 0.05% are not detrimental if the Mn content is over 0.3%. Amounts of Fe up to about 0.01% do not influence self-corrosion if the Mn content is above 0.3%. With additions of Mn, Fe is precipitated from the melt which on solidification is rendered harmless by the formation of Fe crystals with a coating of manganese. The addition of zinc renders the corrosive attack uniform. In addition, the sensitivity to other impurities is depressed. The most important magnesium alloy for galvanic anodes is AZ63, which corresponds to the claims in Ref. 22. Alloys AZ31 and M2 are still used. The most important properties of these alloys are... 

The tubes themselves must be fully characterized in order to ensure that the zirconium alloys, or magnesium alloys, do not contain impurities that may affect the performance of the fuel. For example, the presence of traces of neutron absorbers, like hafnium that always accompanies zirconium in nature, or elements that modify the corrosion resistance of zirconium, must be determined. The ASTM has outlined the specifications for seamless wrought zirconium alloy tubes that are used for nuclear fuel cladding (B811 2013). The exact technical details and analytical test procedures do not directly involve uranium and are beyond the scope of this book. 

Magnesium usage has been limited because of the poor corrosion properties of magnesium alloys (Makar et al., 1988). Corrosion resistance is especially poor when a magnesium alloy contains specific metallic impurities or when the magnesium alloy is exposed to aggressive electrolyte species such as Cl" ions. 

There are two main reasons for the poor corrosion resistance of many magnesium alloys (Makar and Kruger, 1990) - firstly, internal galvanic corrosion caused by second phases or impurities (Chapter XX in Emley, 1966) and, secondly, the quasi-passive hydroxide film on magnesium is much less stable than the passive films which form on metals such as aluminum and stainless steels. This quasi-passivity results in only poor pitting resistance for magnesium and magnesium alloys. 

The early magnesium alloys suffered rapid attack under moist conditions, mainly because of the presence of impurities, notably iron, nickel, and copper. These impurities or their compounds act as minute cathodes in the presence of a corroding medium and create micro-cells with the anodic magnesium matrix (Polmear, 1989). High-purity alloys are a relatively recent development. In high-purity alloys the concentrations of these impurities are controlled to below critical concentrations and as a consequence high-purity alloys are markedly more resistant to salt water than are alloys of normal purity (Shreir, 1965). 

Magnesium alloys are highly susceptible to galvanic corrosion. Galvanic corrosion is usually observed as heavy localized corrosion of the magnesium adjacent to the cathode (Froats et al., 1987). Cathodes can be external, e.g. other metals in contact with the magnesium, or internal, e.g. second or impurity phases. These two kinds of galvan-... 

It is common knowledge that magnesium alloys are more resistant to indoor and outdoor atmospheres than is mild steel, and occasionally they are even more resistant than some aluminum alloys. For example, if the concentration of impurity elements is sufficiently low, the AZ91D alloy corrodes much... 

Different elements have different influences on the corrosion of magnesium alloys. Some elements are beneficial and enhance corrosion resistance whereas other have no significant influence (or their influence remains uncertain). Some elements are, however, extremely detrimental to the corrosion performance of magnesium alloys. They are termed impurity elements. 

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.. 

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). 

Robinson, H. A., George, P. F. (1954), Effect of Alloying and Impurity Elements in Magnesium Alloy Cast Anodes, Corrosion 10(6) 182. 

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