Galvanic magnesium alloys

Some tests indicate that magnesium alloys are resistant to loam sod. However, in the presence of chlorides, corrosive attack may be serious particularly if galvanic couples are present as a result of coupling to iron stmctures. 

D. L. Hawke, J. E. HiUis, and W. Unsworth, Preventive Practice for Controlling the Galvanic Corrosion of Magnesium Alloys, International Magnesium Association, McLean, Va., 1988. 

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... 

Contact of brass, bronze, copper or the more resistant stainless steels with the 13% Cr steels in sea-water can lead to accelerated corrosion of the latter. Galvanic contact effects on metals coupled to the austenitic types are only slight with brass, bronze and copper, but with cadmium, zinc, aluminium and magnesium alloys, insulation or protective measures are necessary to avoid serious attack on the non-ferrous material. Mild steel and the 13% chromium types are also liable to accelerated attack from contact with the chromium-nickel grades. The austenitic materials do not themselves suffer anodic attack in sea-water from contact with any of the usual materials of construction. 

Table 4.70 Relative effects of various metals on galvanic corrosion of magnesium alloys AZ31B and AZ61A exposed at the 24.4- and 244-m (80- and 800-ft) stations, Kure Beach, NCa...

Galvanized steel is a common example of galvanic coupling where steel (Fe), with a standard electrode potential of —0.440 V vs. SHE, is cathodicaUy protected by zinc, which has a more active standard electrode potential of —0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects steel from corrosion through its sacrificial properties. Because zinc is less noble than iron in terms of the standard electrode potentials, it acts as an anode. The sacrificial anode (zinc) is continuously consumed by anodic dissolution reaction and protects the underlying metal (iron in steel) from corrosion. In practice, sacrificial anodes are comprised of zinc, magnesium alloys, or aluminum. 

Intergranular S.C.C. of magnesium alloys has also been reported, attributed to a grain-boundary phase of Mgi7Ali2 that causes galvanic attack of the adjacent matrix [8]. 

TABLE 4—Compatibility of metals in galvanic couple with magnesium alloys AZ31B and AZ61A at Kure Beach, NC [[Pg.543]

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. 

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-... 

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