Magnesium alloys corrosion

Organic compounds normally cause Htde or no corrosion of magnesium. Tanks or other containers of magnesium alloys are used for phenol [108-95-2] methyl bromide [74-96 ] and phenylethyl alcohol [60-12-8]. Most alcohols cause no more than mild attack, but anhydrous methanol attacks magnesium vigorously with the formation of magnesium methoxide [109-88-6]. This attack is inhibited by the addition of 1% ammonium sulfide [12135-76-1] or the presence ofwater. 

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 Alloy, Processesfor Pretreatment and Prevention of Corrosion On, MUitary Specification MIL-M-3171, Dept, of Navy, Naval Air Engineering Center, Philadelphia, Pa., July 11, 1966. 

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... 

Tensile strength diminishes rapidly with increasing temperature above 200°C. The high-magnesium alloys N5, N6 and N8 should not be used above 65°C because higher temperatures make them susceptible to stress corrosion cracking. 

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. 

In considering the corrosion behaviour of magnesium alloys, therefore, it is of the utmost importance to know the nature of the medium to which the metal is to be exposed. In general, atmospheric attack in damp conditions is largely superficial aqueous solutions bring about attack which varies not only with the solute but with the volume, movement and temperature... 

In many ways the corrosion of magnesium alloys in normal atmospheric conditions is a close approximation to the initial formation of rust on mild... 

In conditions where sea-water spray may be deposited regularly on magnesium articles with no alleviating mechanism for its removal, or where breaking waves may drench the components, the effect is quite different. Corrosion of bare metal will be heavy and will be intensified at junctions with other more noble metals. Unless magnesium alloys can be adequately protected in such combinations it is better to avoid their use. This matter is dealt with under the section on protection. 

In considering the corrosion of magnesium and its alloys it is important to examine the methods available for assessing corrosion tendencies and particularly those known as accelerated tests. Tests carried out by immersion in salt water or by spraying specimens regularly with sea-water are worthless as a means of determining the resistance of magnesium alloys under any other than the particular test conditions. Extrapolation to less corrosive conditions is not valid and even the assessment of the value of protective measures by such means is hardly possible. The reason is to be found in the fact that corrosion behaviour is directly related to the formation of insoluble... 

The proneness or otherwise to corrosion is essentially the same in all the magnesium-base alloys and it is important to note that the requirements of protection therefore do not vary for the magnesium alloy under consideration. If conditions are such that any one of the alloys can be used satisfactorily without protection, then any other of the alloys can be so used. On the other hand, if a given protective scheme is found necessary for a particular alloy, then the same protective scheme will be found necessary (and will be equally effective) with any other magnesium-base alloy. 

The Effect of Surface Finishing on the Corrosion Behaviour of Magnesium Alloys... 

By the use of many commercial abrasive processes, the corrosion resistance of magnesium alloys can be reduced to such an extent that samples of metal that may lie quiescent in salt water for many hours will, after shot blasting, evolve hydrogen vigorously, and the corrosion rate, as measured by loss of weight, will be found to have increased many hundred-fold. The effect in normal atmospheres is naturally much less, yet the activation of the surface is an added hazard and is the opposite of passivation which is essential if later-applied paint finishes are to have proper durability. 

Rapid solidification technology has been applied to several magnesium alloy systems and extruded material of some of these systems have exhibited excellent corrosion resistance. 

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