Atmospheric magnesium alloys

Magnesium is thermodynamically one of the less noble metals, and it can protect most other metals when used as sacrificial anodes (see Section 10.4). In the atmosphere the metal is covered by an oxide film. Therefore it resists rural atmospheres but is subject to pitting in marine atmospheres. Magnesium alloys are also liable to SCC and erosion corrosion, and are attacked by most acids. Mg alloys are used in automobile engines, aircraft, missiles and various movable and portable equipment, in all cases primarily because of their low density (1.76 g/cm ). 

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... 

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. 

Chromates are very effective inhibitors of the corrosion of magnesium alloys by saline and other waters, and many treatments have been developed by means of which substantial hlms containing slightly soluble chromate are formed in the metal surface. Except on parts which are to be exposed only to a rural atmosphere, chromate treatment must be supplemented by paint, for which it provides a good base. 

The film of magnesium hydroxide formed can give rise to passivity. This is attacked by anions such as chloride, sulfate and nitrate. The passive film formed gives reasonable protection from corrosion in rural, marine and industrial atmospheres, as evidenced by the corrosion rate data given in Table 4.69. It is obvious from the data that the corrosion performance of magnesium alloy lies between aluminum and carbon steel. 

Papakyriacou, M., Mayer, H., Fuchs, U., Stanzl-Tschegg, S. E., and Wei, R. P., Influence of Atmospheric Moisture on Slow Fatigue Crack Growth at Ultrasonic Frequency in Aluminum and Magnesium Alloys, Blackwell Science Ltd. Fatigue Fract Engng Mater Struct 25 (2002), 795-804. 

M. Jonsson, D. Persson, C. Leygraf, Atmospheric corrosion offield-exposed magnesium alloy AZ91D, Corros. Sci. 50 (2008) 1406-1413. 

Hazardous Decomp. Prods. Sulfuryl and thionyl fluorides heated to decomp., emits highly toxic fumes of F and SOx Uses Dielectric insulating medium for high-voltage elec, and electronic equip. as tracer gas for ventilation measurements in occupied spaces lasers protective atmosphere for casting of magnesium alloys inert filler gas (multiply-glazed windows)... 

Kola (Cola acuminata) extract astringent, skin treatment Zinc oxide astringent, toners Kola (Cola acuminata) extract astringent, topical Aluminum chloride hexahydrate astringent, topical hexahydrate Aluminum chloride anhydrous astringent, veterinary medicine Lead acetate trihydrate asymmetric epoxidation, transition metal catalyzed trans-Stilbene a-terpineol precursor 2-(4-Methyl-3-cyclohexenyl)-2-propanol atmosphere protectant, casting magnesium alloys... 

When considering zinc-aluminum alloys, the surface oxide film normally present is likely to reduce any corrosion current. The risk of bimetallic corrosion is small in atmospheric exposure trials by Noranda have been in progress since 1984 on ZA alloys coupled to other common metals. No visual effects were noted at the 5-year examination (Barmhurst and Belisle, 1992). A zinc-25% aluminum-0.05% magnesium alloy coupled to other materials and exposed on the Noranda Research Center roof showed pitting attack on the zinc-based material (but only up to 0.38 mm deep in 10 years) when joined to copper, brass, or steel, but less when joined to stainless steel or lead and least when joined to aluminum. 

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