Magnesium alloys corrosion forms

Both silicon and aluminium are added to zinc to control the adverse effects of iron. The former forms a ferro-silicon dross (which may be removed during casting). Aluminium forms an intermetallic compound which is less active as a cathode than FeZn,] . Similarly in aluminium and magnesium alloys, manganese is added to control the iron . Thus in aluminium alloys for example, the cathodic activity of, FeAl, is avoided by transformation of FeAlj to (Fe, Mn)Al/. This material is believed to have a corrosion potential close to that of the matrix and is, therefore, unable to produce significant cathodic activity . 

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. 

The major alloying elements are manganese, aluminium, zinc, zirconium, silicon, thorium, and rare earth metals (E). At present E elements are the most promising candidates for magnesium alloys, with high temperature stability as well as improved corrosion behavior. E metals are forming stable intermetallic compounds at high temperature and therefore they decrease casta-bility. Aluminium and zinc are introduced mainly to... 

To date, research efforts regarding protective thin films have primarily focused on compact textures. In fact, smdies have confirmed that mesoporous thin films could confer corrosion protection for magnesium alloys. Furthermore, a mesoporous structure is beneficial to the integrity of the outer surface. This mesoporous structure could effectively release stress in the thin film and ameliorate the mismatch between the substrate and coating thus, crack-free thin films can be successfully prepared. In addition, smdies have reported that mesoporous thin films can easily induce apatite formation and have high bone-forming ability owing to the enhancement of cell activity and protein adsorption. ... 

Dry methyl chloride may be contained in such common metals as steel, iron, copper, and bronze, but it has a corrosive action on zinc, aluminum, die castings, and possibly magnesium alloys. Methyl chloride must not be used with aluminum, since in the presence of moisture it forms spontaneously flammable methyl aluminum compounds upon contact with that metal. No reaction occurs, however, with the drying agent, activated alumina. 

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 and magnesium alloys can suffer from several different forms of corrosion. 

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