Passive properties and stability

The diffusivity D of Mg within the MgO lattice at 400is as low as 2.24 X 10 m /s, justifying negligible weight gains. The oxidation rate of 

3 Schematic showing the three passive layers on polished strips of pure magnesium after immersion in distilled water ifor 48h (Nordlien etal, 1995). 

Dry chlorine, iodine, bromine and fluorine cause little or no corrosion of magnesium at room or slightly elevated temperature. The presence of a small amount of water causes pronounced attack by chlorine, some attack by iodine and bromine, and negligible attack by fluorine. The presence of BF3 or Sp6 in the ambient atmosphere is particularly effective in suppressing high-temperature oxidation up to and including the temperature at which the alloy normally ignites. 

The corrosion product film on magnesium starts at neutral pH and is stabilized with the increase of pH. This causes the lustrous metal to assume a dull gray appearance when exposed to air. The film is amorphous and exposure to humid air or to water leads to the formation of a thick hydrated amorphous film that 

The passive film has the advantage of effective stability for a broad zone of pH in the alkaline medium which is not the case of aluminum alloys that 

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