General and localized corrosion of magnesium alloys: A critical review

Magnesium (Mg) alloys as well as experimental alloys are emerging as light structural materials for current, new, and innovative applications. This paper describes the influence of the alloying elements and the different casting processes on the microstructure and performance of these alloys and corrosion. It gives a comprehensible approach for the resistance of these alloys to general, localized and metallurgically influenced corrosion, which are the main challenges for their use. Exposure to humid air with ∼65% relative humidity during 4 days gives 100–150 nm thickness. The film is amorphous and has an oxidation rate less than 0.01 μm/y. The pH values between 8.5 and 11.5 correspond to a relatively protective oxide or hydroxide film; however above 11.5 a passive stable layer is observed. The poor corrosion resistance of many Mg alloys can be due to the internal galvanic corrosion caused by second phases or impurities. Agitation or any other means of destroying or preventing the formation of a protective film leads to increasing corrosion kinetics. The pH changes during pitting corrosion can come from two different reduction reactions: reduction of dissolved oxygen (O) and that of hydrogen (H) ions. Filiform corrosion was observed in the uncoated AZ31, while general corrosion mainly occurred in some deposition coated alloys. Crevice corrosion can probably be initiated due to the hydrolysis reaction. Exfoliation can be considered as a type of intergranular attack, and this is observed in unalloyed Mg above a critical chloride concentration.