Surface phenomena and protective film growth on magnesium and magnesium alloys

Abstract

Recent research development concerning the filming behavior of magnesium and magnesium alloys is reviewed. The naturally formed film on magnesium in air is thin and dense, and it has an amorphous structure. In humid air, a hydrated layer forms between the metal and initial layer as a result of water ingress through the initial layer. The film formed in water contains an additional top layer with platelet-like morphology, formed by re-deposition of sparingly soluble magnesium. With increasing aluminum content of the alloys, all layers became dehydrated and enriched in aluminum oxide, and they decrease in thickness. These changes are significant especially as the aluminum content of the alloy is increased above 4 wt %, a threshold characterized also by a significant improvement in the corrosion resistance. This transition, corresponds to 35 wt % ofAl in the innermost layer of the oxide film. Alloying of the MgAI alloys with rare earth elements, causes further improvement of the corrosion resistance. This is attributed to a significant dehydration, causing increased stability and passivity of the oxide. For the anodic film growth on pure magnesium, the cylindrical pore structure and barrier layer which are similar to the Keller's model of anodic alumina are confirmed by direct cross- sectional observation. It is assumed that anodic film growth proceeds mainly by the formation of MgF2 and Mgx+2Ox (OH) at the metal/film interface and the dissolution of the film at pore bases. The crystallization of MgF2 and the formation of NaMgF3 simultaneously proceed in the porous layer. The similar pore structure is also found in the film grown on magnesium die cast AZ9ID, however, the film is highly uneven in thickness. The film surface after chemical conversion coating has a granular structure, with each granule corresponding to a single grain. It was revealed that the film was formed by anodic reaction and had a porous cell structure. The porous film is composed of cell colonies in the sub-micron range that have branched fine pores, with central holes (mother pores) sized approximately 50 nm. Square shaped holes about 300 nm in size are believed to be cathodic sites.