Nature of Protective Metal Oxide Films

Regardless of the corrosion type, the major product of iron and steel corrosion is FeOOH, which is referred to as rust. Rust can occur in 4 different crystalUne modifications based on the type of corrosion and the environment that the corrosion takes place a-FeOOH (goethite), p-FeOOH (akaganeite), y-FeOOH (lepidocrocite), and 5-FeOOH (feroxyhite).  

The rust layer formed on unalloyed steel generally consists of two regions an inner region, next to the steel/rust interface often consisting primarily of dense, amorphous FeOOH with some crystalline FejO and an outer region consisting of loose crystalline a-FeOOH and Y-FeOOH.- -  

Aluminum initially forms a few nm thick layer of aluminum oxide, mainly y-Al Oj (boehmite), which in humidified air is covered by aluminum oxyhydroxide, y-AlOOH due to hydrolysis, resulting in a double-layer structure. Related reactions that occur within the passive film when in contact with humidity or water are as follows  

The probable composition of the outer layer is a mixture of AljOg and hydrated AI2O3, mostly in the form of amorphous A1(0H)3 or a-Al(OH)j (bayerite). This outer coating of AlOOH-A1(0H)3 is colloidal and porous with poor corrosion resistance and cohesive properties. The inner layer on the other hand 

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