Adsorption of oxygen atoms on sublayer sites such as octahedral and tetrahedral sites were therefore, identified in the studies. For both the Fe(100) and Fe(110) surfaces, octahedral sites remained stable at higher oxygen coverages, while tetrahedral sites were the opposite, which is more stable at lower oxygen coverages. It was found that oxygen atoms adsorbing at sublayers of iron were less favourable comparing to on-surface adsorption. Based on their DFT studies, Ye et al. proposed an oxygen adsorption mechanism describing the initial stage of iron corrosion as illustrated in Figure (d). At a low concentration of oxygen in the environment, partial oxygen coverage was observed on the iron surfaces rather than diffuse into the bulk. However, at high oxygen levels with increased coverage, the oxygens were diffusing into the bulk which is promoted through the repulsion of adjacent oxygen atoms.