Rearrangement of surface atoms due to impingement or adsorption of CO and O
2 gas was studied for W, Mo, Ir and Re, using the field ion microscope. Mechanism of adsorption and desorption was also studied. In the case of the surfaces of W, Mo, Ir and Re on which CO or O
2 adsorbed, a clean surface could only be obtained after fiield-evaporation of several atomic layers. Corrosion is mainly infulenced by a process of surface migration of CO and O
2, to which the zone of spherical metal greatly contributes : for instance, [111] zone of W and Mo, [110] zone of Ir and [2110] zone of Re. The contribution to corrosion decreases in the sequence of Mo>W>Ir>Re. When CO or O
2 adsorbs on the metal surface, the field-evaporation voltage reduces for most metals. The extent of this voltage decreases in the order Mo, W, Ir and Re.
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