The Oxidation of Tin: II . The Morphology and Mode of Growth of Oxide Films on Pure Tin

Abstract

Oxide films formed during the study of the kinetics of oxidation of pure tin, under different conditions of time, temperature, and oxygen pressure, have been examined by means of electron microscopy and electron diffraction. Under all the conditions investigated, the oxide nucleates at numerous sites on the tin surface and spreads laterally, until the surface is essentially completely covered with oxide.For oxygen pressures of 1 mm Hg and above, at all temperatures investigated, the oxide nuclei develop into microscopic, wheel‐shaped “growth centers” about 1 micron in diameter. Segments of the rims of some of these growth centers grow outward as spikes or spines, generally in a spiral pattern and rapidly develop into oxide platelets. The platelets from adjacent growth centers extend laterally until they mutually impinge. This occurs as the oxidation rate reaches a maximum.Cavities have been found at the oxide‐metal interface. They extend nearly through the oxide film, but are closed near the oxide‐oxygen interface by a thin membrane of oxide. These cavities are first detected at the junctions of growth lamellae within the oxide platelets and enlarge with time, as the oxidation rate decreases in a logarithmic manner, until they reduce the area of contact between the oxide and metal by as much as one‐third.At oxygen pressures below 1 mm, the oxide grows in a dendritic manner. The dendrites spread laterally but evidently do not cover the tin surface in a compact layer, since the oxidation rate increases continually during the period of time investigated. The main direction of growth of these dendrites from various centers appears to be the same within the area of a given tin grain in the substrate, but is different for adjacent areas corresponding to tin grains of generally different orientations. The electron diffraction results show that only one crystalline oxide, , is formed under all of the oxidation conditions investigated.