酸素中における金属表面の薄膜の構造と成長

抄録

Reaction of oxygen with a clean metal surface follows the sequence (1) adsorption, (2) formation of oxide nuclei, (3) growth of continuous oxide. For stage (1) physical adsorption of O₂ is followed by chemisorption of atomic O. Low energy electron diffraction data show that for Ni a specific number of metal atoms move into the plane of adsorbed O atoms, forming a stable structure which remains on the surface even after heating Ni to near the melting point. The stable chemisorbed atomic O layer together with an overlying chemisorbed O₂ layer are considered to make up the passive film, accounting for the corrosion resistance of many passive transistion metals including Ni, Fe, Cr, Ti and the stainless steels. The adsorbed film functions not as a diffusion barrier, but increases instead the activation energy for hydration and dissolution of the metal lattice by displacing adsorbed H₂O and anions.
On continued exposure to low pressure oxygen, oxide nuclei grow rapidly to a thickness limited by the electron tunneling distance. Thereafter, electron transfer at the metal-oxide interface assumes control of oxide thickness, accounting for predominant lateral growth of nuclei until the film becomes continuous. Growth of the continuous film follows logarithmic oxidation kinetics, with a constant density nagative space charge in the oxide progressively slowing down electron transfer at the metal-oxide interface. At a critical thickness of oxide, trapped charge dissociates, resulting in a diffuse space charge and increase of oxidation rate, leading to two-stage logarithmic behavior.
The stable chemisorbed oxygen-metal atom monolayer probably persists during growth of continuous oxide, the work function of which, rather than of clean metal, determining electron transfer and oxidation rates. Hence the (111) surface of Ni on which O is adsorbed has the highest work function and is also the face which oxidizes least.