INITIAL STAGES

11.2 INITIAL STAGES

The processes that take place at a clean, reactive metal surface exposed to oxygen obey the following sequence: (1) adsorption of oxygen, (2) formation of oxide nuclei that grow laterally, and (3) growth of a continuous oxide fi lm. Because the free energy of adsorption of atomic oxygen exceeds the free energy of dissociation of oxygen, the fi rst - formed adsorbed fi lm consists of atomic oxygen. Low - energy electron - diffraction data show that some metal atoms enter the approximate plane of adsorbed oxygen to form a relatively stable two -

dimensional structure of mixed O ions (negative) and metal ions (positive). As was discussed earlier in relation to the passive fi lm (Section 6.5 ), this initial adsorbed partial monolayer is thermodynamically more stable than the metal oxide; for example, for nickel, the adsorbed fi lm resists decomposition up to the melting point of nickel [3] , whereas NiO decomposes accompanied by oxygen dissolving in the metal. * Continued exposure to low - pressure oxygen is followed

by adsorption of O 2 molecules on metal atoms exposed through the fi rst adsorbed layer. Since the second layer of oxygen is bonded less energetically than the fi rst layer, it is adsorbed without dissociation to its atoms. The resultant structure is usually more stable on transition than on nontransition metals [4] . Any additional layers of adsorbed oxygen are still less strongly bonded, and the outer layers, at

* J. Moreau and J. B é nard [ C. R. Acad. Sci (Paris) 242 , 1724 (1956)] showed, through observation of the metal surface in H 2 O–H 2 mixtures at elevated temperatures, that oxygen adsorbed on an 18% Cr stainless steel is thermodynamically more stable than the metal oxide. For analogous data on iron, see A. Pignocco and G. Pellissier, J. Electrochem. Soc. 112 , 1118 (1965); E. Hondros, Acta Metall. 16 , 1377 (1968).

INITIAL STAGES

elevated temperatures, eventually become mobile, with the corresponding dif- fraction pattern being that of an amorphous structure.

Because the free energy of adsorption per mole of oxygen decreases with amount of oxygen adsorbed (the O – substrate bond becomes weaker), multilayer adsorbed oxygen on metal M eventually favors transformation to a crystalline stoichiometric oxide. In other words, ΔG for O · M ads

+ nO 2 → (nO 2 )·O·M ads becomes less negative per mole O 2 as n increases, whereas ΔG for 2 n M + (nO 2 )·O·M ads → (2 n + 1)MO to form the oxide becomes correspondingly more negative. Therefore, oxide formation nucleates preferably at surface sites where multilayer adsorption is favored, such as at surface vacancies, ledges, or other imperfections. When conditions are favorable, oxide nuclei form at specifi c sites of the surface, aided by rapid surface diffusion of M and O ions (Fig. 11.1 ) [5] . Because the amount of adsorbed oxygen on any likely site increases with oxygen pressure, more sites are brought into play, and the density of nuclei increases. Similarly, since multilayer adsorption decreases at elevated temperatures, the density of nuclei decreases with increase of temperature [6, 7] . Nuclei grow rapidly to a certain height on the order of nanometers, * and then they grow more

Figure 11.1. Nuclei of Cu 2 O formed on copper surface at 10 −1 mmHg oxygen pressure (13.3 Pa), 525 °C, 20 s (17,600 ×). Black lines are bands of imperfections (stacking faults) in the copper lattice (Brockway and Rowe [5]).

* Limited perhaps by the electron tunneling distance [4] below which electrons can penetrate the metal - oxide – interface barrier without fi rst acquiring the usual activation energy.

218 OXIDATION

rapidly in a lateral rather than in a vertical direction. On copper exposed to

1 mm Hg (133 Pa) oxygen pressure at 550 ° C, a continuous fi lm of Cu 2 O forms within 6 s; at 1 atm O 2 , the time required is still less. Using transmission electron microscopy and computer modeling, it has been shown that the metal – scale interface is very tightly bonded as a result of epitaxial relations between the metal and the scale, at least for thin scales [8] ; that is, as the crystalline oxide grows on the metal substrate, the crystal structure of the oxide is aligned with the structure of the metal. Growth of the resulting thin continuous oxide fi lm may be controlled by transfer of electrons from metal to oxide [9] , or by migration of metal ions in a strong electric fi eld set up by nega- tively charged oxygen adsorbed on the oxide surface [10] . When the continuous fi lm reaches a thickness in the order of several hundred nanometers, diffusion of ions through the oxide may become rate controlling instead. The latter situation prevails so long as the oxide remains continuous. Eventually, at a critical thick- ness, the stresses set up in the oxide may cause it to crack and detach — called

spalling — and the oxidation rate increases.