Oxide nuclei

A furdrer complication is that in these slowly growing oxide films, tire spread of the oxide across the metal surface is limited in the early stages by nucleation and growth control. The bare patches of metal between the oxide nuclei will clearly be exposed to a higher oxygen potential and new oxide nuclei will grow at a different initial rate than on the existing nuclei. 

The Ni-base alloy surface is exposed to an oxidizing gas, oxide nuclei form, and a continuous oxide film forms (Ni) (Cr203, etc.)- This oxide film is a protective layer. The metal ions diffuse to the surface of the oxide layer and combine with the molten Na2S04 to destroy the protective layer. Ni2S and Cr2S3 results sulfidation) ... 

The uniformity of film thickness is dependent upon temperature and pressure. The nucleation rate rises with pressure, such that at pressures above atmospheric the high rate of nucleation can lead to comparatively uniform oxide films, while increase in temperature reduces the density of oxide nuclei, and results in non-uniformity. Subsequently, lateral growth of nuclei over the surface is faster than the rate of thickening until uniform coverage is attained, when the consolidated film grows as a continuous layer ... 

In the very early stages of oxidation the oxide layer is discontinuous both kinetic and electron microscope" studies have shown that oxidation commences by the lateral extension of discrete oxide nuclei. It is only once these interlace that the direction of mass transport becomes of importance. In the majority of cases the metal then diffuses across the oxide layer in the form of cations and electrons (cationic diffusion), or as with the heavy metal oxides, oxygen may diffuse as ions with a flow of electrons in the reverse direction (anionic diffusion). The number of metals oxidising by both cationic and anionic diffusion is believed to be small, since a favourable energy of activation for one ion generally means an unfavourable value for the other... 

We define a nucleation overpotential rjN EN E0 (Fig. 36) required to make the N0 oxidation nuclei appear. The nucleation overpotential is related to the degree of closure (compaction) of the polymeric entanglement ( ), expressed as the fraction of interchain free volume destroyed after polarization at a given potential Ec, compared with the amount of free volume present at Es. 

There are two possible explanations for this behavior (1) the signals come from the oxygen of minority species, such as incipient oxide nuclei, which continue to grow with exposure even though total coverage barely changes ... 

Figure 4.8 Oxygen adatom mobility resulting in the growth of oxide nuclei at Mg(0001) at 295 K (a-c) separation of oxide bilayer at Mg(0001) at 295 K (d-f). (Reproduced from Ref. 41).

It is quite clear, of course, that the dislocations in the metal and those much more numerous in the oxide crystals composing the oxide layer play a role in the growth of the oxide, but because of the "back stress" effect mentioned earlier, they should produce a very nearly uniform film. The formation of these oxide nuclei remains, therefore, unexplained. 

Some experimental data obtained in high temperature oxidation studies indicate that the above might be a fruitful field of investigation. Harris et al (72) have shown that the nucleation and growth of cuprous oxide on copper are closely related to dislocations in the metal. Gulbransen and Andrews (73) report data which indicate that the growth of oxide nuclei on iron is related to the number and arrangement of dislocations in the metal crystal. 

Mixed y-Ga203-Al203 oxides of different stoichiometry were prepared by the solvothermal method from Ga(acac)3 and Al(OPr-i)3 as starting materials and were used as catalysts for selective reduction of NO with methane. The initial formation of gallium oxide nuclei controls the crystal structure of the mixed gallium-aluminum oxides. It is found that the acid density per surface area is independent of the Al Ga feed ratio but depends on the reaction medium (diethylenetriamine, 2-methylaminoethanol, toluene, 1,5-pentanediol etc.), whereby in diethylenetriamine the catalyst had lower densities of acid sites and showed a higher methane efficiency. 

Figure 6. Oxide nuclei on (111) iron foil after oxidation at 540°C and 1.1 X 10" ...

Figure 8.23 Oxide formation on a metal surface (a) physical adsorption of oxygen (O2) molecules from the air (b) chemical adsorption of separated oxygen atoms (O) strongly hound to the surface (c) penetration of some oxygen atoms into the metal to form a subsurface layer, as more oxygen arrives at the surface (d) saturation of the surface and subsurface with oxygen, leading to formation of oxide nuclei on the surface (e) surface layer of oxide grains...

Adsorption has been assumed to be the rate-determining process during early oxide-film formation. When a clean surface is exposed to an oxidizing gas, each molecule impinging on the surface may either rebound or adsorb. The fraction, a, that remains adsorbed on the metal surface should be constant for a constant temperature and oxygen partial pressure. Therefore, under these conditions a constant reaction rate is expected. However, the value of a is markedly lower on those parts of the surface covered with a monolayer or oxide nuclei. Thus, as adsorption or oxide nucleation proceeds, the reaction rate is expected to decrease accordingly until complete coverage of the surface by oxide has been achieved, when a much lower rate is expected to be observed. 

In the low-temperature region (approx 250 "C), the oxidation rate of iron is sensitive to crystal face, decreasing in the order (100) > (111) > (110) [47]. The oxide nuclei, apparently consisting of Fe304, grow to form a uniform film of oxide. Subsequently, a FeiOs nucleates and covers the Fc304 layer [48,49]. 

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