Oxygen diffusion into metals

On metal surfaces, well-ordered structures are often formed [7, 79-82], At sufficiently high temperatures (> 300 K) and pressures (> 10 6 Torr), oxygen diffuses into the catalyst bulk [8, 61, 73 76, 83-91] in Pd, as many as 300 monolayers are dissolved [83] whereas in Ir and Pt the number of dissolved monolayers is slightly lower [78, 85 87]. The oxygen dissolved in the subsurface layer changes the reaction rate constants considerably (see Table 3 [86]). Finally, under certain conditions, oxygen adsorption can lead to surface reconstruction [7, 92]. Various types of oxygen adsorption over Pt metals have been studied in detail by Savchenko [7]. 

The rate of selective internal oxidation is limited by the rate of oxygen diffusion into the metallic matrix. With increasing oxidation time, the reaction slows because the zone in which it takes place moves towards the interior. The thickness of the two-phase layer (B + AO, -) increases gradually as the element A is transformed into oxide. The oxidation of CoTi alloys with very low Ti content illustrates this mechanism, titanium having a much greater affinity for oxygen than cobalt. 

The series of events leading to the formation of a severe crevice can be summarized in the following three stages. Firstly, crevice corrosion is believed to initiate as the result of the differential aeration mechanism mentioned earlier. IDissolved oxygen in the liquid which is deep in the crevice is consumed by reaction with the metal [Fig. 6.21( )]. Secondly, as oxygen diffusion into the crevice is restricted, a differential aeration cell tends to be set up between the... 

In addition to oxidation itself, gas diffusion into the base metal can be more damaging than the actual loss of metal from the surface. Thus the loss in mechanical properties owing to diffusion of oxygen into niobium makes it more difficult to protect niobium against oxidation damage than molybdenum, even though molybdenum has less resistance to normal oxidation effects than niobium. 

After leaving our samples in contact with C02 for about 16 hr, we found that the bands below 1950 cm-1 had increased in intensity, while the band at 2030 cm-1 had remained essentially constant. Obviously, some of the CO had migrated to other parts of the surface, while, in addition, some of the oxygen may have migrated, or even diffused into the bulk of the metal, whereupon some further chemisorption has taken place on the Bs sites, which have thus become vacated. When the C02 is frozen out by cooling part of the cell to the temperature of liquid nitrogen, no changes in the spectra are observed. Evacuation of the cell with a diffusion pump for a... 

Oxidative UPD involves the oxidation of species to form an atomic layer where the precursor contains the element in a negative oxidation state. A classic example is the formation of oxide layers on Pt and Au, where water is oxidized to form atomic layers of oxygen. Halide adsorption can be thought of similarly, where a species such as I oxidatively adsorbs on a metal surface as the halide atom. In that case, a bulk film is not formed at more positive potentials, but the diatomic is generated and diffuses into solution. With respect to compound formation, oxidative UPD from a sulfide solution is a good example ... 

Injection of air the oxygen in the injected air will prevent sulfate-reducing conditions in the sewer. The DO concentration in the wastewater establishes an aerobic upper layer in the biofilm, and sulfide produced in the deeper part of the biofilm or the deposits that may diffuse into the water phase will be oxidized (cf. Figure 6.2). The oxidation of sulfide will mainly proceed as a chemical process, although microbial oxidation may also take place (Chen and Morris, 1972). Factors that affect the oxidation rate of sulfide include pH, temperature and presence of catalysts, e.g., heavy metals. 

In the first scheme the metal boiling point is less than the oxide boiling point and the model consists of a vaporising droplet of metal surrounded by a detached reaction zone where condensed oxides appear as fine droplets. The reaction rate is said to be controlled by the vapour phase diffusion of metal and atmospheric oxygen into the reaction zone as in Figure 5.6. 

If these conditions are not satisfied, some process will be involved to prevent accumulation of the intermediates at the interface. Two possibilities are at hand, viz. transport by diffusion into the solution or adsorption at the electrode surface. In the literature, one can find general theories for such mechanisms and theories focussed to a specific electrode reaction, e.g. the hydrogen evolution reaction [125], the reduction of oxygen [126] and the anodic dissolution of metals like iron and nickel [94]. In this work, we will confine ourselves to outline the principles of the subject, treating only the example of two consecutive charge transfer processes O + n e = Z and Z 4- n2e — R. 

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