Stages of oxide formation

An introduction to the fundamental concepts 1.1 STAGES OF OXIDE FORMATION 

As is the case for most natural phenomena, oxide films often have a rapid initiation, a vigorous youthful growth period, a fruitful middle age, a declining later period and, finally, destruction. In this vein, we can list at least ten overlapping stages which may characterize an oxide film at one time or another during its lifetime. 

Both categories of transport discussed above involve the motion of defects relative to an otherwise ordered array of ions, so the transport is referred to as defect transport. The concentrations of such defects (as contrasted with the total atom concentrations within the oxide lattice) are the important quantities for mass transport, charge transport, and space—charge effects, so it is the species defect concentration which appears in the diffusion equation. Likewise, the diffusion coefficients 

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Oh Ni85Cri5(l 11) different stages of oxide formation are found by Jeng et al [68]. At 200-300 °C, initially a chromium oxide is formed and only at higher O2 exposures is NiO also observed. On Ni-Ru [69], XPS and UPS unambiguously indicated the formation of an oxide overlayer. 

At least two anodic peaks depicting the initial stages of oxide formation are observed, followed by a very wide plateau at potentials > 1 V. 

De, A. K., A. Mukhopadhyay et al. 2004. Numerical simulation of early stages of oxide formation in molten aluminium-magnesium alloys in a reverberatory furnace. Modell. Simul. Mater. Sci. Eng. 12(3) 389-405. 

In 1929 Pfeil" published a most interesting account of the way layered structures form and the manner in which they influence oxidation rates. From detailed studies of the growth and composition of scales he was able to show clearly how the formation of barrier layers reduced scale formation by hindering outward diffusion of iron through the scale. Naturally, this work had to be largely based on the study of scales of sufficient thickness so that the mechanism of the early stages of oxidation could not be studied in this way. Pfeil analysed the outer, middle and inner layers of scales formed... 

The transition from non-protective internal oxidation to the formation of a protective external alumina layer on nickel aluminium alloys at 1 000-1 300°C was studied by Hindam and Smeltzer . Addition of 2% A1 led to an increase in the oxidation rate compared with pure nickel, and the development of a duplex scale of aluminium-doped nickel oxide and the nickel aluminate spinel with rod-like internal oxide of alumina. During the early stages of oxidation of a 6% A1 alloy somewhat irreproducible behaviour was observed while the a-alumina layer developed by the coalescence of the rodlike internal precipitates and lateral diffusion of aluminium. At a lower temperature (800°C) Stott and Wood observed that the rate of oxidation was reduced by the addition of 0-5-4% A1 which they attributed to the blocking action of internal precipitates accumulating at the scale/alloy interface. At higher temperatures up to 1 200°C, however, an increase in the oxidation rate was observed due to aluminium doping of the nickel oxide and the inability to establish a healing layer of alumina. 

Assuming the formation of N0 nuclei at the first stages of oxidation, the effective relaxed area (taking into account the overlap between neighboring expanding conductive regions) at every overpotential tj can be estimated by means of the Avrami equation.177 We arrive at... 

Tremiliosi-Eilho G, Jerkiewicz G, Conway BE. 1992. Characterization and significance of the sequence of stages of oxide film formation at platinum generated by strong anodic polarization. Langmuir 8 658-667. 

Later work by several research workers, and most particularly Greef (1969), showed that the very early stages of oxide film formation could be detected using more sensitive instrumentation and the changes in A, 4 and the light intensity correlated very well with the charge passed both in the anodic and cathodic sweeps (see Figure 3.13). 

The reaction of ions with peroxyl radicals appears also in the composition of the oxidation products, especially at the early stages of oxidation. For example, the only primary oxidation product of cyclohexane autoxidation is hydroperoxide the other products, in particular, alcohol and ketone, appear later as the decomposition products of hydroperoxide. In the presence of stearates of metals such as cobalt, iron, and manganese, all three products (ROOH, ROH, and ketone) appear immediately with the beginning of oxidation, and in the initial period (when ROOH decomposition is insignificant) they are formed in parallel with a constant rate [5,6]. The ratio of the rates of their formation is determined by the catalyst. The reason for this behavior is evidently related to the fast reaction of R02 with the... 

At this point the oxidation stage of quinonediimine has been fully reached its (very unstable) salts have scarcely any colour. The production of colour only takes place when quinonoid and benzenoid systems are present together. The molecular union of the two substances at different stages of oxidation produces the intense absorption which is a prerequisite for the formation of a dye (Willstatter and Piccard). This union need not take place in the proportion 1 1, which obtains in the present case. The relations between quinhydrone and quinone-quinol are quite similar (p. 314). 

Despite the vast quantity of data on electropolymerization, relatively little is known about the processes involved in the deposition of oligomers (polymers) on the electrode, that is, the heterogeneous phase transition. Research - voltammetric, potential, and current step experiments - has concentrated largely on the induction stage of film formation of PPy [6, 51], PTh [21, 52], and PANI [53]. In all these studies, it has been overlooked that electropolymerization is not comparable with the electrocrystallization of inorganic metallic phases and oxide films [54]. Thus, two-or three-dimensional growth mechanisms have been postulated on the basis that the initial deposition steps involve one- or two-electron transfers of a soluted species and the subsequent formation of ad-molecules at the electrode surface, which may form clusters and nuclei through surface diffusion. These phenomena are still unresolved. 

Many vent microbes are symbionts. Others form dense filamentous mats, such as the sulfide oxidizers Beggiatoa (oxic) and Thioploca (hypoxic). Similar mats probably develop subsurface within hydrothermal conduits. These subsurface mats are ejected into the water column during first stage of vent formation. The resulting explosive discharge of this biomass, which has the appearance of a white floe, has given rise to the term snowblower vent. The ejected microbes are thought to eventually settle back down onto the seafloor where they increase in number to form surficial mats that support the successional colonization of vent animals. 

Other indices measure a secondary stage of oxidation, such as the anisidine value (ANV), pointing to formation of carbonyl compounds, capable of undergoing condensation reactions with p-anisidine, and the thiobarbituric acid reactive substance (TBARS) pointing to the presence of malondialdehyde (MDA) in particular. In biological systems, TBARS is of widespread use as a measure for the extent of oxidation damage. Another test for stability of oils to oxidation is based on the development of acidity as secondary product, for example, standards using the Rancimat equipment or a similar setup. 

The results of methanol addition to the system H202—H20—CH4 lead to an unambiguous conclusion about the non-intermediate role of CH3OH in the selective oxidation of CH4 with hydrogen peroxide [36,37], This reaction does not produce CO and C02, therefore, in conditions of relatively low temperatures and short contact times the elementary stages of their formation were not taken into account in the suggested mechanism of CH4 oxidation. Moreover, the following reaction was also excluded from the oxidation scheme ... 

In recent years, modern instrumental methods have been developed to monitor lipid oxidation in biological samples, including dairy products. These include use of electron spin resonance (ESR) spectrometry, direct measurement of secondary oxidative products such as malonaldehyde, static and dynamic GC/MS methods. ESR spectrometry permits detection of free radicals formed in the very early stages of oxidation prior to the formation of peroxides. The method has been applied successfully to dairy products such as milk powders and processed cheese (Nielsen et al., 1997 Stapelfeldt... 

Organic salts of nickel do not themselves act catalytically, but are reduced by hydrogen when suspended in the heated oils, yielding nickel in various stages of oxidation, and these products effect the hydirogena-tion. Thus nickel formate is reduced by hydrogen at 210° C. in the presence of the oil to nickel suboxide, whilst at 250° C. complete reduction to the metal is effected. 

The reaction of 1,2-diaminonitrobenzenes with aldehydes is a widely accepted synthetic route to nitrobenzimidazoles [57, 62,63,66, 350,372-383], This reaction passes sequentially through a stage of the formation of azomethines (Schiff s base) and benzimidazolines. On oxidation the latter forms the corresponding benzimidazole derivatives (Scheme 2.44). 

The coloured borax beads are due to the formation of coloured borates in those cases where different coloured beads are obtained in the oxidizing and the reducing flames, borates corresponding to varying stages of oxidation of the metal are produced. Thus with copper salts in the oxidizing flame, one has ... 

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