Composite oxide scale

Fig. 3. Wagner s model of the transition from the internal to external oxidation of alloy A-B under the condition where both A and B can oxidize transition from internal oxide (a) to external composite oxide scale (b) to external exclusive oxide scale (c) [44,45,110-112].

The transition phenomena between external and internal oxidation of alloys are very complex. For simplification, such transitions can be divided into two main types according to the composition of the oxides formed on or in the alloys [6]. In the first type of transition, the external oxide scale and the internal oxide particle have the same composition, as shown in Fig. 3.1. This type of transition takes place between the growth processes of internal oxide particles in the alloy substrate (Fig. 3.1a) and an exclusive oxide scale on the alloy surface (Fig. 3.1b), under the condition that only the solute metal in the alloy can oxidize. In the second type of transition, the external oxide scale has a composite structure, as shown in Figs 3.2a and 3.2b. The composite oxide scale (AO + BO) represents mixtures, compounds or layers of AO and BO under different oxidizing conditions. This type of transition takes place between the formations of the internal oxide particles beneath the composite oxide scale (Fig. 3.2a) and a composite oxide scale on the alloy surface (Fig. 3.2b), under the condition that oxidation of all elements in the alloy can occur. In addition, there is a possibility of another type of transition between the growth processes of a composite oxide scale (Fig. 3.2b) and an exclusive oxide scale on the alloy surface (Fig. 3.2c) [7]. 

Different types of oxides can form. Under the conditions of Po, > P t and Eq. (3.30), a composite oxide scale can form on the alloy surface, as shown in Fig. 3.2b. Under the conditions of Pbo < P>2 - Po ° and Eq. (3.30), a transition from temporary external oxidation to intemad oxidation can take place (Fig. 3.10b). Under the conditions described by Eq. (3.29), however, this is the transition from permanent external to internal oxidation (Figs 3.10c to 3.10b). Therefore, under the condition of - Po 4 0- (3.28) can be used as the criterion for the transition between the growth processes of an exclusive oxide scale and a composite oxide scale while under the conditions of Pbo < Vq, fo °, it can be used as the criterion for the transition from... 

Directed Oxidation of a Molten Metal. Directed oxidation of a molten metal or the Lanxide process (45,68,91) involves the reaction of a molten metal with a gaseous oxidant, eg, A1 with O2 in air, to form a porous three-dimensional oxide that grows outward from the metal/ceramic surface. The process proceeds via capillary action as the molten metal wicks into open pore channels in the oxide scale growth. Reinforced ceramic matrix composites can be formed by positioning inert filler materials, eg, fibers, whiskers, and/or particulates, in the path of the oxide scale growth. The resultant composite is comprised of both interconnected metal and ceramic. Typically 5—30 vol % metal remains after processing. The composite product maintains many of the desirable properties of a ceramic however, the presence of the metal serves to increase the fracture toughness of the composite. 

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... 

Many barium aluminosilicate-based compositions will eventually react with the chromium oxide or aluminum oxide scales on the metal interconnect or metal edge rails to form barium chromate or a celsian phase at the interface [6], This can cause a mechanical weakness that is easily delaminated. Also, compositions that contain boron can react over time with water (steam) to produce B2(OH)2 or B(OH)3 gas. This can decompose the glass and greatly limit the lifetime of the seal. Thus many of the new investigations have emphasized low or no boron glass compositions. 

Both rust and oxide scales are usually mixtures of iron oxides vith other Fe (e. g. siderite) and non-Fe compounds (CaCOs). In some cases there is a more or less random mixture of components, vhereas in others, the different oxides are arranged in layers to form duplex or triplex scales. Layer-type rust arises as a result of potential or chemical gradients across the film. As these gradients vary ivith film thickness, the composition of the rust changes with the distance from the metal. On the whole, if Fe " and Fe" are present, the oxide containing Fe" is found in the inner layer of the rust. 

The oxidation behaviour of Si3N4 ceramics strongly depends on impurities in the gas atmosphere. Impurities like alkaline or alkaline earth metals, S02, and vanadium drastically decrease oxidation [431, 433, 434]. The main influence of the different impurities is caused by a change of the viscosity or the destruction of the oxide scale, accelerating the diffusion of oxygen or water vapour into the ceramic and increasing the corrosion. Of coarse, the effect strongly depends on temperature and gas composition. 

Long-term exposure of composites to oxidative environments can have deleterious effects on short-term mechanical behavior, such as resistance to crack initiation. This is particularly true in the case where the composite oxidizes to form an oxide surface scale. Although such reactions can be beneficial in limiting oxidation reactions, when the composite is subsequently cooled to room temperature, the reaction product can be a source of flaws and increase the composite s susceptibility to crack initiation. The following example illustrates this point. 

Enhanced oxidation resistance was also found at elevated temperatures for Co-Cr3C2,35 Ni-Cr 15 and Ni-Si3N415 composites. In contrast Ni-SiC15 and Ni-TiC23 composites have a higher hot oxidation rate than nickel. During hot oxidation porous metal oxide scales are formed at the metal-air interface. At elevated temperature interdiffusion between the particles and the metal in composites affects the formation of these scales. The break down of TiC particles in Ni-TiC composites accelerates corrosion by favoring the formation of nickel oxide.23 In... 

Typical micrographs of Fe-Al-Cr-alloys after oxidation testing are shown in Fig. 4. All alloys without mischmetal show a similar shape of the oxide scale. The scale is thin, with very few protrusions and a slightly wavy morphology. The scale appearance was affected neither by chromium nor by aluminium within the composition range investigated. None of the alloys showed significant void formation beneath the scale (Fig. 4a,b). 

The observed differences in scale thickness and microstructure between the oxide scales and subsurface zones at the various oxidation temperatures seem to be mainly attributed to the different diffusion rates at the respective temperatures. Since the oxidation products formed do not show any differences in the temperature range of 800°C to 1000°C it is concluded that no significant effect of the thermodynamic stability on the composition and structure of the oxidation products occurs. From the calculations of Rahmel and Spencer [21] it is known, however, that the activity of A1 and Ti in the system Ti-Al varies depending on the temperature. Thus it has to be taken into account that the temperature may have an influence on the expansion of the phase fields of some important phases in the system Ti-Al-N-O. Nevertheless it is evidently the temperature which mainly influences the kinetics because the structure of the metal/oxide interface, the formation of titanium nitrides, A127039N and an aluminium depleted metal phase is on principle always very similar. In this way the effect of different temperatures can, to a certain degree, be interpreted as that of a shift in the different stages of the oxidation process. 

A result of these hot salt corrosion and hot salt stress corrosion studies of titanium alloys was the importance of alloy composition in determining a titanium alloy susceptibility to attack. It was shown that alloys high in aluminium were preferentially attacked by hot salt [24, 25], with both A1 and Zr being incorporated in the internal and external oxide scales [26]. This early work raised the importance of aluminium in the hot salt corrosion of titanium alloys. 

In most cases [2,6] influence of alloying elements on oxidation resistance of titanium is explained by formation of protective film with a structure and phase composition that differs from film formed on the unalloyed titanium. In our researches the phase composition of scale consisted mainly of oxide TiOi (rutile) for all alloys. The composition of scale did not practically depend on alloying and duration of exposure. Therefore the reason of rising heat-resistant in the alloys is presumably due to influence of their structure and phase composition on the scale formation. 

In general, the solubility of oxides is a function of melt basicity and depends on the chemical composition of the passive layer. Hence, the corrosion process of alloys in molten salts is quite complicated as a result of the heterogeneous composition of the oxide scales formed on alloys. 

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