Oxide scales structure

Hydrogen permeation tests on ferritic stainless steels indicated that hydrogen can diffuse through the alloys, though the permeation was drastically decreased by formation of chromia scale on the alloys. - The mechanisms by which the presence of hydrogen or protons at the air side affects the oxide scale structure and growth are not clearly understood at this time. Several mechanisms have been proposed to tentatively explain the observed anomalous oxidation behavior. ... 

According to these thermodynamic considerations, the observed oxidation behavior is complex leading to a layered oxide scale structure with Ti02 on the outside and oxides with higher metal contents underneath, including AljOj (Khobaib and... 

Sintering aids are often added to SiC and Si3N4 to promote densification of these materials. Sintering aids have several noteworthy effects on the oxidation of silica forming materials including increased oxidation rates, change in rate controlling mechanism, and alteration of the oxide scale structure. 

Figure 4.12 Summary of oxide scale structure and composition formed on austenitic Fe-Cr-Ni alloys after exposure in 25 MPa SCW at 550°C for 500 h in a static autoclave [45].

A typical two-layer oxide scale structure was observed on the surface of the Ni-based alloys studied an outer layer of Cr203 and below it AI2O3 (Alloy... 

Oxide scale structures observed were found to be different on the two sides of the samples used (steel samples 25 x 9 x 8 mm iron samples 25 x 9x2 mm) when the reaction gas was supplied at a rate of 1 litre/min. In all cases, it was found that the typical three-layered (hematite/magnetite/wustite) oxide scale structures with a smooth scale surface were observed on the sample surfaces facing the gas flow, whereas hematite and magnetite were absent at the back surfaces and the scale had the idiomorphic [56,64], multi-faceted appearance associated with scaling in pure carbon dioxide and steam, as will be further discussed later. The relatively slow gas flow rate... 

With the improved understanding of the oxide scale structure entering the finishing mill, some related questions can be raised ... 

Electron backscatter diffraction (EBSD) analyses of oxide scale structure... 

R. Y. Chen and W. Y. D. Yuen, Oxide-scale structures formed on commercial hot-rolled steel strip and their formation mechanisms , Oxid. Met. 56, 89-118 (2001). 

Since the paper by Pilling and Bedworth in 1923 much has been written about the mechanism and laws of growth of oxides on metals. These studies have greatly assisted the understanding of high-temperature oxidation, and the mathematical rate laws deduced in some cases make possible useful quantitative predictions. With alloy steels the oxide scales have a complex structure chromium steels owe much of their oxidation resistance to the presence of chromium oxide in the inner scale layer. Other elements can act in the same way, but it is their chromium content which in the main establishes the oxidation resistance of most heat-resisting steels. 

Traditional alloy design emphasizes surface and structural stability, but not the electrical conductivity of the scale formed during oxidation. In SOFC interconnect applications, the oxidation scale is part of the electrical circuit, so its conductivity is important. Thus, alloying practices used in the past may not be fully compatible with high-scale electrical conductivity. For example, Si, often a residual element in alloy substrates, leads to formation of a silica sublayer between scale and metal substrate. Immiscible with chromia and electrically insulating [112], the silica sublayer would increase electrical resistance, in particular if the subscale is continuous. 

The ability of the STM to achieve atom-resolved real-space images of localized regions of a surface and to directly resolve the local atomic-scale structure has provided essential insight into the active sites on catalysts and emphasized the importance of edges, kinks, atom vacancies, and other defects, which often are difficult to detect with other techniques (46-49). It is evident, however, that STM cannot be used to image real catalysts supported on high-surface-area, porous oxide carriers. 

In this article I will elaborate on the knowledge, which was obtained by Professor Leroy Eyring and his colleagues in more than 50 years of research on the lanthanide higher oxides and their ideas for the applications of these oxides. Also the unique non-stoichiometric characteristics of the lanthanide higher oxides are emphasized and the intrinsic relationship between the macroscopic properties and nano-scale structures is demonstrated. 

The molecular-scale structure and chemistry of crystalline and X-ray amorphous metal oxide precipitates... 

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