Fundamental Studies on the Transient Stages of Scale Growth in Fe-22 wt.% Cr Alloys

Abstract

It is known that additions of reactive elements such as Ce, La or Y improve the properties of protective oxide-scales on Ni and Fe based alloys [ - ] by increasing oxide adhesion, decreasing the transient time until a continuous Cr2O3 layer is formed and decreasing the parabolic rate constant. Nevertheless, the precise roles played by these reactive elements to improve scales and the precise mechanisms by which they are incorporated into the scale during the surface treatment processes are unknown. Although they are believed to be associated with transport properties in the scale, it is not clear how this occurs or why it improves oxidation resistance. This project is aimed to gain understanding of the scale evolution in Fe-22 wt.% Cr alloys at 800 oC in dry air during the transient stage after 15 minutes of oxidation. The effect of La (120 and 290 ppm) and Ce (270 and 610 ppm) additions added during melt-stage processing are investigated. The surface oxidation process was imaged in-situ through a Confocal Scanning Laser Microscope (CSLM) and the results were correlated with post-experiment characterization through FEG-SEM and FIB-SEM combined with 3D reconstruction. The roles of rare-earth oxide particles on nucleation of Cr2O3 and blockage of short-circuit diffusion paths in the oxide scale and underlying metal are discussed.

On the Relation Between Oxide Ridge Evolution and Alloy Surface Grain Boundary Disorientation in Fe–22 wt % Cr Alloys

“…Type C represents situations in which diffusion occurs exclusively through the GB, and any case between the two extremes is type B, in which volume diffusion and GB diffusion simultaneously occur. While the presence of ridges coinciding with the underlying alloy GBs suggests that the kinetics are of either type C or more likely B ͑as shown below͒, it was not established in the earlier study 7 if there was a correlation between the GB characteristics and the presence or size of ridges. The purpose of the present investigation is to examine the relationships between fast-path diffusion during transient oxidation with respect to the geometry of alloy GBs by analyzing the scale ridge features and the underlying alloy GB characteristics.…”

“…However, some knowledge on the growth mechanism of the scale formed in the initial stage of oxidation is necessary because the nature of this transient scale, such as its spatial distribution, morphology, composition, structure, and/or any pertinent features, can influence the nature of the steady-state scale. 6 In an earlier paper, the authors observed scale ridge formation at the very early stage of oxidation, 7 and the scale ridge formation related to the alloy grain boundaries ͑GBs͒ beneath. Fast-path diffusion through GBs can be categorized into three kinetic types, A, B, and C, according to Harrison.…”

An Electron Microscopy Investigation of the Transient Stage Oxidation Products in an Fe-22Cr Alloy with Ce and La Additions Exposed to Dry Air at 1073 K (800 °C)

“…The fast grain-boundary diffusion of the scale-forming elements resulted in the formation of scale ridges, [17] and the relation between scale ridges and alloy grain-boundary characteristics has been analyzed in Reference 16.…”