Oxide Structures Produced on Iron-Chromium Alloys by a Dissociative Mechanism

Abstract

The oxidation of Fe-Cr alloys (1 to 15 wt % Cr) in a 0.1H20-0.9Ar atmosphere at 700~176 produces a characteristic two-layer scale overlaying a thin subscale. The outer ~s t i t e scale is extensively detached from the i n n e r scale containing an Fe-Cr spinel phase in a wfistite matrix. Phase identification by x -r a y diffraction, chemical analyses by conventional and electron microprobe techniques, and detailed metallographic studies have shown these structures to be consistent with the phase equilibria of the F e -C r -O constitution diagram. The extensive detachment of the outer w/istite scale layer and the continued high rate of oxidation of these alloys after such detachment occurs suggest that a dissociative process is responsible for the maintenance of this high oxidation rate. The experimental evidence for this process is presented, the importance of such a mechanism is discussed, and examples of other gas-metal systems in which a similar process has been recognized are cited.

A Comparison of the Oxidation of Fe&&bond;Cr, NiCr and CoCr alloys in oxygen and water vapour

“…The form of the kinetic curves is a shallow curve which can be approximated to a linear rate law. The activation energy of oxidation is such as to support the rate-controlling process being a phase-boundary reaction, occurring at the oxide/gas interface and involving the incorporation of oxygen into the lattice, in agreement with earlier extensive work (53).…”

“…Firstly, the effective oxygen potential or availability at the alloy/oxide interface in water vapour is always sufficiently high to ensure that the oxygen flux into the alloy is sufficient to precipitate the chromium as discrete oxide particles in the alloy. A 'carrier', involving hydrogen or water vapour, in the pores in the inner layer (53) presumably promotes adequate transport to the interface. Secondly, the rate of encroachment of the outer scale on to the internally oxidized zone is generally less than in oxygen.…”

Extreme‐Value Statistics Reveal Rare Failure‐Critical Defects in Additive Manufacturing

“…Several others have elucidated the environmental circumstances under which Fe-Cr alloys form such whisker-like oxides during thermal exposure. [40][41][42] However, it is unclear if the presence of this brittle oxide contributes to the low resistance to fracture, or if the premature failures are due solely to the morphology of the lack-of-fusion cavities.…”

Effects of oxidation-nitridation in the presence of water vapor on ASTM A335 P92 steel using SEM-EDS and XPS characterization techniques

“…On the other hand, it has been determined that the presence of water vapor stimulates the formation of volatile species in the form of hydroxides, stimulating the increase in the speed of corrosion of the material. One of the main reasons because the steels with nine percent of chromium (9% Cr) present such high corrosion rates in the presence of combustion gases, is due to the presence of water vapor [7][8][9][10]. In these steels, the negative effect of water vapor on oxidation is greater, as during the dry oxidation a thin layer of chromite (Cr2O3) is produced, which is continuous and adherent, which is a protective layer.…”