A Critical Compressive Stress for Increasing the Oxidation Kinetics of Fe–20Cr Alloy Oxidized at 900 °C

Abstract

The effects of compressive stresses on the oxide-scale morphologies formed on an Fe-20Cr alloy were investigated by comparison of the oxidation behavior in air under classical conditions, i.e., without any applied mechanical stresses and under static compressive stresses, at 900°C. The study was carried out mainly by comparisons of oxidation kinetics gained by thermogravimetric analysis (TGA), surface morphologies of oxidized specimens observed by scanning electron microscopy (SEM), oxidized products examined by X-ray diffraction (XRD). It was found that the application of compressive stresses induced an increase in oxidation rate, but a decrease of oxide grain size. When the stresses are in the range of 5-8 MPa, both chromium-and iron-oxides formed but, at other stresses, only chromia was present. In particular, there was a maximum in oxidation rate when the applied stress was 5 MPa. The paper places emphasis on analyzing the cause of this phenomenon.

Oxidation Kinetics Researches under the Condition of Compressive Loading

“…The change of surface morphologies were approved that in the previous work [11][12][13]. Besides, the growth of integrity oxide scale on Fe-20Cr alloy was increased by compressive loading based on the short-circuit diffusion along oxide grain boundary [10]. Ni,700 o C,10MPa Figure 6.…”

“…Where D eff is effective diffusion coefficient, D b is diffusion in the bulk, D gb is diffusion along grain boundaries, D gb >>D b [10]. Thus, the induced dislocations and defects by some methods will supply more nuclei and increase the diffusion, and therefore increase the oxidation rate of a metal/alloy.…”

“…And the smaller grain size of oxide induced by the compressive loading can be as an ex situ evidence on the increasing nuclei at the introduced dislocations and defects. This phenomenon is also reflected in pure Ni oxidized at 700 ℃ and Fe-16Cr alloy oxidized at 900 ℃ under the condition of compressive loading [10][11][12][13]. …”

Phase Identification and Internal Stress Analysis of Steamside Oxides on Plant Exposed Superheater Tubes

“…[1,12,13,[16][17][18] As a consequence of its effect on stress formation, the surface curvature of the sample also affects the kinetics and the morphology of the growing oxide. [19][20][21] In addition to the different stress states in planar vs nonplanar samples, the surface topography and the microstructure of laboratory samples is usually different from real superheater tubes. The flat laboratory samples may be deformed by machining (grinding and/or polishing), and the resulting lattice defects enhance diffusion in the surface near region strongly.…”

Effect of tensile load on high temperature oxidation of CoNiCrAlY coating

“…Calvarin-Amiri et al [29] investigated the effect of tensile load on the oxidation of Ni-20Cr alloy and they found that the tensile load accelerated hightemperature oxidation and the oxidation products were different between the specimens with and without tensile load. Zhou et al [30][31][32] [33], and the results showed that tensile stress accelerated high-temperature oxidation, whereas compressive stress had opposite effect, which were explained via theoretically deriving the effect of stress on ion diffusion.…”