Microstructure Evolution During Steam Oxidation of a Nb Stabilized Austenitic Stainless Steel

Abstract

The oxidation behaviour of TP 347H FG in mixtures of water, oxygen, and hydrogen was investigated at 500, 600, and 700°C for a fixed oxidation time of 336 h. The samples were characterised using X-ray diffraction, reflective light and electron microscopy methods. Thin discontinuous double-layered oxide scales developed during oxidation at 500°C, whereas continuous double-layered oxide scales covered the entire sample surface after oxidation at 600 and 700°C. The major part of the inner oxide layer developed within the former alloy grains, whereas a Fe-Cr spinel formed along the former alloy grain boundaries. Transmission electron microscopy and electron energy loss spectroscopy investigations revealed that the part of the scale that grows into the alloy grains consists of particles of Fe-Cr spinel embedded in a metallic Fe-Ni matrix, which indicates that this part of the scale grows by an internal oxidation mechanism. The thickness of the inner oxide zone at high humidity (46%) is not significantly affected by the type of carrier gas used, whereas this thickness at low humidity (8% H 2 O) is sensitive for the carrier gas and increases in the following order: air \ Ar?7% H 2 \ Ar, indicating that the presence of oxygen or hydrogen in addition to a relatively low content of water vapour counteracts the effect of water vapour on the development of the inner oxide zone.

Effect of Water Vapor on High-Temperature Corrosion under Conditions Mimicking Biomass Firing

“…(c) Assisting the internal oxidation of Cr and as a result aiding the outward growth of Fe-rich oxide [31,33].…”

“…Effects of H2O vapour on high temperature corrosion of materials have been widely investigated in the absence of deposits (relevant to steamside oxidation of superheater tubes) [28][29][30][31][32][33][34][35][36][37][38]. Generally the corrosion results in an outward growing magnetite layer and an inward growing Fe-Cr spinel phase.…”

Effect of Element Diffusion Through Metallic Networks During Oxidation of Type 321 Stainless Steel

“…However, the concept has been widely used for analyzing oxidation in concentrated alloys in many Fe-and Ni-based alloy systems with high concentrations of active elements ( Ref 7,39,[41][42][43][44][45]. Although internal oxidation is always possible during high-temperature oxidation, the question is whether internal oxidation is the primary cause for breakaway oxidation.…”

High-Temperature Oxidation of FeCr(Ni) Alloys: The Behaviour After Breakaway

“…Refs. [1][2][3][4][5]. The useful life of an alloy component that suffers breakaway oxidation depends both on the incubation time prior to breakaway and on the scale growth rate after breakaway.…”

“…Refs. [1][2][3][4][5]. A recent paper describes the detailed microstructure of several cases of ''breakaway scales'' observed on a FeCrNi stainless steel that had been exposed to a variety of environments at 600°C [6].…”