M. Hänsel

Effect of water vapour on growth and adherence of chromia scales formed on Cr in high and low pO2-environments at 1000 and 1050°C

Abstract The oxidation behaviour of pure Cr at 1000 and 1050°C was studied in Ar–O2 and Ar–H2–H2O mixtures. It was found that in the low-pO2 gases the oxide scales exhibited higher growth rates than in the high-pO2 gases. The scales formed in the low-pO2 gases showed substantially better adherence during cooling, than scales formed in the high-pO2 gases. These differences in growth rate and adherence can be correlated with differences in size and location of the in-scale voids formed during the isothermal exposure. Exposures in Ar-O2-H2O mixtures revealed that the differences in scale growth rates as well as in scale void formation and growth are not primarily related to differences in the oxygen partial pressure of the atmosphere but to the presence of water vapour in the test gas. At sufficiently high H2O/O2-ratios, water vapour promotes oxide formation at the scale/metal interface thereby suppressing excessive growth of existing voids, and also as a consequence improved scale adherence. Whether the enhancement of inward scale growth is related to transport of H2O- or H2-molecules or due to OH− ions, cannot be derived with certainty from the present results.

Carburization of Cr‐based ODS alloys in SOFC relevant environments

Chromium based ODS alloys are being discussed as interconnector materials in solid oxide fuel cells (SOFCs). One of the major requirements for the interconnect material is sufficient corrosion resistance in the anode and cathode side gases at the operating temperature of around 950 °C. In the present study the corrosion behaviour of a number of chromium based ODS alloys has been investigated in carbon containing simulated anode gas of an SOFC. Under these experimental conditions all alloys studied are prone to carburization. The carbon uptake, which mainly occurs in the early stages of exposure, leads to the formation of a mixed carbide/oxide layer beneath the external chromia scale. The carburization resistance of the alloys depends on the type and concentration of the oxide dispersion but even to a larger extent on its distribution. Therefore the careful control of the alloy manufacturing process is of great significance for obtaining sufficient carburization resistance under SOFC conditions.

Effect of in-situ gas changes on thermally grown chromia scales formed on Ni–25Cr alloy at 1000°C in atmospheres with and without water vapour

Abstract Regarding oxidation resistance, most high temperature alloys rely on the formation of a protective chromia surface scale during service. In the present study, the oxidation behaviour of a Ni–25%Cr model alloy was investigated during single- and two-stage oxidation in Ar–O2 and Ar–H2O gas mixtures at 1000°C. In the two-stage experiments, the test gas was changed after a predefined oxidation time from dry to wet gas, and vice versa, without intermediate specimen cooling. Oxidation kinetics were measured using thermogravimetry and post exposure characterisation was accomplished using scanning and transmission electron microscopy, focused ion beam and energy dispersive X-ray spectroscopy techniques. The single stage exposure to Ar–O2 resulted in the formation of a voided, poorly adhering chromia scale exhibiting a coarse, equi-axed morphology. In Ar–7%H2O a fine, columnar grained oxide scale was formed which was free of microvoidage within the scale and at the scale/metal interface and therefore exhibited excellent adherence to the metallic substrate. During two-stage exposure to Ar–O2 and subsequently to Ar–H2O, a fine grained, columnar shaped oxide developed on top of the coarse oxide scale formed in the first oxidation stage. The opposite effect occurred when the exposure started in the wet environment. The mechanisms which are responsible for the observed oxidation features are discussed on the basis of classical oxidation theory in combination with previous results on oxidation of chromia forming alloys in oxygen rich gases and water vapour containing, low p(O2) environments.

Enhanced Internal Oxidation as Reason for Breakdown of Protective Chromia Scales on FeCr-Alloys in Water Vapour Containing Gases

The oxidation behaviour of binary Fe-Cr alloys containing 10 and 20 mass % Cr, respectively, was studied in Ar-20%O2, Ar-7%H2O and in Ar-4%H2-7%H2O at temperatures between 800 and 1050°C. Thermogravimetric analyses in combination with analytical studies using SEM/EDX and Raman Spectroscopy revealed, that in atmospheres in which water vapor is the source of oxygen, Cr exhibits a higher tendency to become internally oxidized than in the Ar-O2 gas. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the surface oxide scale, the present results reveal that the presence of water vapor also affects the transport processes in the alloy. The enhanced internal oxidation, which is likely the result of water vapor increasing the solubility and/or the diffusivity of oxygen in the alloy, explains the frequently observed effect that Fe(Ni)Cr alloys with intermediate Cr contents (e.g. 10-20%, depending on temperature) exhibit protective oxidation in dry gases but breakaway type oxidation in steam. The temperature dependence of the change from protective to non-protective behaviour in Ar-H2O differs quantitatively, but not qualitatively from that in Ar-O2.

Effect of H2/H2O ratio on thermally grown chromia scales formed on Ni25Cr alloy in Ar–H2–H2O atmospheres at 1000°C

Abstract The oxidation behaviour of a NiCr-base model alloy with a 25% chromium content is investigated at 1000°C in different low p(O2) environments (Ar –H2–H2O) with the emphasis of how hydrogen andyor water vapour affects the oxidation behaviour of chromia forming alloys \in low p(O2) environments containing H2 and H2O. After equilibrium conditions are achieved, chromia growth on Ni25Cr in low p(O2) type test gases containing H2 and H2O is ruled by the concentration of Cr interstitials available for Cr outward transport. The chromia microstructure, therefore, consists largely of columnar shaped oxide grains. The chromia scales are growing predominantly from oxygen coming from the water vapour in the test gas. H-species may facilitate oxide growth in pores at the metal – oxide interface. H-defects affect the transport properties of chromia scales formed on Ni25Cr. At low p(O2) the H-defects in the chromia scale are compensated by a lower concentration of Cr interstitials. Depending on the nature of the test gas, i.e. p(O2), p(H2) and p(H2O), as well as on the Cr activity in the metal substrate, the growth of chromia scales on NiCr alloys may be featured and shaped differently.

Effect of specimen thickness on chromia scaling of Ni25Cr in N2–O2–H2O test gases at 1000°C

Abstract The isothermal oxidation behaviour of a Ni25Cr model alloy was studied using specimens of different thicknesses at 1000°C in dry and wet N2–1%O2. The oxidation mechanisms were evaluated using thermogravimetry and SEM/electron backscatter diffraction analyses of oxide scale cross-sections. The oxidation rates decreased with increasing specimen thickness and increasing water vapour additions in the gas. The findings can be explained by considering the effect of H-defects and in-scale stress state on point defect concentrations in the chromia lattice.

Hydrogen Uptake and Hydrogen Profiles in Chromia Scales Formed on Ni25Cr(Mn) in Low pO2 Test Gases at 1000°C

Two chromia forming materials have been exposed to different test gases at low pO2. All used test gases contained hydrogen species in the form of molecular hydrogen and water vapour. To insure the validity of the measured H-profiles H2 was replaced by with D2 and H2O by D2O. The Deuterium profiles in the oxides formed were established using SIMS and GDOES techniques. The profiles were quantified and correlated with the growth rate and the microstructures of the chromia scales formed on in low pO2 test gases. The presence of H2/H2O in the low pO2 test gas alters the properties of thermally growing chromia scales.