Zbigniew Żurek

Diffusion Rates of51Cr,54Mn and 59Fe in MnCr2O4 and FeCr2O4 Spinels

As the result of oxidation of Cr-Mn steels in SO2 the three layer scale is formed. The intermediate layer of this scale is composed mainly of MnCr2O4 spinel whereas FeCr2O4 spinel is present in small amount. MnO dominates in the outer layer. The inner, very thin scale layer contains oxides/sulfides mixture. The aim of this study was to examine self-diffusion processes in both spinels by multitracer method of diffusion measurements to know which of the transport processes during oxidation is the smallest one and deciding on the corrosion rate. In diffusion experiments the radioisotopes 54Mn, 51Cr and 59Fe were used. The serial sectioning method was applied to simultaneous evaluation of diffusion rates of chromium, manganese and iron in both spinels at 1073 K and 1173 K under the pressure of 105 Pa in SO2 containing 10 Pa O2. These spinels were obtained by modified sol-gel method from nitrates. Structures of the spinels were examined by X-ray spectrometry. It was found, that the diffusion rates of metals are higher in MnCr2O4 spinel. Moreover the dominant mechanism of manganese transport (the highest one) in studied samples is the volume diffusion while chromium and iron are transported mainly through the high diffusivity paths.

Influence of Temperature and Composition of SO2/O2 Atmosphere on the Surface Morphology of Alumina Scales Formed on FeCrAl Steel

The paper deals with the oxidation of an FeCrAl-type steel in SO2 and in SO2/O2 gas mixtures in the temperature range 800 850°C. It has been found that at 850°C, independently of the composition of the oxidizing atmosphere, the α-Al2O3 scale is strongly deformed and poorly adherent to the metallic substrate. At lower temperatures (800-830°C), the scale is adherent and composed of α-Al2O3 and θ-Al2O3, with a characteristic plate-like morphology of the external surface. At 820°C, in the atmospheres containing less than 60% SO2, the plate-like structure tends to disappear and the rate of scale formation increases with the concentration of O2 in the SO2/O2 mixtures. In SO2 and in the SO2/O2 mixtures containing more than 95% SO2, a well-developed plate-like structure is observed and its mass is comparable to the mass of the inner compact layer of α-Al2O3.

The Oxide Scale Growth Mechanism on Fe20Cr5Al+RE Alloy in SO2+O2

The oxidation mechanism of FeCrAl (+RE), RE: reactive elements: Y and Hf) thin foils was studied at temperatures ranging from 1093 K to 1173 K in SO2+1%O2 atmosphere. Materials were subjected to isothermal and thermal cycling exposures as well as to the so-called two-stage-oxidation. In the latter, an oxygen isotope 18O2 was used as a tracer. Starting materials and scales were characterized using Grazing Angle X-Ray Diffraction (GA-XRD), EDX, SEM, XPS and High Spatial Resolution Secondary Ion Mass Spectrometry (HSR-SIMS). The obtained results showed within the studied range of exposure conditions the scales on all the studied alloys grow via outward mechanism typical for transient oxides and not for the -Al2O3 which is consistent with phase composition results and scale morphology and/or microstructure. It was also found that ‘as received’ foils are not bare metals but complex oxide-on-metal systems resulting from their manufacturing procedure. The obtained results are discussed in terms of the diffusion-related transport properties of the scale and of the scale phase composition.

High Temperature Sulphidation Behavior of Fe20Cr6Al-Y,Hf Steel

The high temperature corrosion behavior of Fe20Cr6Al-Y,Hf steel was studied in the range of temperature 800-1000oC in H2/H2S atmospheres at pS2=10-2 ,10-3 and 10-4 Pa sulphur vapor pressures. Kinetics depend on the temperature and sulphur vapor pressure. After 24 hours the whole specimen was practically consumed because the samples were 0,06 cm thick. Morphology of the scales have been performed by SEM techniques. Phase and chemical composition have been studied by EDX and XRD techniques. It was found that scale formed on Fe20Cr6Al-Y,Hf alloy was built with porosity sulphides layer. EDX analysis of the scale surface show that the any aluminum, hafnium and yttrium sulfides were found in the formed scale layer, however small amounts of Al2S3 was detected in scale/steel interface. Also internal sulphidation was observed. A phase analysis of the formed scale revealed that it is composed mainly of an FeS, Fe7S8 phases and CrS, Cr5S6. Result were compared with data obtained on the pure Fe and Cr samples.

Investigation of the Scale Growth Mechanism on FeCrAl Thin Foil in SO2 Using Tracer Method

The results of the study of the scale growth mechanism on the 50 µm thick foil of Fe18Cr5Al are reported. The experiments were carried out at 1093 K using a two-stage-oxidation approach. In the first stage, the samples were exposed to the SO2 + 1 vol. % O2 mixture for 1.5 h and 24 h in order to produce the primary scale. Subsequently, the samples were cooled down to the room temperature and moved to the furnace where they were exposed to the atmosphere rich in 18O2. The duration of the second stage was 0.5 h and 4.5 h for samples previously oxidized for 1.5 h, 8 h and 72 h, for the samples oxidized previously for 24 h. The elemental distributions were determined using high resolution SIMS, while the scale morphology was observed by means of SEM. In addition the radioisotopic studies using 35SO2 were carried out in order to assess whether sulfide phases developed at the initial stage of oxidation. All the results were interpreted in terms of the transport processes in the oxide scale.