Papers by Author: Lars Gunnar Johansson

Abstract: The oxidation of iron in dry O2 and in wet O2 (40% H2O) has been studied at 600°C. The oxide microstructure was investigated by SEM/EDX, FIB and XRD. Iron forms a layered scale in dry and wet oxygen at 600°C. The scale consists of a top hematite layer, a middle magnetite layer and a wüstite layer close to the scale metal interface. All three layers grow with time, but with different growth rates, the overall growth being approximately parabolic. The presence of water vapour increases the rate of oxidation and affects the evolution of the oxide microstructure. The higher rate of oxidation in the presence of water vapour is due to an increased growth rate of the magnetite layer and, especially, of the hematite layer, while the growth of the wüstite layer is not affected. It is suggested that water vapour influences grain boundary transport in the hematite layer.

Abstract: Two FeCrAlRE alloys, a commercial, 0C404, and a model alloy in the form of thin foils, with different Mn, Nb, Mo and Ti concentrations were subjected to cyclic oxidation in lab air at 1100°C. The oxidized samples were studied by gravimetry, Grazing-Incidence X-ray Diffraction (GI-XRD), Scanning Transmission Electron Microscopy (STEM), and Energy Dispersive X-ray (EDX) analysis. The two FeCrAl alloys exhibit different oxidation kinetics; however, both alloys have the same weight gain after 500 hours exposure. During the early stages the scale consists mainly of α-Al2O3 together with some oxide particles containing Mn, Al, Fe and Cr formed on the alloys. After 500 hours the 0C404 scale locally also consists of larger polycrystalline regions of Mn-Cr-Al spinel. In addition, Si-rich oxide, chromia and Al-Cr oxide could be observed at the metal/oxide interface.

Abstract: The influence of KCl, K2CO3 and K2SO4 on the initial stages of corrosion of 304-type (Fe18Cr10Ni) stainless steel was investigated at 600°C in 5% O2 + 40% H2O. Small amounts of salt (1.35 .mol K+/cm2) were added before exposure. The exposures were carried out in a thermobalance. Exposure time was 24 hours. Reference exposures were carried out in 5% O2 and in 5% O2 + 40% H2O. The oxidized samples were analyzed by SEM/EDX, XRD and IC. KCl and K2CO3 are very corrosive towards 304L, producing thick non-protective scales. Corrosion is initiated by the reaction of the potassium salts with the protective, chromium-rich oxide forming K2CrO4. This depletes the oxide in chromia and converts it into iron-rich non-protective oxide. In contrast, K2SO4 does not accelerate corrosion significantly.

Abstract: Corrosion field tests have been carried out in the superheater region of a commercial waste-fired 75MW CFBC boiler using air cooled probes. Exposure time was 24 and 1000 hours. The effect of adding sulphur to the fuel on the corrosion of two high alloyed steels and a low alloyed steel was studied. The fuel consisted of 50% household waste and 50% industrial waste. The exposed samples were analyzed by ESEM/EDX and XRD. Metal loss was determined after 1000 hours. Both materials suffered significant corrosion in the absence of sulphur addition and the addition of sulphur to the fuel reduced corrosion significantly. The rapid corrosion of the high alloyed steel in the absence of sulphur addition is caused by the destruction of the chromiumcontaining protective oxide by formation of calcium chromate. Adding sulphur to the fuel inhibited chromate formation and increased the sulphate/chloride ratio in the deposit. Iron(II) chloride formed on the low alloyed steel regardless of whether sulphur was added or not.

Abstract: Corrosion/deposition field tests have been carried out in the superheater region of a commercial waste-fired 75MW CFBC boiler using air cooled probes. The influence of material temperature (450-500°C), flue gas temperature, temperature variations (i.e. thermal cycling) and additives to the fuel (elemental sulphur and dolomite) on deposition and corrosion was studied. The results presented here mainly consider the influence of sulphur additions to the fuel. The fuel was a mixture of 50% household waste and 50% industrial waste. After exposure the samples were analyzed by ESEM/EDX, XRD, AAS, FIB and IC. With no additional sulphur, alkali chlorides made up a large part of the deposit/corrosion product layer and in some cases chromate (VI) was detected. It is suggested that the chromate (VI) has formed by reaction of the protective oxide with alkali chlorides in the deposit. Adding sulphur to the fuel changed the composition of the deposits, alkali chlorides being largely replaced by alkali sulphates. No chromates(VI) were detected in the sulphur-added runs. It is suggested that adding sulphur to the fuel may decrease fireside corrosion because it changes the composition of the deposit. Alkali sulphates are much less corrosive than alkali chlorides partly because they do not form chromate(VI).

Abstract: The influence of gaseous KCl on the high temperature oxidation of alloy Sanicro 28 (27Cr31Ni) at 600°C in 5% O2 (N2 in balance) is reported. The samples were exposed isothermally in flowing gas, the dew point of KCl being 590°C corresponding to a partial pressure of KCl of about 2·10-6 atm. The exposure time was 24, 72 and 168 hours. The samples were investigated by gravimetry, grazing incidence XRD, SEM/EDX and AES. The results show that the oxidation of Sanicro 28 at 600°C is accelerated by KCl(g) at metal temperatures above the dew point of the salt. KCl(g) reacts with the protective chromium rich oxide ((Fe1-xCrx)2O3) forming K2CrO4. The resulting chromium depletion of the oxide gives an increasing oxidation rate but does not trigger “breakaway” corrosion. The distribution of potassium chromate on the sample surface is strongly flow-dependent, showing that the rate of formation of potassium chromate is limited by the rate of transport of KCl(g) to the surface. No evidence for chlorine was found on the corroded samples by AES profiling or EDX.