Effects of Alkali Salts on the High Temperature

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The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
Effects of Alkali Salts on the High Temperature
Corrosion of Stainless Steels
J. Pettersson*, 1, J. Froitzheim1, C. Pettersson1, T. Jonsson1, M. Halvarsson2, L.-G.
Johansson1 and J.-E. Svensson1
1
Chalmers, Dep. of Chemical and Biological Engineering, Göteborg, Sweden
2
Chalmers, Dep. of Applied Physics, Göteborg, Sweden
* [email protected]
ABSTRACT
The effect of alkali salts KCl, K2CO3 and K2SO4) on the high temperature corrosion of the austenitic
stainless steels, 304L and Sanicro 28, has been investigated in the laboratory. The exposures were
performed in a well controlled environment containing O2 and H2O. The lab results are compared with
samples exposed in commercial waste-fired and biomass-fired boilers using cooled probes. The metal
temperature was 600°C and the exposed samples were analyzed by SEM/EDX, XRD, FIB, TEM and IC.
K2CO3 and KCl strongly accelerate the corrosion of both materials. The protective, chromium rich oxide
reacts, forming K2CrO4.
½(CrxFe1-x)2O3(s)+ xK2CO3(s)+ xH2O(g)+ 0.75xO2(g) → ½(1-x)Fe2O3(s)+ xK2CrO4(s) + xCO2(g)
½(CrxFe1-x)2O3(s)+ 2xKCl(s)+ xH2O(g)+ 0.75xO2(g) → ½(1-x)Fe2O3(s)+ xK2CrO4(s)+ 2xHCl(g)
Hence, potassium chromate(VI) is a sink for chromium in the oxide. This results in a destruction of the
protective oxide and an acceleration of oxidation. The resulting thick scale formed on 304L consists of an
outer hematite layer and an inner layer consisting of FeCrNi spinel oxide. Micrometer size potassium
chromate particles are found in the upper part of the hematite layer. For the Sanicro 28 sample, the
corrosion attack was less severe. The corrosion morphology described above is true for both KCl and
K2CO3 exposed samples and there are only small differences in corrosion morphology between them.
Hence, the role of chlorine on the initial corrosion attack of these steels is small and the corrosion is
initiated by the destruction of the protective chromium-rich oxide and the formation of K2CrO4.
In addition to KCl and K2CO3, the corrosive effect of K2SO4 was investigated on 304L. In contrast to KCl
and K2CO3, the presence of K2SO4 did not increase the corrosion rate of 304L exposed at 600°C.
Accordingly, no chromate was detected on the sample surface after exposure. This is in line with
predictions using thermodynamic calculations. Thus, potassium sulphate remains unreacted on the surface
throughout the exposure time and does not take part in the corrosion attack.
Keywords: alkali chlorides, high temperature corrosion, stainless steel
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