Corrosivity of KCl(g) at Temperatures above Its Dew Point - Initial Stages of the High Temperature Corrosion of Alloy Sanicro 28 at 600°C

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.