KCl-Induced High Temperature Corrosion of the Austenitic Stainless Steel 304L – The Influence of SO2

The effect of SO2 on the oxidation of alloy 304L in O2+H2O and O2+H2O+KCl environment has been investigated at 600°C. Exposure time was 1-168 hours. The exposed samples were analyzed by SEM/EDX, XRD and IC. In dry O2, a protective and chromium-rich corundum-type oxide forms. In the presence of H2O(g), chromium is volatilized in the form of CrO2(OH)2(g). The corresponding chromium depletion of the protective oxide triggers a partial loss of protective properties resulting in the formation of oxide islands on the alloy grain centers. The oxide islands consist of an outward growing hematite layer and an inward growing FeCrNi spinel layer. By coating the samples with KCl the chromia depletion of the protective oxide dramatically increases due to the formation of K2CrO4. This leads to breakaway corrosion, a rapidly growing scale forming all over the surface. The resulting thick scale has a similar structure as the oxide islands formed in the absence of KCl. The addition of 300 ppm SO2 to the O2+H2O and O2+H2O+KCl environments results in a drastic reduction of corrosion rate. In O2+H2O environment the effect of SO2 is attributed to the formation of a thin sulphate film on the oxide surface that impedes chromium volatilization and decreases the rate of oxygen reduction on the oxide surface. In O2+H2O+KCl environment the corrosion mitigating effect of SO2 is mainly attributed to the rapid conversion of KCl to K2SO4. In contrast to KCl, K2SO4 does not deplete the protective oxide in chromium by forming K2CrO4.

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

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.

A Burner Rig Investigation of the Hot Corrosion Behavior of Several Wrought Superalloys and Intermetallics

The hot corrosion behavior of wrought Ni- and Co-base superalloys and nickel aluminide intermetallics was studied in a burner rig at 900°C (1650°F) injected with 5 and 50 ppm sea salt. Nickel aluminides (IC-50 and IC-218) were found to be extremely susceptible to hot corrosion attack. Some state-of-the-art wrought superalloys, such as HAYNES alloys 230 and 188, on the other hand, were extremely resistant to hot corrosion. The behavior of different alloy systems, breakaway corrosion and corrosion morphology are discussed.