KCI-lnduced High Temperature Corrosion of the Austenitic Stainless Steel 304L - The Influence of SO_2

摘要

The effect of SO_2 on the oxidation of alloy 304L in O_2+H_2O and O_2+H_2O+KC1 environment has been investigated at 600℃. Exposure time was 1-168 hours. The exposed samples were analyzed by SEM/EDX, XRD and IC. In dry O_2, a protective and chromium-rich corundum-type oxide forms. In the presence of H_2O(g), chromium is volatilized in the form of CrO_2(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 KC1 the chromia depletion of the protective oxide dramatically increases due to the formation of K_2CrO_4. 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 KC1. The addition of 300 ppm SO_2 to the O_2+H_2O and O_2+H_2O+KC1 environments results in a drastic reduction of corrosion rate. In O_2+H_2O environment the effect of SO_2 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 O_2+H_2O+KC1 environment the corrosion mitigating effect of SO_2 is mainly attributed to the rapid conversion of KC1 to K_2SO_4. hi contrast to KC1, K_2SO_4 does not deplete the protective oxide in chromium by forming K_2CrO_4.