The Influence of KCl on the Corrosion of an Austenitic Stainless Steel (304L) in Oxidizing Humid Conditions at 600 °C: A Microstructural Study

The effect of KCl on the corrosion behavior of the austenitic stainless steel 304L was studied at 600 °C in 5% O2 + 40% H2O + N2. The breakdown of the protective oxide was investigated. This was done through a detailed microstructural characterization of the oxide scales formed after 1, 24 and 168 h. The oxidized samples were investigated by SEM/EDX, FIB and STEM/EDX. The presence of KCl(s) causes a breakdown of most of the protective scale, even though it is not in direct contact with KCl(s) particles, starting after just 1 h exposure. A fast growing porous oxide formed in direct contact with (former) KCl(s) particles and an about 2 μm thick scale covered most of the surface. Only some regions were covered by a thin scale. K2CrO4 particles were randomly distributed all over the scale after 1 h exposure. The particles are situated above the oxide scale and are not in direct contact with the subjacent metal. The thin scale contains lower Cr levels than has been observed in corresponding scales formed in the absence of KCl. The breakdown of the protective scale is suggested to be caused primarily by the formation of K2CrO4, depleting the protective oxide in chromium. In addition, chromia evaporation contributes to chromia depletion and breakdown of the protective scale. Very little or no transition metal chlorides were found after breakaway oxidation. Cl is suggested to play a minor role in the initial breakdown of the protective scale. The presence of KCl particles caused local rapid oxidation, which results in an outward growing Fe and Fe–Cr rich porous oxide.