The Effects of KCl, K2SO4 and K2CO3 on the High Temperature Corrosion of a 304-Type Austenitic Stainless Steel

The oxidation of 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated at 500 and 600 degrees C in 5% O(2) + 40% H(2)O. Prior to exposure the samples were sprayed with KCl, K(2)CO(3) or K(2)SO(4), the amount of salt corresponding to 1.35 mu mol K(+)/cm(2). For reference, salt-free samples were exposed in 5% O(2) + 40% H(2)O and in 5% O(2) (N(2) was used as carrier gas). The oxidized samples were analyzed with SEM/EDX, XRD, IC and FIB. KCl and K(2)CO(3) strongly accelerate the corrosion of 304L while K(2)SO(4) has little influence on the corrosion rate and on the morphology of the corroded surface. KCl and K(2)CO(3) react with the chromium-rich oxide on the sample surface, forming K(2)CrO(4). The resulting chromium depletion of the protective oxide causes rapid oxidation and the formation of a thick duplex scale consisting of an outer hematite layer and a inner layer made up of FeCrNi spinel-type oxide. The differences in the corrosivity of the three salts are directly connected to their ability to form chromate on the surface and, hence, to the relative stability of the corresponding leaving groups (HCl, CO(2) and SO(3))