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The influence of KCl on the oxidation of the 304-type (Fe18Cr10Ni) austenitic stainless steel at 600 • C in 5% O 2 and in 5% O 2 + 40%H 2 O is investigated in the laboratory. The samples are coated with 0.1 mg/cm 2 KCl prior to exposure. Exposure time is 1-168 h. Uncoated samples are exposed for reference. The oxidized samples are analyzed by ESEM/EDX, XRD and AES. The results show that small additions of potassium chloride strongly accelerate high temperature corrosion, the oxide thickness being up to two orders of magnitude greater after exposure in the presence of KCl. The rapid corrosion is initiated by the formation of potassium chromate through the reaction of KCl with the protective oxide. Chromate formation is a sink for chromium in the oxide and leads to a loss of its protective properties. The resulting rapidly growing scale consists of an outer hematite layer with embedded K 2 CrO 4 particles and an inner layer consisting of spinel oxide, (Fe,Cr,Ni) 3 O 4 . Little or no chlorine is found in the scale or at the scale/metal interface.

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The effect of KCl on the corrosion behavior of the austenitic stainless steel 304L was studied at 600°C in 5% O 2

The oxidation of 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated at 500 and 600°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 lmol 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 ).