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This paper reports the oxidation behaviour of a low-alloyed steel in the presence of small amounts of KCl(s) at 400 and 5008C. Cleaned and polished sample coupons were exposed in a horizontal tube furnace with exposure times ranging from one hour to one week. The flue gas composition was 5% O 2 with 40% H 2 O in N 2 , and 0.1 mg KCl/cm 2 was deposited on the samples prior to exposure. X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis were used to characterize the samples. Cross sections were prepared by means of focused ion beam (FIB) and broad ion beam (BIB). At 4008C there was a modest mass gain in the absence of KCl. In the presence of KCl, mass gain is significantly increased and a rapid attack is evident already after short exposure times. At 5008C, the mass gains in the absence of KCl were considerably higher than at 4008C, especially after 168 hours of exposure. KCl had an effect at shorter exposure times, but after one week the samples exposed with and without KCl had mass gains of comparable magnitude.

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 ).

Polished 310 stainless steel coupons were exposed isothermally in a horizontal tube furnace at 500°C for 1-168 h. The exposure gas consisted of N 2 with 5% O 2 and 500 ppm HCl͑g͒. The corroded samples were analyzed by X-ray diffraction, focused ion beam microanalysis, scanning electron microscopy with energy-dispersive X-ray analysis, and by Auger electron spectroscopy. Mass gain was recorded. As expected, HCl͑g͒ is a corrosion accelerator, the dominating corrosion products being FeCl 2 , Fe 2 O 3 , and Cr 2 O 3 . A tentative mechanism for the initial stages of the HCl-induced corrosion of stainless steel is presented.

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