In biofuel combustion corrosion of the superheater tubes induced by alkali chlorides in the deposits limits the efficiency in electricity production. The most severe corrosion generally occurs at the edge of the deposits. This location may be governed by the transport through the deposit. While most of the literature is focused on the effect of the deposit composition this study examined how the morphology of solid KCl deposits affects the attack. Coupons of two austenitic alloys (Alloy 3 10 and Sanicro 28) inside tablets of pressed KCl with different density and thickness were exposed to N(2)5%O(2)10%H2O at 500degreesC for 168h. Prior to the exposure tablets were shaped to examine the effect of thickness gradients, edges and cracks. Potassium chromate and iron-chromium oxides formed for all deposit morphologies and chlorine was frequently observed at the interface to the metal. The thicknesses of the deposit clearly affected that of the reaction products, especially for Alloy 310. The thickest products formed at intermediate deposit thickness. This behaviour is similar to that observed for these alloys in a field test. Cracks in the deposits enhanced the attack. At least for Sanicro 28, the chromate formation was observed to break down the protective chromia and thus accelerate the attack. Both alloys were preferentially attacked at metal grain boundaries.