Alkali chlorides deposited on superheater tubes are known to cause corrosion in boilers burning biofuels and waste. The objective of this work was to compare the corrosivities of KCl and NaCl on commonly used superheater steels at typical superheater tube temperatures (400-650 degrees C). Two austenitic stainless steels and one low alloy steel were tested under laboratory conditions in air. The results show some differences between the two salts in terms of corrosion behaviour and internal degradation. However, for practical applications, the two chlorides are equally corrosive on the tested steels under the experimental conditions considered. (C) 2012 Elsevier Ltd. All rights reserved.