One of the major problems during combustion of biomass is corrosion of superheaters, caused by the deposition of alkali chloride containing ashes. Alkali chloride particles are formed during biomass combustion and transported via aerosols or in the vapour phase within the combustion gas, subsequently depositing on the metallic surface or on the already formed oxide layer. By reaction with the metal or the oxide layer the oxidation process is accelerated and metal is heavily consumed. To investigate these reactions several treatments, e.g. pre-oxidation, KCl deposition by thermophoresis and oxidation, were combined to produce various local deposits of KCl particles on iron samples. The deposit-carrying iron samples were exposed to N-2-20 vol. % O-2 and N-2-20 vol. % O-2- 500 vppm HCl atmospheres for short times at 300 degreesC. In N-2-O-2, deformation and local spreading of the particles was observed, probably by melt formation in contact with the metal. Oxidation with HCl addition led to a significant increase of the chlorine and oxygen content on the sample surfaces under KCl deposits.