In order to control ash deposition on heat exchanger tubes in coal or woody biomass combustion boilers, surface treatment of the tubes, using a thermal spraying technique, was proposed in this study. The ash deposition mechanisms were elucidated, based on physical and chemical aspects, under the condition of long exposure period. First, adhesive force (Sb) between ash and alloy surface at high temperature was measured. As the physical aspect, Sb increased with an increase of the exposure time and could be predicted by an improved logistic function. Moreover, Sb depended on the temperature, so that an interface reaction controlled the increasing rate of adhesive force. As the chemical aspect, the interface between the ash deposition and alloy was observed and analyzed by a SEM-EDS and a FE-EPMA to consider the mechanisms of an increase of the adhesive force. The result showed that S and Na contained in the ash composition diffused into the alloy and that Fe mainly composing of the alloy specimens also diffused into the ash deposition. The diffusion coefficient of Fe into the ash deposition was also calculated by the results of a point elemental analysis. These results suggested that the interfacial reactions of the ash particles with the alloy affected the increase of adhesive force.
