The aim of this paper is to obtain and discuss the mechanical properties and adhesion of corrosion product films on mild steel in high temperature corrosive environments by a particle impact method using plastic strain analyses of indentation behaviour. Corrosion/oxidation tests were conducted in high temperature corrosive environments of wet air, water steam with hydrogen chloride to look into basic behaviour of material degradation and the growth of corrosion product films. Weight gain and loss of the samples and film thickness was measured. Surfaces and cross-sections of the materials were observed in order to characterize physical properties of the films. Impact tests were performed on the surface of mild steel covered with corrosion product films under the corrosive environments to investigate destruction and exfoliation of the films around impact craters. Some equations with assumptions and based on mechanics of materials were derived to obtain mechanical properties and adhesion of the films from the experiments.