The corrosion/oxidation tests for mild steel were performed in the high temperature corrosive environments. The behaviours of weight gain, loss and film thickness and cross-sectional appearances of product films by SEM were investigated and characterized with properties of the films. It can be found that the films formed on the metal surface were composed not of iron chlorides but only iron oxides and that the corrosiveness of hydrogen chloride accelerates the oxidation. A method of single particle impact on the mild steel covered with the oxide films was useful for obtaining mechanical properties and adhesion of the oxide films at the high temperature corrosive environments. Adhesion and the mechanical properties of the oxide films such as elastic modulus, compressive fracture stress and shearing fracture stress could be obtained in this study.