XPS is a surface chemical analysis technique that can be used to analyze the surface chemistry of a material in its "as received". It is a powerful technique to investigate the nature of passive film in nanoscale. In this study, the XPS technique is being used to measure the elemental composition of the passive film that is formed on the surface of carbon steel at different depths and also to determine the ionic state of each element. In Fig. 3, a typical XPS spectrum of the passive film on the surface of carbon steel is demonstrated.