Microalloyed steels show a greater atmospheric corrosion resistance than plain carbon steels through the formation of protective films, designated as patinas. During the development of these films, they present a continuous double layer structure. The inner phase is the only one left after long exposure periods and is determinant of the low atmospheric corrosion rates observed. In the present work a new technique was applied for the separation of phases. Characterization of both phased film and inner phase alone was performed by IR analysis, electrochemical reduction, and scanning electron microscopy. Test pieces of a weathering steel (C, 0.11; Mn, 0.38; Si, 0.23; P, 0.9; Cr, 0.69; Ni, 0.11; and Cu, 0.38) were exposed for 9 and 22 months at a semi-industrial atmosphere. Characterization techniques applied to samples of both phases and inner phase of patinas showed little difference in composition, noticeable morphological differences suggesting on the contrary greater barrier effect due to the inner phase rather than the outer one.