The passivation process was carried out and monitored employing a three electrode electrochemical glass cell. Surface analysis using grazing incidence X-ray diffraction (GIXRD) elucidated that trace magnetite in the dominant siderite (FeCO3) was responsible for the passivation. Transmission electron microscopy (TEM) with the energy dispersive X-ray fluorescence (EDX) technique was used to determine the structure of the passive layer and confirm its chemistry. A passive phase tens of nanometers thick was observed beneath iron carbonate scale and at the crystal boundaries. Scanning transmission electron microscopy (STEM)/EDX profiles suggest that this phase is not a carbonate containing compound since only oxygen and iron were observed. This indicates that the possible chemical compound for the passive phase is an iron oxide, agreeing with the previous GIXRD surface analysis. Fe3O4 as a passive film was confirmed.