Combined ultrahigh vacuum-electrochemical (UHV-EC) methods were utilized to expose polycrystalline iron and mild steel surfaces to 0.01 M NaOH and for the subsequent transfer back to the UHV analysis chamber without exposure to air. X-ray photoelectron spectroscopy (XPS) was then employed to elucidate a chemical description of the iron surfaces as a function of the emersion potential. The mild steel electrodes display lower corrosion resistance in the alkali solution, underlining a crucial role played by minor alloying components, such as sulfide and/or carbide. Carbonate, an unavoidable contaminant of caustic solutions, is strongly adsorbed to both iron surfaces, its concentration generally being unaffected by a subsequent water wash. Polycrystalline iron extracted in the soluble ferrate domain shows enhanced corrosion resistance, which is suspected to arise through surface impurity enrichment following the anodic leaching of the FeO42- species. In contrast, mild steel extracted in the ferrate domain exhibits poor corrosion resistance, further highlighting the detrimental effect of certain alloying elements.