Carbon, CS, and weathering steels, WS, were totally immersed in a NaCl containing solution. The influence of steel composition and the presence/absence of air flux into the solution on the physical properties of spinel and other iron phases were investigated. Large amounts of defective magnetite were formed only on CS, whereas little amounts of small grain-sized defective maghemite were detected only on WS. The chemical composition of the steels greatly affects the type of spinel phase being formed and their relative abundance. A non-aerated environment favored the formation of magnetite in CS. The protective ability of the rust was unfavored in the presence of large amounts of spinels.