Steel coupons were subjected to 100 percent relative humidity and were inoculated every day with 100 microliters of 0.01 N solutions of sodium chloride, sodium sulfate, lithium chloride, or cesium chloride. The first solid rust constituent that formed contained significant amounts of both gamma-FeOOH and ferrihydrite. In contrast, only gamma-FeOOH was observed in the rust formed during atmospheric corrosion and during wet-dry cycling with distilled water in the laboratory. The ferrihydrite in time was converted to gamma-FeOOH, alpha-FeOOH, and gamma-Fe₂O₃. The fractions of femihydrite plus gamma-FeOOH in the rust formed as a function of time during atmospheric exposure and during rusting in the laboratory environment were the same in the two cases. For related study on corrosion of iron, see AATA 20-1006. -- AATA