Using eight-electrode electrochemical sensors of the rate of atmospheric corrosion, we study the specific features of the corrosion of carbon steel when adsorption and phase moisture films are formed. We have established that, with decrease in the air relative humidity from 100 to 80%, the corrosion rate falls down by almost three orders of magnitude: from 10−2 mm/yr to 10−5 mm/yr. The corrosion rate of carbon steel on a cooled surface at an air relative humidity of 100% is almost an order of magnitude lower than on a noncooled surface, but, at an air relative humidity of 80%, it is an order of magnitude higher. If the air relative humidity is 80%, then the moisture condensation on a cooled surface begins at a temperature of 50°C, whereas it is not observed on a noncooled surface. In crevices of height 1 mm, where the temperature of their bottom and top surfaces is lower than that of ambient air, the moisture condensation on the sensor surface begins at 40°C, which accelerates the corrosion processes.