Fe plates with rust layers containing various MgCl₂ amounts were prepared as specimens. Each specimen was subjected to dry−wet repeated cycle test beyond 50 cycles, and then subjected to electrochemical hydrogen absorption test under atmospheric corrosion in the air with controlled relative humidity (RH). For an MgCl₂ amount of 39.8 g·m⁻², a hydrogen absorption rate (i_H) started to increase from an RH around 15%, steeply increased with an increase in RH up to about 30%, steeply decreased up to about 35%, gradually increased up to 65% and gradually decreased up to about 92%. A decrease in MgCl₂ amount in the range between 0.514 and 39.8 g·m⁻² induced a decrease in i_H in the whole RH range. The maximum i_H at an RH around 30% increased with an increase in MgCl₂ amount in the rust layer. Besides, the RH where the maximum i_H was obtained beyond an RH of 40% increased with a decrease in MgCl₂ amount. From theoretical relationship between RH and thickness of MgCl₂ solution film on the Fe plate without rust layer, it is found that the solution film thicknesses at the RHs were about 0.18 mm, almost independent of MgCl₂ amount. In addition, thicknesses of the rust layers containing 25.7 and 39.8 g·m⁻² MgCl₂ were measured to be almost 0.18 mm each other. The trends of i_H depending on MgCl₂ amount were tried to be explained using nature of deliquescence for MgCl₂.