Kelvin force microscopy (KFM) as one of the AFMs is very useful for corrosion study, because it provides the surface morphology and potential distribution with high accuracy. The mechanism of initial atmospheric corrosion of steels accelerated by sea salt particles is not well known. In-situ observations of the corrosion initiated with small droplets of artificial sea water and dilute MgCl₂ solution were made under low relative humidity condition by using KFM and super Kelvin force microscopy (SKFM) which we have developed recently. At the same time, EDX analysis and optical microscope observation were made. The results show that the morphology of corrosion can be classified into 3 types. The potential of corroding part is more negative than those of other parts. Iron chloride is formed at the corroding part and water is supplied by the deliquescence of iron chloride.