Effect of Magnesium Chloride Liquid Thickness on Atmospheric Corrosion of Pure Iron

The recently developed super Kelvin force microscopy (SKFM), which is an atomic force microscopy (AFM) measurement method, is very useful for corrosion study because surface morphology and potential distribution can be obtained at high accuracy. In most cases, sea salt particles accelerate atmospheric corrosion. However, the initial corrosion mechanism of steels by sea salt particles is not well known. In-situ observations of corrosion initiated from small droplets of magnesium chloride (MgCl₂) solution with different liquid thicknesses were performed under low RH conditions by super Kelvin force microscopy (SKFM). At the same time, energy-dispersive x-ray analysis (EDX) and optical microscope observation were done. The results show that the corrosion morphology can be divided into three types. The size and ratio of the anode and cathode areas changed with the thickness of the MgCl₂ liquid layer. That is, the anode and cathode areas were very small when the liquid thickness of MgCl₂ was < 100 nm while the anode and cathode areas were large when the liquid thickness of MgCl₂ was > 1 μm. The ratio of anode area to cathode area increased when the liquid thickness of MgCl₂ was 7 μm. The potential of the corroding part was more negative than other parts. Enrichment of the chloride ion occurred at the corroding part, and water was supplied by the deliquescence of iron chloride (FeCl₃).