Influences of concentration and thickness of MgCl₂ solution layer on pit initiation of type 304 stainless steel in atmospheric environment

This paper aimed to investigate conditions for pit initiation on type 304 stainless steel under MgCl₂ solution layer in atmospheric environment using a CCD camera system and a surface potential measurement equipment. A droplet of dilute MgCl₂ solution was put on the specimen. The specimen was set in the chamber with a relative humidity of 33% in order to make the droplet dry into the layer. The CCD camera system was employed to observe the change in the shape of the droplet and in the specimen surface during the drying process. In addition, the surface potential measurement equipment was applied for monitoring the surface potential of the specimen with the layer. When the droplet of dilute solution became thin and saturated, pitting corrosion occurred and the surface potential rapidly lowered. By contrast, thick and saturated layer did not induce pitting corrosion and the surface potential indicated the relatively low and steady. Therefore, it was revealed that the atmospheric corrosion occurred on the steel under saturated MgCl₂ solution layer whose thickness less than 0.2 mm.