Grain boundary corrosion of SUS304 and SUS304L stainless steels sensitized for 0.1 to 24h at 675°C was studied by atomic force microscopy (AFM) and an electrochemical potentiokinetic reactivation (EPR) technique. The specimens for AFM observations were polished carefully and exposed to 10% oxalic acid. The width, depth and angle of the grain boundary grooves of variously sensitized specimens were measured from the profile of grain boundary groove observed by AFM. It is shown that both the depth and width of grain boundary grooves increase and the angles of grooves become narrower with an increase in sensitizing time. The relationship between the degree of intergranular corrosion, profiles of grain boundary groove and estimated grain boundary energy of the sensitized stainless steels was discussed. It is suggested that the grain boundary energy obtained from the grooves angle becomes large with an increase in sensitizing time. This result coincides with that obtained from EPR measurment. AFM is a powerful technique for investigating the mechanism of grain boundary corrosion.
