Influence of Microstructure on Initial Corrosion Behavior of Steel Materials Using Kelvin Force Microscope

Improvement of the efficiency of power engines and reduction of the vehicles weight are important to boost the fuel efficiency of transportation equipment such as automobiles. One of the countermeasures is to expand the application of ultra-high tensile steel materials to structural members. However, the surface of ultra-high tensile steel becomes often non-uniform due to the metallographic control during manufacturing. Surface non-uniformities are more likely to lead to corrosion. Therefore, in order to further expand the application of ultra-high tensile steel materials, it is necessary to fully understand local corrosion behavior such as selective corrosion depending on the metal structure and grain boundaries. The purpose of this study is to research an influence of the non-uniformity of surface microstructure on corrosion of steel material, and the surface potential of each metallographic structures was measured by the Kelvin force microscope (KFM). Carbon steel (S45C) was heat-treated under conditions of holding it at 1250°C for 30 minutes and cooling in the furnace. After the sample surface was mirror-polished, the sample surface was smoothed by performing ion milling as a pretreatment for the KFM measurement. The KFM measurement was conducted for a predetermined range with 0.30 Hz of a scanning frequency, and the surface shape image was also measured in AFM mode. Furthermore, a corrosion test was conducted by immersion in a borate buffer containing 0.01 mol dm -3 NaCl for 1 hour, in order to clarify the relationship between the surface potential distribution obtained by KFM measurement and the initial corrosion behavior. After that, the sample surface was thoroughly washed with distilled water, dried, and KFM measurement was performed again. The surface morphology and the potential of the samples before the corrosion test changed periodically corresponding to the ferrite and cementite phases in pearlite. The potential of the cementite phase was about 10 mV lower than that of the ferrite phase. It was suggested that the interface between the cementite phase and the ferrite phase becomes an origin of corrosion, since the place showing the large potential difference is likely to be the starting point of corrosion. In comparing the surface morphology of the samples before and after the corrosion test, the width of the cementite phase was clearly narrower than that before the corrosion test. The surface potential of this part of the sample before the co