Surface Treatment Lab., Steel Research Laboratories, Nippon Steel Corp. , 20-1, Shintomi, Futtsu-Shi, Chiba-Ken, 293-0011, Japan
Keywords: scanning Kelvin probe, filiform corrosion, under-film corrosion, painted steel, electric potential distribution measurement
Figure 1 shows the Kelvin scanning equipment. The substrate was part of an xy-stage, whose position was computer controlled. As the probe (vibrating reference electrode) an NiCr steel was used. The probe tip diameter was 150 micrometers. The z-position of the probe was also computer controlled. The vibrating reference electrode was calibrated versus the standard hydrogen electrode (SHE) measuring the electric potential difference to a Cu/Cu2+ electrode. The measurement chamber was provided with gas inlet and outlet ports. During the analysis with the scanning Kelvin probe the humidity in the measurement chamber was kept over 90%.
In order to maintain a constant distance between the surface of specimen and the probe tip, surface profile measurement was made. After the profile measurement, an electric potential measurement was made of the same area. The tip-to-surface working distance was approximately 50 micrometers.
The scanning Kelvin probe technique was applied to measure the electric potential of filiform corrosion inside the area surrounded by the white line in Figure 2. Figure 3 shows the profile and electric potential distribution of filiform corrosion: (a) is the profile, and (b) is the electric potential distribution. Contour lines are drawn at a pitch of one micrometer in Figure 3(a). Contour lines are drawn at a pitch of 30 mV in Figure 3(b). The comparison of Figure 3(a) and Figure 2 shows that they correspond well. Besides, comparing the profile in Figure 3(a) with the electric potential distribution in Figure 3(b), the electric potential on the head of filiform corrosion is less noble. This less noble area shows anodic site of the corrosion.
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Fig.1 Schematic diagram of the Kelvin sensor.
Fig.2 Appearance of filiform corrosion tracks.(a) Profile(b) Electric potential distributionFig.3 Profile and Electric potential distribution of filiform corrosion.
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