Effect of Ultrasonic Wave on the Growth of Corrosion Pits on SUS304 Stainless Steel

Abstract

Anodic polarization testing of SUS304 stainless steel was carried out in an aqueous solution of 3.5 mass% NaCl at a potential increasing at a constant rate of 20 mV/min. When the current density reached 2, 10 or 50 A/m² within the pitting corrosion region, the potential was kept at a constant value for 600 s at each current density. The ultrasonic wave (UW) was applied to the specimen from the beginning of test or just after the current density reached one of these constant values. The current density in these cases was compared with that without the application of UW. The results indicated that when the UW was applied from the beginning of test the current density was not affected by the application of UW in both the cathode and passive regions, but the increase in current density was suppressed by UW when the potential was kept at the constant value after the current density reached 10 or 50 A/m². Also when the UW was applied just after the current density reached 2, 10 or 50 A/m² and the potential was kept at the constant value, the increase rate of current density was reduced by the application of UW. At the same time, the number and size of pits were decreased by the application of UW. The reason for the decrease in pitting corrosion was considered to be that UW destroyed the corrosion product on pits and the stirring effect of UW decreased the concentration of hydrogen and chloride ions in the pits, which accelerated the formation of a passive film on the pit wall.