Pitting Corrosion of SUS304 Steel Covered with Agar Film Containing Thiosulfate in Chloride Solution

In this study, SUS 304 stainless steel specimen was covered with an agar film containing Na2S2O3 as a quasi-biofilm in a 0.1 kmol·m−3 NaCl solution, and the relationship between the Na2S2O3 concentration in the film and the pitting potential of the specimen was determined by the potentiodynamic polarization test. In addition, the Na2S2O3 concentration of the film surface in contact with the specimen was estimated from the pitting potential, and its change with the pre-immersion time was examined. As a result, the following findings were obtained. The Na2S2O3 concentration in the film matched that in the Na2S2O3 solution used in preparing the film from the powder. When the specimens with films containing 0.1 kmol·m−3 or more Na2S2O3 were immersed in the NaCl solution, the pitting potential once decreased, then increased, and finally showed a steady value with increasing the immersion time. The Na2S2O3 concentration in the film estimated from the pitting potential decreased rapidly, then slowly, and rapidly again with the immersion time. When a constant potential was applied for a specified period to the specimen covered with a film containing 1.0 kmol·m−3 Na2S2O3, then pitting potential was obtained. The relationship between the estimated Na2S2O3 concentration in the film and the potential holding time was similar the relationship between the concentration and the immersion time.