Localized Corrosion Behavior of Stainless Steels Under Loading Stress Environment

Abstract

Stainless steels are used in various situations because of its high corrosion resistance. For instance, the environments are inside the human body, building materials in seashore, and nuclear power plants. In such an environment, the allowable amount of corrosion is very few. Although stainless steels have high corrosion resistance, it has a number of damage reports caused by its corrosion. Many of stress corrosion cracking (SCC) cases have been mentioned as a factor involved in serious accidents. It is difficult to elucidate the SCC of the generating mechanism, because three elements ( the environment, the material, and the stress ) relate closely to the occurrence of the SCC. There are not so many reports of the influence that stress affects directly to not progress of crack but corrosion. In this study, the purpose is to elucidate the influence of stress loading in localized corrosion of type 304 and 316 stainless steels. As the test materials, type 304 and 316 stainless steel that was a general austenitic stainless steel was used. And the test pieces were covered by epoxy resin after the test pieces were adhered lead cable by Ag paste. Test surface was 10mm × 60mm × 0.05mm. It was grinded in water by emery paper #1000. The test pieces were bent in a U-shape so as to be wide 58.5mm to load stress. In addition, the test pieces of the same shape that was not bending in a U-shape were used as a comparative test piece. All test pieces were cleaned by ultrasonic cleaning in ethanol before its measurement. The Test solution was 0.5M, 0.154M NaCl solutions. Counter electrode was Pt. The reference electrode was Ag / AgCl / KCl sat.. Potential sweep range was -1V to 1.5V, potential sweep speed was 1mV / s. Test temperature was 25℃. In case of type 316 stainless steel in 0.5M NaCl solution, the stress loading caused the pitting potential shift to the ignoble potential and the passive current density was increased. Since the stress loading made surface passive film deformed, and the defects appeared on the surface of passive film, the defects caused that the newly formed surface is exposed. Although cathodic polarization behavior did not indicate the difference of loading stress, it is obviously that the current density increased caused by loading stress in anodic one. And the polarization behavior of type 316 stainless steel in 0.154M NaCl solution was similar to 0.5M one. Both are regarded that the decrease of pitting corrosion potential is caused by loading stress and the promotion of localized corrosion reaction. In the environment in the presence of chloride ions, pitting corrosion potential decreases with increasing chloride ion concentration because of that chloride ion promotes incidence of pitting corrosion. Since there was a significant difference in the pitting potential to the type 304 and 316 stainless steel, the type 316 stainless steel has a higher localized corrosion resistance than the type 304. However, the type 316 stainless steel is than the 304, pitting corrosion potential difference of the test piece which is not loaded with the specimen loaded with stress was large. It is indicates that the type 316 stainless steel is easily influenced by loading stress than the 304 for pitting corrosion sensitivity in this experiment.