Electrolyte and temperature effects on pitting corrosion of type 316LN stainless steels

摘要

The influence of electrolyte composition and temperature on the pitting corrosion resistance of nitrogen bearing (0.015, 0.198, and 0.56 percent N) type 316L stainless steels has been investigated. Anodic polarization curves were determined in neutral chloride solution at room temperature and at elevated temperatures of 308, 318, and 333 K. Similar polarization studies were also conducted at room temperature in 1N H_2 SO_4 and in an acidic chloride solution containing 1N H_2 SO_4 and 0.5 M NaCl. The results show that the critical pitting potential E_(pp) in neutral chloride and acidic chloride media increases as the nitrogen content of the alloy increases, indicating that resistance to pitting increases with the addition of nitrogen. However, in 1N H_2 SO_4 solution, the transpassive potential was almost independent of the nitrogen content. As the temperature of the neutral chloride medium was increased, the E_(pp) values decreased, irrespective of nitrogen content. The decrease in E_(pp) is attributed not only to the temperature induced modification of the passive oxide film but also to chloride induced activity at the passive film/solution interface. Nevertheless, the alloy containing 0.56 percent nitrogen showed better resistance to pitting at 333 K than did the alloy with 0.015 percent nitrogen at room temperature. SEM examination of the pitted specimens showed clear evidence of pitting for the alloy with 0.015 percent nitrogen, but insignificant pitting attack for the alloy with 0.56 percent N. However, the alloy with 0.56 percent N displayed some pitting attack when the experiments were conducted at 333 K.