Effect of Addition of Boron and Nitrogen on the Corrosion Resistance of Modified 9Cr-1Mo Ferritic Steel

Electrochemical noise (EN) studies were conducted on three different sets of modified 9Cr-1Mo steels containing different contents of nitrogen and boron in air-saturated 0.05 M NaCl solution at 1 Hz sampling frequency for 72 h continuously, in order to study the metastable pitting corrosion behaviour of these steels. These specimens had different combinations of boron and nitrogen (60 ppm boron and 110 ppm nitrogen, 60B + 110N), (90 ppm boron and 100 ppm nitrogen, 90B + 100N) and (100 ppm boron and 47 ppm nitrogen, 100B + 47N) respectively, while base metal composition was same. Current signal analysis was carried out to know the total number of current transients as a function of current amplitude. The statistical parameter like σI was studied to indicate the extent of localized corrosion activity. It was observed that the total number of current transients was highest for the specimen with comparable concentrations of boron and nitrogen, 90B + 100N; also it showed highest number of high current transients, suggesting its tendency towards pitting corrosion. This is mainly due to the formation of boronitrides which reduces dissolved boron and nitrogen causing increased pitting corrosion activity. Low boron (60 ppm) and high nitrogen specimen showed lowest number of high current transients, indicating improved passivation. The plots of σI as a function of time as well as cumulative probability (CP) vs. σI showed inferior pitting corrosion resistance for the former while the latter showed the least corrosion activity.