Effect of boron and nitrogen on the corrosion properties of modified 9Cr–1Mo steel in alkali and chloride medium

The present investigation involved the execution of electrochemical corrosion tests on three distinct modified 9Cr–1Mo steel alloys, each characterised by varying nitrogen and boron contents. These alloys, denoted as P91 (B=0, N=330 ppm), P91B (B=100 and N=20 ppm) and P91BN (B=60 and N=110 ppm), underwent exposure to varying concentrations of NaOH (0.1, 0.2 and 0.5 M) solutions. Additionally, the study explored the influence of chloride (0.1 M NaCl) on the occurrence of pitting corrosion within an alkali environment. The results indicated that the passive current density in all the concentrations of NaOH studied is maximum for alloy P91 followed by P91B and P91BN, respectively, indicating alloy P91 is more prone to corrosion than alloy P91B and P91BN. Electrochemical impedance spectroscopy demonstrated a higher polarisation resistance value for P91BN and the lowest for P91. Scanning electron microscopy analysis revealed that in alloy P91B and P91BN, pits are formed at the M23C6 carbide/matrix interface whereas in alloy P91 apart from carbides, pits are formed at inclusions. Energy-dispersive X-ray mapping identified the elemental composition of the inclusion in P91 which is found to be complex Al2O3–MnS inclusions enriched with Mo. P91BN demonstrated better pitting corrosion resistance compared to alloys P91 and P91B when exposed to NaOH+NaCl medium.