Effect of boron addition on pitting corrosion resistance of modified 9Cr–1Mo steel: Application of electrochemical noise

9Cr–1Mo steels indigenously melted with the addition of boron (Alloy B) and without it (Alloy D) along with Alloy C (without boron addition with minor changes in the trace element concentrations) were studied for their pitting corrosion resistance in 0.001 M, 0.01 M, 0.05 M and 0.1 M sodium chloride solutions using electrochemical noise (EN) technique. Weibull probability plots were used to determine the pit embryo generation rates. Gumbel extreme value analysis was conducted to determine the maximum metastable as well as stable pit radii. The analysis of the data showed superior pitting corrosion resistance of the Alloy B compared to Alloy C as well as Alloy D.

► Weibull probability plots separate pitting and passive corrosion events.
► Gumbel distribution analysis gives maximum metastable pit depths.
► Addition of boron results in superior pitting corrosion resistance in 0.1 M NaCl.
► Incorporation of B into M23C6 carbides refines them and improves pitting resistance.
► Coarse M23C6 carbides and delta-ferrite result in inferior pitting resistance.