The effects of Nb on grain refining and pitting corrosion behavior of Fe(balance)18Cr10Mn0.3C0.3NxNb (x = 0-0.5 wt%) alloys were investigated through microstructure analysis, potentiodynamic polarization tests, and passive film analyses. As Nb content increased, the grain size decreased and the volume fraction of Nb(C,N) particles increased. The pitting potentials of the alloys increased as Nb content increased up to 0.2 wt%, but further addition of Nb caused a decrease in the pitting potential. On the grain-refined alloy with the addition of 0.2 wt% Nb, a thick and less defective passive film was formed resulting in the rise of the pitting potential. On the other hand, the decrease in the pitting potentials of the alloys with the addition of Nb more than 0.3 wt% Nb was primarily attributed to the increased volume fraction of coarse Nb(C,N) which provided initiation sites for pitting corrosion. (C) 2017 The Electrochemical Society. All rights reserved.