Plasma nitriding treatment was applied at 603 K to introduce interstitial nitrogen into carbon steel and to form a precipitation free nitrogen solid solution layer. The nitrogen solid solution layer contained 0.05-0.1 mass% (0.2-0.4 at%) interstitial nitrogen and resulted in ca. 0.7% expansion of the lattice parameter. Anodic polarization measurements of the nitrogen solid solution layer were taken to reveal the effect of interstitial nitrogen on the polarization behavior of carbon steel. The nitrogen solid solution layer was passivated in Na2SO4 solutions above pH 6.0. The effectiveness of alkalization and NO3- ions, which were the expected dissolution by-products or products of interstitial nitrogen, was confirmed using buffer solutions at pH 6.0 and 8.45 with and without NO3- ions. In addition, the work function of carbon steel increased 0.08 eV with the interstitial nitrogen, suggesting the interstitial nitrogen made it difficult for the carbon steel to dissolve into solutions. Therefore, it was concluded not only did the interstitial nitrogen affect the exposed environments due to the alkalization and NO3- ions, but also that the work function of the carbon steel changed. The combination of these three factors resulted in the passivation of the nitrogen solid solution layer in a neutral environment. (C) The Author(s) 2016. Published by ECS. All rights reserved.