Carbon steels are known to be passivated in alkaline environments and to dissolve actively in neutral environments. With the aim of improving the corrosion resistance of carbon steels in neutral environments, a precipitation-free nitrogen solid solution layer containing 0.05-0.1 mass% interstitial nitrogen was formed on a carbon steel in this study. Interstitial elements, such as nitrogen and carbon, have been reported to improve the localized corrosion resistance of austenitic stainless steels.^1,2 Alkalization of the environment of the metal surface with interstitial nitrogen is likely to be a factor contributing to this improved localized corrosion resistance.^1,3 The expansion of the lattice parameter of austenitic stainless steel in the presence of interstitial carbon is considered to be related to the improved localized corrosion resistance.²Since lattice parameter expansion has also been reported in the presence of interstitial nitrogen, it is possible that the interstitial nitrogen affects both exposed aqueous environments (by dissolution products and/or by-products) and the solid solution metal itself (by expanding lattice parameter).

In order to evaluate the effect of interstitial nitrogen on the corrosion resistance of carbon steels, a plasma nitriding treatment was carried out at 603 K for 12 h in a 50 vol% N₂ - 50 vol% H₂gas mixture at 250 Pa pressure for a commercial carbon steel. The plasma nitriding treatment was carried out at a lower temperature than that used in the conventional plasma nitriding treatments of carbon steels in order to avoid the formation of nitride precipitates in the nitrogen solid solution layer. A precipitation-free nitrogen solid solution layer with 0.05-0.1 mass% interstitial nitrogen was formed on the inner side of a mechanically removable nitrogen compound layer. The plasma nitrided carbon steel was polished with 6 μm and 1μm diamond pastes to remove the nitrogen compound layer and to expose the nitrogen solid solution layer. The nitrogen solid solution layer had a ferrite and pearlite structure and evenly-dispersed sulfide inclusions, such as MnS, just like the untreated carbon steel.

Anodic polarization curves of the nitrogen solid solution layer that formed on the carbon steel indicated passivation in 0.1 M Na₂SO₄ solutions above pH 6.0. The electrode surfaces (ca. 10 mm × 10 mm) remained silver or were only slightly stained after the polarization. In contrast, the nitrogen solid solution layer rotated at 200 rpm to prevent the dissolution products from remaining near the electrode surface failed to be passivated in 0.1 M Na₂SO₄ at pH 6.0. The predicted dissolution products and by-products of interstitial nitrogen are nitrate ions (NO₃^-) and alkalization^1,3. Rather than become passivated, the nitrogen solid solution layer actively dissolved in a buffer solution at pH 6.0. This suggests that the alkalization of the environment near the metal surface due to the dissolution of interstitial nitrogen resulted in the passivation of the nitrogen solid solution layer in 0.1 M Na₂SO₄ solutions above pH 6.0. The anodic polarization curve of the untreated carbon steel measured in a buffer solution at pH 8.45 with NO₃^- ions demonstrated the effect of NO₃^- ions on the passive current density of the carbon steel. The passive current density of the untreated carbon steel was inhibited with NO₃^- ions to ca. 1.5 × 10^-2 A m^-2, which was less than one tenth that of the untreated carbon steel without NO₃^- ions. The anodic polarization behavior of the untreated carbon steel in the buffer solution with NO₃^- ions corresponded to that of the nitrogen solid solution layer in the buffer solution without NO₃^- ions. The work functions of the untreated carbon steel and the nitrogen solid solution layer, which were measured to quantify the effect of interstitial nitrogen on the carbon steel itself, were 5.02 and 5.10 eV, respectively. Since a difference of about 0.1 eV in work function is indicative of different kinds of metals, the improvement in the properties of the bulk metal itself provided by the interstitial nitrogen is clearly substantial. It is therefore reasonable to conclude that the interstitial nitrogen affects not only the exposed environments due to the alkalization and NO₃^-ions, but also the work function of carbon steel, contributing to the passivation of carbon steel with the nitrogen solid solution layer in the neutral environments studied here.

1. H. Baba, T. Kodama, and Y. Katada, Corros. Sci., 44, 2393 (2002).

2. A. Chiba et al., J. Electrochem. Soc., 162, C270 (2015).

3. I. Flis-Kabulska, Y. Sun, and J. Flis, Electrochem. Acta, 104, 208 (2013).