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 alkalization1,3. 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.
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