Electrochemical Aspects of Interstitial Nitrogen in Carbon Steel: Passivation in Neutral Environments

Abstract

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 Na 2 SO 4 solutions above pH 6.0. The effectiveness of alkalization and NO 3 − 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 NO 3 − 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 NO 3 − 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. Interstitial nitrogen and carbon have been reported to successfully improve the corrosion resistance of iron-based metals. Because the solid solubility limits of nitrogen and carbon in austenite phase is higher than those in the ferrite and martensite phases, interstitial elements are typically used in austenitic stainless steels. Low temperature plasma nitriding treatments have been known to result in the formation of a precipitation free nitrogen super saturated solid solution layer on austenitic stainless steels. These are characterized by a crystal structure similar to that of face centered cubic (fcc) austenite with interstitial nitrogen, and are known to exhibit high localized corrosion resistance. Many researchers have demonstrated that plasma nitriding treatments below 723 K form a precipitation free nitrogen solid solution layer which results in the improvement of pitting and crevice corrosion resistance, whereas plasma nitriding treatments above 723 K form nitride precipitates, such as chromium nitride (CrN), in a nitrogen solid solution layer and decrease corrosion resistance.1-6 Other nitriding treatments without nitride precipitates, such heat-treatment in a nitrogen atmosphere and a nitrogen gas pressurized electro-slag remelting process, also improve the pitting corrosion resistance of austenitic stainless steels. 7,8 Baba et al. detected ammonia (NH 3 ) as a dissolution product of interstitial nitrogen in austenitic stainless steels by absorptiometry, and proposed that the hydroxide ions (OH − ) generated as a by-product of the NH 3 production reaction improved the localized corrosion resistance of austenitic stainless steels with interstitial nitrogen. 7 In an experiment on a low temper...

Interstitial Carbon Enhanced Corrosion Resistance of Fe-33Mn-xC Austenitic Steels: Inhibition of Anodic Dissolution

“…A difference of about 0.1 eV in work function indicates different kinds of metals, 45,46 and the interstitial carbon clearly resulted in a decrease in the work function of the Fe-33Mn-C steels. An increase in the work function of a carbon steel with interstitial nitrogen was demonstrated in a previous study; 31 however, the interstitial carbon led to a decrease in the work function of the Fe-33Mn-C steels in this study. Figure 9 exhibits the Fe 2p 3/2 and Mn 2p 3/2 X-ray photo-electron spectra of the 0 C and 1.1 C steels.…”

“…25,30 The enhanced corrosion resistance of carbon steel with interstitial nitrogen was demonstrated in our previous work. 31 Passivation occurred in carbon steel with approximately 0.1 mass% interstitial nitrogen in Na 2 SO 4 solutions above pH 6.0 during anodic polarization measurements, whereas carbon steel without interstitial nitrogen actively dissolved in Na 2 SO 4 solutions below pH 10.0. It was confirmed that the dissolution by-products and products of interstitial nitrogen, such as alkalization and nitrate ions (NO 3 − ), played important roles in improving the corrosion resistance of carbon steel, as in the case of austenitic stainless steels with interstitial nitrogen.…”

Pack Rust Identification and Mitigation Strategies for Steel Bridges

“…A plasma nitride treatment on carbon steel forms a nitrogen solid solution layer on the surface which increases the corrosion resistance of the carbon steel. The anodic polarization curves show a passivation behavior (Chiba, Nagataki, & Nishimura, 2016). A plasma nitride treatment on the components of the bridge which will be in contact can prevent crevice corrosion.…”

Decrease in Pitting Corrosion Resistance of Extra-High-Purity Type 316 Stainless-Steel by Cu2+ in NaCl

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