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

Abstract

Five Fe-33Mn-xC steels, referred to as 0 C, 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels according to their carbon content in mass%, were prepared to clarify the effect of interstitial carbon on the dissolution behavior of steel. The 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels indicated a fully austenitic structure with no carbide precipitate. The lattice parameters of the 0.6 C, 0.8 C, and 1.1 C steels calculated from the γ(111) and γ(200) diffraction peaks increased by up to around 0.8% over that of the 0.3 C steel, suggesting that the added carbon was present as interstitial carbon in the steels. The 0.6 C, 0.8 C, and 1.1 C steels were passivated during the anodic polarization measurements in 0.1 M Na 2 SO 4 solution at pH 12.0, whereas the 0 C and 0.3 C steels actively dissolved. The anodic polarization measurements in a buffer solution at pH 10.0 demonstrated a lower dissolution current density for the 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels with higher amounts of interstitial carbon. The dissolution current density at 0.3 V vs. Ag/AgCl (3.33 M KCl) of the 1.1 C steel was reduced to approximately 1 × 10 −2 A m −2 , which was one hundredth that of the 0.3 C steel. The dissolution current density of the steels was not inhibited by the presence of 0.1 M CO 3 2− ions, which is an expected dissolution product of interstitial carbon, implying that the interstitial carbon improved the electrochemical property of the steels themselves. The work function of the 1.1 C steel, which showed improved corrosion resistance with interstitial carbon, was 0.12 eV lower than that of the 0 C steel. Interstitial carbon has been reported to successfully improve the corrosion resistance of austenitic stainless steels. Low-temperature carburizing treatments have been typically applied to introduce a substantial amount of interstitial carbon at the surface region of austenitic stainless steels to form a carbide precipitation free carbon super saturated solid solution layer. [1][2][3][4][5][6][7][8][9] The carburizing treatments are known to dramatically improve the pitting corrosion resistance of austenitic stainless steels in chloride-containing environments. Anodic polarization measurements of the specimens with an exposed area of around 1 cm 2 indicated an increase in the pitting corrosion potential of the carburized stainless steels with interstitial carbon compared to untreated stainless steels. 1,[3][4][5] The microscopic anodic polarization measurements of the carburized stainless steel with a micro-scale electrode area (around 150 μm × 300 μm) containing an MnS inclusion, which is known to act as an initiation site of the pitting of stainless steels, 10-21 demonstrated that no pitting was initiated on the carburized stainless steel despite the presence of the MnS inclusion.6 The micro-scale electrode areas were in situ observed by an optical microscope with a water immersion objective lens (a lateral resolution of ca. 350 nm)19,22 during the microscopic anodic polarization. The microscopic polarization measurements revealed that the carbur...

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