Interstitial carbon has been reported to successfully improve the corrosion resistance of austenitic stainless steels.^1,2 Low-temperature carburizing treatments have been applied to introduce a substantial amount of interstitial carbon at the surface region of austenitic stainless steels. The carburizing treatments are known to dramatically improve the pitting corrosion resistance of austenitic stainless steels in chloride-containing environments. It has been proposed that the formation of a stable Cr₂O₃-rich passive film on the surface of the carburized stainless steel improves the corrosion resistance of these steels. ³ However, it has yet to be clarified whether interstitial carbon improves the corrosion resistance of non-chromium containing steels. With the aim of analyzing the effect of interstitial carbon on the corrosion behavior of steel, five Fe-33Mn-C steels, which were 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 in this study.

The XRD patterns of the five steels indicated that the 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels had a fully austenitic structure with no carbide precipitate, and the 0 C steel had a mechanical-grinding-induced ε-martensite in an austenite matrix. The increase in the lattice parameters of the 0.6 C, 0.8 C, and 1.1 C steels up to 0.78% over that of the 0.3 C steel calculated from the γ(111) peaks suggested that the added carbon was presented as interstitial carbon in the steels. Dot-like MnO and Mn-S-O inclusions with a diameter of less than 10 μm were evenly dispersed in the five steels.

The 0.6 C, 0.8 C, and 1.1 C steels were passivated during the anodic polarization in 0.1 M Na₂SO₄ solution at pH 12.0, whereas the 0 C and 0.3 C steels actively dissolved. The anodic polarization measurements of the 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels in 0.05 M Na₂B₄O₇-NaOH buffer solution at pH 10.0 with 0.1 M Na₂SO₄ revealed that the dissolution current density of the steels decreased 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 about 1 × 10^-2 A m^-2, which was one hundredth that of the 0.3 C steel. A new finding demonstrated here is that a higher interstitial carbon content in the Fe-33Mn-C steels resulted in a stronger inhibition in the dissolution current density of the steels, resulting in an improvement in the corrosion resistance of the steels.

The dissolution current density of the steels was not inhibited by CO₃^2- ions, which is the expected dissolution product of the interstitial carbon, during the anodic polarization in Na₂CO₃-NaHCO₃ buffer solution (0.1 M CO₃^2-) at pH 10.0 with 0.1 M Na₂SO₄. It was confirmed that the decrease in the dissolution current density of the steels as a function of the interstitial carbon content measured in 0.05 M Na₂B₄O₇-NaOH buffer solution at pH 10.0 with 0.1 M Na₂SO₄ was not the effect of the formation of CO₃^2- ions in the solution. The XPS analysis of the 1.1 C steel detected the chemical shifts approximately 0.1 eV higher in the Fe 2p_3/2 electron binding energy and approximately 0.2 eV higher in the Mn 2p_3/2 electron binding energy compared with the peak positions of the Fe 2p_3/2 and Mn 2p_3/2 spectra measured for the 0 C steel. This can likely be attributed to the partial chemical bonding of interstitial carbon to iron and manganese, respectively.

References.

1. Y. Sun, Corros. Sci., 52, 2661 (2010).
2. A. Chiba, S. Shibukawa, I. Muto, T. Doi, K. Kawano, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 162, C270 (2015).
3. A. H. Heuer, H. Kahn, F. Ernst, G. M. Michal, D. B. Hovis, R. J. rayne, F. J. Martin, and P. M. Natishan, Acta Mater., 60, 716 (2012).