For the improvement of mechanical properties of steels, nanocrystallization is a center of interest. Nanocrystallization greatly influences on the mechanical properties such as tensile strength, ductility, and wear resistance. Severe plastic deformation (SPD) is one of the effective methods to generate the nanocrystalline structure. It was reported that nanocrystallization also affects corrosion behavior of steels. When the grain size of stainless steels is less than 1 μm, it would appear that the grain boundaries act as barriers against the growth of pitting corrosion¹⁾. Interstitial carbon atoms also affect the corrosion behavior²⁾. In this study, the SPD layers were fabricated on Fe plates by ball-milling, and electrochemical measurements were conducted to investigate the corrosion behavior of the SPD layers.

Planetary ball-milling was applied to fabricate the SPD layers on Fe and stainless steel plates. The Fe and type 304 stainless steel plates were polished by SiC paper through a 1500 grid and put into a stainless steel pot with Fe powders, and mechanical alloying of Fe and C powders was accomplished. Grinding balls were made of stainless steel. In the milling process, the SPD layers of Fe-C alloy were fabricated on the Fe and stainless steel plates. To investigate the effect of the carbon concentration, the amount of the carbon powders put into the pot was changed.

Potentiodynamic anodic polarization curves were measured in deaerated H₃BO₃-Na₂B₄O₇-1 mM NaCl solution (pH 8.0) at 298 K to elucidate the influence of the SPD and the carbon concentration on the corrosion resistance of the Fe-C steels. With the exception of the electrode area (ca. 5 mm × 5 mm), the surfaces of the specimen were coated with an epoxy resin and paraffin. The measurements were performed in a conventional three electrode cell. The reference electrode was Ag/AgCl (3.33 M KCl) and the counter electrode was a Pt plate.

The results of potentiodynamic anodic polarization for the SPD layers on the Fe plates are shown in Fig. 1. In the case of pure Fe, the increase of the anodic current density at ca. -0.5 V was due to the anodic oxidation and/or dissolution of Fe. In the potential range of -0.3 to 0 V, the current density decreased by passivation. And then, the current density drastically increased. This appeared to be attributed to localized corrosion. On the other hand, at around -0.5 V, the current densities for Fe-0.1mass%C and Fe-0.5mass%C were higher than that of pure Fe. However, the passivation current density of Fe-0.5mass%C specimen was the lowest. Therefore, the increase of carbon concentration possibly causes the improvement of corrosion resistance.

References

1. A. Chiba, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 159, C345 (2012).

2. A.Chiba, S. Shibukawa, I. Muto, T. Doi, K. Kawano, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 162, C278 (2015).