(Invited) Beneficial Role of Interstitial Carbon on Corrosion Resistance of Carbon Steels

Martensitic carbon steels with fine grains are used as important industrial materials. Interstitial carbon atoms are located in the octahedral sites in the body-centered tetragonal (bct) crystal structure, and bring high strength to martensite. Although it has been reported that the interstitial carbon has the beneficial effect on the corrosion resistance of steels,1,2 the role of interstitial carbon is still unclear. In this research, first-principles calculations were applied to explore the role of interstitial carbon on the corrosion resistance of martensite. The specimens were martensitic carbon steels with three carbon contents: 0.001 mass%, 0.44 mass%, and 0.88 mass% C. The specimens were heat-treated at 1223 K and then, quenched in water to form martensitic structure. Potentiodynamic polarization of the specimens was conducted using 1 M H2SO4. In addition to the electrochemical measurements, the first-principles calculations based on density functional theory (DFT) were carried out. For the calculation, the Vienna Ab initio Simulation Package (VASP) was used with the projector augmented wave (PAW) method 3 and GGA-PBE potential for both of the bulk and surface phases of martensite. The bct-based supercells are adopted and the interstitial carbon atoms were located at the octahedral sites in bct crystal structures. In the potentiodynamic polarization curves of the martensitic specimens in 1M H2SO4, the current densities of all specimens increased continuously with the electrode potential. This indicates that the specimen surfaces dissolved actively and were not passivated in this solution. The active dissolution current decreased with increasing in carbon content, suggesting that the active dissolution of martensite was suppressed by the interstitial carbon. By using the first-principles calculations, the effect of interstitial carbon atoms on the electronic structure of bct iron (martensite) was investigated. It was clarified that the electronic density of states (DOS) of iron atoms at and near the Fermi level decreased by increasing carbon concentration. Considering that the valence electrons near and at the Fermi level are responsible for the electrochemical reactivity (reduction/oxidation reactivity), this is evidence that interstitial carbon enhances the electrochemical stability of bct iron. Also, the effect of interstitial carbon on the work function of bct iron was investigated by the first-principles calculations. The work function on bct (110) and bct (100) surfaces increased with increasing interstitial carbon content, which might result in the suppression of the active dissolution of martensite. References; M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 164, C962 (2017). M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 165, C711 (2018). G. Kresse and J. Furthmüller, Phys. Rev. B, 54, 11169 (1996).