Effects of 211 and 413 ordering on the corrosion behavior of V-Al-C MAX phases prepared by spark plasma sintering

In the present study, two V-Al-C based MAX phases, i.e., V₂AlC and V₄AlC₃ having two types of ordering were successfully manufactured by spark plasma sintering and the corrosion behavior of sintered samples was evaluated. Al, V and C metal powders were mixed with the desired molar ratios by a mixer mill, and sintered at 1300 °C. The relative density calculation revealed almost full densification for both prepared MAX phases. The measurements of mechanical properties showed a low increase in bending strength and Vickers hardness of V₄AlC₃ compared to V₂AlC MAX phase. Evaluation of corrosion behavior of developed MAX phases was carried out in 6.5 M HCl solution using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. Corrosion current density and corrosion potential of V₂AlC (5.3 ± 0.21 μA/cm² and -0.451 ± 0.01 V, respectively), and V₄AlC₃ (1.07 ± 0.22 μA/cm² and -0.091 ± 0.02 V, respectively) were measured and no passivation behavior was observed in their potentiodynamic polarization curves. However, EIS tests at open circuit potential confirmed more corrosion resistance of V₄AlC₃compared to V₂AlC. These tests also revealed the active dissolution of MAX phases in 6.5 M HCl solution at anodic potential of +0.1 V, while the impedance values of V₄AlC₃ were larger than those of V₂AlC. Microstructural investigation revealed the preferential dissolution of V₂AlC phase in grain boundaries after corrosion test. Moreover, the layered structure of V₂C MXenes was observed in some regions. After corrosion test, V₄C₃ MXene layers had larger thickness compared to V₂AlC. It was found that V₄AlC₃ with higher amount of Al₂O₃ and thicker layers has more corrosion resistance than V₂AlC MAX phase.