3.4. Corrosion Characterization

The anti-corrosion of the samples were evaluated via potentiodynamic polarization curves (PDP) and electrochemical impedance spectroscopy (EIS) using an electrochemical device (zennium, Zahner, Germany). A three-electrode cell with the samples was taken as the working electrode with an exposed area of 1 cm²; a saturated calomel electrode (SCE) was taken as the reference electrode, and an electrode was taken as the counter electrode. All electrochemical surveys were conducted in a SBF solution (6.5453 g/L NaCl, 2.2683 g/L Na₂CO₃, 0.3728 g/L KCl, 0.1420 g/L Na₂HPO₄, 0.3050 g/L MgCl₂·6H₂O, 0.2780 g/L CaCl₂, 0.0711 g/L Na₂SO₄, 6.057 g/L tris) at 37 °C. The PDP was obtained at a scan rate of 1 mV/s. In EIS, the frequency range was set as 10⁵ Hz to 10⁻² Hz. The polarization resistance (R_p) was calculated using the Equation (1):

The hydrogen evolution test was conducted to evaluate the long-term corrosion behavior of the samples immersed in a SBF solution at 37.5 °C for 192 h. The hydrogen evolution rate VH of the sample was calculated according to the volume of hydrogen precipitation, and then the corrosion rate PH was calculated according to Equation (2).