*2.3. Electrochemical Tests*

An electrochemical experiment was carried out to study the corrosion behavior. The self-prepared simulated body fluid (SBF) was used at testing solution. A three-electrode system was adopted in electrochemical tests. The nominal chemical composition of SBF was listed in Table 1. The system consisted of platinum as counter electrode, saturated calomel as reference electrode and the test part as working electrode. The initial open-loop circuit (OCP) tests were firstly performed. Then, the Tafel polarization curve was recorded at a rate of 0.05 mV/s. The corrosion rate (*Pi*) was determined by corrosion current (*Icorr*):

$$\text{Pi} = 3.27 \times 10^{-3} \times I\_{corr} \to /\text{\textdegree} \tag{1}$$

**Table 1.** Chemical composition of SBF.


E was the weight equivalent, and ρ was the material densigty. Besides, the electrochemical impedance spectroscopy (EIS) testing was carried out within the scope of 0.01 Hz to 1000 kHz. Zsimpwin software was adopted to analyze the result. Furthermore, the transient time-current curve was determined at 1 mV/s. The Mott-Schottky curve was recorded to study the semiconductor properties of the corrosion film.

#### *2.4. Immersion Tests*

SBF immersion testing was performed to further study the degradation behavior of as-built parts. The parts were immersed in SBF at an exposure ratio of 0.1 cm2/mL. After immersion for 7, 14, and 28 days, the parts were washed with distilled water and then observed by SEM. The samples were washed using 200 g/L of CrO<sup>3</sup> solution to remove corrosion products. Subsequently, the surface morphology was investigated by an atomic force microscope (AFM, Veeco Instruments, Plainview, NY, USA). Meanwhile, the corrosion rate (*Cr*) was calculated by using the weight loss method after immersion tests.
