*2.3. Electrochemical Tests*

All the electrochemical tests were carried out using a CS 350 Corrtest electrochemical workstation (Wuhan Corrtest, Wuhan, China). The working electrode (10 <sup>×</sup> 10 <sup>×</sup> 10 mm3) was sealed with epoxy resin (working area = 1.0 cm2), which was ground with 2000 grit SiC paper and rinsed in distilled water and ethanol. A saturated calomel electrode (SCE) and a platinum (Pt) electrode were used as the reference electrode and the counter electrode, respectively. Polarisation curves were generated and electrochemical impedance spectroscopy (EIS) was performed, respectively, at different corrosion times. At free corrosion potentials (Ecorr), the EIS tests were performed with an AC voltage amplitude of 10 mV in the frequency range of 100 kHz–0.05 Hz. The EIS results were fitted with the Zview2.0 software. The polarization curves were generated at the scan rate of 0.5 mV s−1, scanning towards the positive direction. The corrosion current density (*i*corr, mA cm<sup>−</sup>2) was estimated by the cathodic Tafel extrapolation method, according to [5,44]. The corresponding corrosion rate (*P*i, mm y−1) was converted by the equation [5,45]:

$$P\_{\rm i} = 22.06 i\_{\rm corr} \tag{1}$$

where the corrosion current density *i*corr is estimated by the Tafel extrapolation of the cathodic branch of the polarization curves, and *P*<sup>i</sup> is related to the average corrosion rate.

All the electrochemical tests were repeated at least three times in this study.

#### *2.4. Weight Loss Tests*

The cast ZK60 specimens (20 <sup>×</sup> 20 <sup>×</sup> 4 mm3) were ground with 2000 grit SiC paper, rinsed in distilled water and ethanol, dried with cold air, and kept in a vacuum desiccator before the weight loss test. The specimens were corroded in 0.1 M NaCl for different times and then immersed in a CrO3 solution (180 g L<sup>−</sup>1, ~25 ◦C) for 10 min to remove corrosion products. At least three parallel tests were performed under each test condition. The average weight loss rate of the cast ZK60 alloy (Δ*W*, mg cm−<sup>2</sup> h<sup>−</sup>1) can be converted to a general corrosion rate Pw (mm y<sup>−</sup>1) using [5,44]:

$$P\_W = 3.6524 \Delta W / \rho \tag{2}$$

where ρ is the metal density (g cm<sup>−</sup>3). For the cast ZK60 alloy, ρ = 1.8 g cm<sup>−</sup>3; thus, Equation (2) becomes:

$$P\_W = 48.67 \Delta W \tag{3}$$
