2.3.3. XRD

The specimens prepared in 2.2.2 were tested using an X-ray di ffractometer (D/MAX-Ultima<sup>+</sup>, Rigaku, Tokyo, Japan) with Co K α radiation. The di ffraction angle range was from 10◦ to 100◦. The specimens that had been tested by XRD were immersed in artificial seawater. After immersion for 240 h and 720 h, they were removed, rinsed with deionized water, dried, and tested by XRD.

#### 2.3.4. Electrochemical Test

Electrochemical measurements were conducted in a typical three-electrode system. The working electrode was the surface of the specimen with an area of 0.38 cm<sup>2</sup> prepared in 2.2.1. A platinum net and a saturated calomel electrode served as the counter and reference electrode, respectively. The test medium was 100 mL of artificial seawater. The polarization curves were recorded at a sweep rate of 1 mV/s from -500 mV to 500 mV vs. the open circuit potential using an IM6ex Electrochemical Workstation (IM6ex, ZAHNER, Kronach, Germany). The voltage perturbation was 10 mV. To avoid any inconsistency of the experiment, two or three repeated tests were carried out for each sample. The electrochemical corrosion parameters in this paper were then obtained by fitting a linear polarization region of approximately 10 mV around the resting potential [19].
