**2. Experiment**

#### *2.1. Materials and Corrosive Medium*

The experimental material was UNS C95810 and its chemical composition is shown in Table 1.


The measured yield strength, tensile strength and elongation were 266 MPa, 679 MPa and 29.5%, respectively. Specimens with and without defects were taken from a UNS C95810 on a large as-cast sample, and they were cut out (of 10 × 10 × 10 mm3) and observed under optical microscopy to determine whether there was a defect on the surface of each sample. For simplicity, the specimens with and without defects are represented by S-Defect and S-Cast, respectively. To e ffectively control the uncertainties of the test, artificial seawater was used as a corrosive medium, and its main chemical composition from ASTM D1141 was listed in Table 2.



#### *2.2. Preparation of Test Specimens*

#### 2.2.1. Specimens for Electrochemical Tests

Each of the specimens was mounted in plastic tubes by a two-component epoxy resin (WSR618, Nantong Xingchen Synthetic Material Co., Ltd, Nantong, China) with a Cu wire welded on their back, leaving an area of 0.38 cm<sup>2</sup> to be in contact with the artificial seawater. The study surface of each specimen was ground to 1000 grit by silicon carbide abrasive paper and polished. Then, the sample was cleaned with absolute ethanol to remove organics, embedded polishing media, etc.

#### 2.2.2. Specimens for the Corrosion Test and Microstructure Analysis

Each specimen used for the weight loss measurements, scanning electron microscopy (SEM, Supra-55-sapphire, Carl Zeiss AG, Jena, Germany), confocal laser scanning microscopy (CLSM, OLS4000, Olympus, Tokyo, Japan) and X-ray di ffraction (XRD, D/MAX-Ultima+, Rigaku, Tokyo, Japan) was mounted in plastic tubes by a two-component epoxy resin, leaving an area of 1 cm<sup>2</sup> to be in contact with the artificial seawater. The test surface of each specimen was ground to 1000 grit by silicon carbide abrasive paper and polished. Then, the sample was cleaned with absolute ethanol to remove organics, embedded polishing media, etc.

#### *2.3. Measurement and Characterization*

### 2.3.1. Weight Loss

The specimens prepared in 2.2.2 were ultrasonically cleaned in ethanol, dried with blowing air and weighed as m1. They were then immersed in artificial seawater. After immersion for 72 h, 168 h, 432 h, 720 h and 960 h, the specimens were removed, rinsed with deionized water, dried, and weighed as m2. Subsequently, they were immersed in HCl solution (containing 500 mL of commercially available 32% HCl and 500 mL of deionized water) for 2 min to remove the corrosion products and weighed as m3 after ultrasonically cleaning and drying. The weight loss of a fresh sample after immersion in this solution for 2 min was less than 0.1 mg to ensure the base metal was not attacked vigorously by the acid. The weight loss of the specimen and the weight of the corrosion products were noted as m1–m3 and m2–m3, respectively. The weight loss rate = *m*1−*m*3 *S*·*t* , where *S* is the surface area of the sample in contact with artificial seawater and *t* is the immersion time.

#### 2.3.2. Microstructure Analysis and Corrosion Morphology Observation

The surface morphology of the specimen prepared in as described in Section 2.2.2 was observed by SEM, and the defects and their surrounding as-cast microstructures were analyzed by an energy dispersive spectrometer (EDS, Ultim Extreme, OXFORD Instruments, Oxford, UK) before and after immersion in artificial seawater. After immersion for 72 h and 240 h, they were removed, rinsed with deionized water, dried, and observed by CLSM. After immersion in artificial seawater for 720 h, the washed and dried specimens were subjected to SEM observation and EDS analysis.
