*2.1. Materials*

The chemical compositions of the samples were measured by a direct reading spectrometer (ARL-3460L, Thermoelectric Corporation of America, Chicago, IL, USA): C: 0.07, Si: 0.20, Mn: 1.42, Mo: 0.22, Ni: 0.79, Cr: 0.07, Ti: 0.013, Nb: 0.04, V: 0.006, Al: 0.03, P: 0.006, S: 0.001, Fe balanced.

## *2.2. Electrochemical Test*

In order to ensure the accuracy of the experiments, the electrochemical experiments were conducted in triplicate, meanwhile, the specimens were 4 × 3 = 12, every set needed 4 specimens, and there were 12 specimens in total.

The experimental samples were polished with W28, W20, W14 and W10 sandpaper, then polished with a metallographic polishing machine, and finally wiped with ethanol and acetone cotton balls. After welding the copper wire on the non-working surface, the non-working surface was sealed with epoxy resin, and a 10 mm × 10 mm electrode working surface was reserved.

Electrochemical experiments were carried out on a weekly basis after 7, 14, 28 and 49 days of total immersion corrosion in seawater.

All electrochemical tests were performed using a CHI760E electrochemical workstation produced by Shanghai ChenHua instrument Co., LTD (Shanghai, China). The sample was used as the working electrode, the saturated calomel electrode (SCE) was used as the reference electrode, and the platinum wire around the working electrode was used as the auxiliary electrode.

EIS (electrochemical impedance spectroscopy) measurements were performed at the open circuit potential. The open circuit potential was monitored for 60 s before electrochemical measurement. The frequency range was 100 kHz to 100 mHz and the voltage amplitude was 10 mV. Potentiodynamic polarization curves were obtained in the scanning range of −600 mV to 1400 mV with respect to the Open Circuit Potential (OCP) at a scanning rate of 0.5 mV/s.

The corrosion morphology was detected by scanning electron microscope (SEM, EV018, Carl Zeiss AG, (Oberkochen, Germany).

X-ray diffraction (XRD X'Pert Powder, Malvern Panalytical, Malvern, UK) was used to determine the surface phase compositions of the samples. The working voltage was 40 kV, the working current was 40 mA, the radiation target was Cu Ka, and the scanning speed was 6/min. The scanning ranged from 10◦ to 90◦ (2*θ*). The numbers of the JCPDS (Joint Committee on Powder Diffraction Standards) cards are used in this paper as references to identify the crystalline phases in the XRD patterns. The JCPDS of Fe3O4 is 98-015-8506, Fe2O3 is 98-010-890, and FeOOH of JCPDS is 96-100-8763.
