*2.6. Raman Spectroscopy*

Raman estimations were performed using a RAMAN HR800 confocal micro-Raman spectrometer (Horiba Scientific, North Edison, NJ, USA) with backscattered calculation through a 10× (NA = 0.25) magnifying lens objective. An argon laser transmitting at a frequency of 514.5 nm was used as the excitation source, and the quantity of grating in the Raman spectrometer was 1800 grooves/mm. The Raman band of a silicon wafer at 520 cm<sup>−</sup><sup>1</sup> was utilized to align the spectrometer. Raman spectra were recorded in the broad range of 200–2000 cm<sup>−</sup>1. Corrosion products formed on the specimens were characterized with the help of Raman spectrometer using an excitation laser of 514.5 nm wavelength coupled with an Olympus optical microscope.

#### *2.7. Scanning Electron Microscopy (SEM) and Energy-Dispersive Spectroscopy (EDS)*

In this present work, scanning electron microscopy (SEM) was used to study the surface morphology of the maraging steel with different wt% of Mo, and EDS was used to determine the composition of the corrosion products on the surface of maraging steel. All the studies were carried out with Model JEOL JSM-6360. In this present study, 200× magnification was used. All the SEM images before and after corrosion are presented in results section.

#### **3. Results and Discussion**

#### *3.1. Optical Microscopic Observation*

Figure 1 shows the optical micrographs of the maraging steel produced by ESR and with different Mo contents. The microstructure displayed a lamellar morphology with packets of martensite within prior austenite grains. As the Mo concentration increased, the martensite packets became visible both due to the special etching along their boundaries and the martensite packets inside an austenite grain that failed to extend beyond the austenite grain boundary. The microstructure of M9.8 differed from the other samples according to the existence of interlath austenite, which was not fully resolved. However, we speculated that this interlath structure affected the corrosion-resistance properties.

These findings suggested that increasing the concentration of the alloying element (Mo) resulted in an increased tendency to form retained austenite. Compared with conventional casting methods, ESR enables lower local solidification time, allowing the formation of a very fine and well-distributed microstructure not present in other production methods used to produce maraging steel.

**Figure 1.** *Cont*.

**Figure 1.** Microstructure analyses of (**a**) M0, (**b**) M2.9, (**c**) M4.6, and (**d**) M9.8.
