*3.2. XRD Analysis*

Figure 2 shows the XRD patterns for the as-received and differently heat-treated maraging-steel samples produced using ESR. The diffraction peaks α(110), α(200), and α(211) of the martensitic phase corresponded to diffraction angles of 44.5◦, 65◦, and 82.2◦, respectively. Additionally, there was a weak diffraction peak (ࢢ)111 ((for the austenite phase at a diffraction angle of 43.5◦. The samples mainly comprised a martensitic phase and a small amount of retained austenite phase, which arose from the microsegregation of solute elements (particularly Ni) at cellular boundaries during solidification. The enrichment of Ni stabilized the retained austenite, thereby allowing the detection of the austenite phase. Moreover, the diffraction peak intensity for the austenite phase in the sample became higher as Mo concentration increased. In the M9.8 sample, we observed reversion of the martensitic phase into the austenite phase. The austenite in maraging steel comprises retained austenite, and the reverted austenite forms mostly during the aging process by a diffusion-controlled reaction for overaging conditions.

**Figure 2.** XRD analysis of maraging steel with different molybdenum concentrations.

It is clear from Figure 2 that the amount of retained austenite in ESR remelted steels depends mainly on the chemical composition of investigated steels. An increase in the amount of alloying element, i.e., Mo, is accompanied by an increase in the tendency to form retained austenite.

#### *3.3. Potentiodynamic Polarization Analysis*

We then assessed the effect of Mo on the corrosion-resistance properties of the developed steel. We found that this component improved the passive behavior of the protective films. Previous studies report that Mo improves chromium enrichment in the film without being incorporated [22,23]. Additionally, studies sugges<sup>t</sup> that Mo in the compound breaks down into Mo particles, speeding the repassivation rate (anodic inhibitor) [24–26]. Furthermore, Mo expands the stability of the inner layers of oxide film [26] and diminishes the unfavorable action of sulfides on pitting resistance [27]. Figure 3 shows the potentiodynamic polarization curve of maraging steel in 3.5% NaCl solution.

**Figure 3.** Potentiodynamic polarization curve of maraging steel in 3.5% NaCl solution.

In 3.5% NaCl solution, as seen in Table 2, the icorr value of the maraging steel gradually increased along with increases in the Mo content in the alloy, with similar results observed in 1 M H2SO4 solution. In both cases, with 9.8% Mo content, the M9.8 sample showed an increase in the icorr value. By studying the X-ray patterns, it can be seen that retained austenite of experiment steels increases with the increase in Mo content. On the other hand, the microstructure in Figure 1 shows that even at high magnification, retained austenite is not recognized in the studied steels except in specimen M9.8. In samples with lower Mo contents, austenite presents as lath austenite; on the other hand, the specimen containing 9.8 wt% Mo retained austenite aggregate in separate grains. So, the high corrosion of sample M9.8 may be attributed to the large amount of retained austenite and its morphology. Previous studies reported that Mo alone does not directly affect corrosion resistance but rather indirectly influence this property [28–32].

Figure 4 and Table 3 present the results of the potentiodynamic polarization study of maraging steel in1MH2SO4 solution. Interestingly, the rate of corrosion for M9.8 was worse in 1 M H2SO4 solution as compared with 3.5% NaCl solution, possibly due to the NaCl solution allowing Mo to produce a more protective layer.

**Figure 4.** Potentiodynamic polarization curve of maraging steel in1MH2SO4 solution.
