*3.4. EIS Analysis*

We then used EIS to examine the corrosion resistance of the ESR steel samples in 1MH2SO4 solution to determine their mechanistic and kinetic parameters and compare them with the results of potentiodynamic polarization. The Nyquist plots (Figures 5 and 6) were examined by fitting the trial results to an equivalent circuit model (Figure 7). The circuit comprises a parallel combination of a consistent phase element (CPE, Q), the charge transfer resistance (Rct) relating to the corrosion response at the metal/electrolyte interface, and the solution resistance (Rs) between the working and reference electrode [33,34]. To diminish the effects of surface anomalies in the metal, CPE (Q) was brought into the circuit instead of a pure double-layer capacitance (Cdl), allowing a more precise fit [35,36]. Among these parameters, Rct is the factor that determines the corrosion resistance of composites. Because Rct is conversely relative to icorr, a higher Rct value correlates to a lower icorr value. The impedance of CPE can be presented as follows:

$$\text{ZCPE} = \mathbf{1} / \text{YO}(\text{j}\omega)\mathbf{n},\tag{1}$$

where Y0 is the CPE constant, n is the exponent (phase shift), ω is the angular frequency, and j is the imaginary unit. CPE can represent resistance, capacitance, and inductance according to the estimations of n [37]. In all analyses, the estimated n ranged from 0.8 to 1.0, suggesting a capacitive response of CPE. From this circuit, we determined Rs, Rct, and Cdl (Tables 4 and 5).

The Nyquist plots indicated that the impedance response of the samples gradually increased along with increases in Mo concentration. However, the smallest capacitive loop in a high-frequency region was observed in the M9.8 sample in both 3.5% NaCl and 1 M H2SO4 solutions, indicating that this sample showed the highest corrosion rate. Tables 4 and 5 present all EIS parameters for all of the maraging steels. We found that the Rct value decreased gradually for samples up to M4.6, with the highest value observed for M9.8, confirming the lowest corrosion-resistance property in M9.8 sample as Rct is inversely proportional to the icorr value as mentioned earlier. Furthermore, this was supported by the potentiodynamic polarization results for M9.8. Comparing the diameters of the Nyquist plots, the curves in 3.5% NaCl solution showed increased diameters relative to those in 1MH2SO4 solution, verifying the potentiodynamic results.

**Figure 5.** Nyquist plot curves of maraging steel with different molybdenum (Mo) concentrations in 3.5% NaCl solution.

**Figure 6.** Nyquist plot curves of maraging steel with different Mo concentrations in 1 M H2SO4 solution.

**Figure 7.** Equivalent circuit diagram.


**Table 4.** Electrochemical impedance parameters of maraging-steel samples in 3.5% NaCl solution.

**Table 5.** Electrochemical impedance parameters of maraging-steel samples in1MH2SO4 solution.


#### *3.5. Scanning Electron Microscopy (SEM) Analysis*

Figures 8 and 9 show SEM images of the samples after corrosion in 3.5% NaCl solution and 1 M H2SO4 solution, respectively. It is well known that Mo affects the pitting resistance of the maraging steel by reducing pitting on the sample surface. Chloride is primarily responsible for surface pitting on materials. However, in the present study, the presence of more salt resulted in decreased corrosion resistance in NaCl solution relative to H2SO4 solution. Moreover, Mo increased the stability of the inner layers of the oxide film, resulting in minimal effect on the Ecorr value in both solutions. Furthermore, the surface of the M9.8 sample showed significant effects from both solutions relative to the other samples as a result of the decreased corrosion resistance associated with Mo.

**Figure 8.** Scanning electron microscope (SEM) micrographs of maraging-steel samples: (**a**) M0, (**b**) M2.9, (**c**) M4.6, and (**d**) M9.8 in 3.5% NaCl solution.

**Figure 9.** SEM micrographs of maraging-steel samples: (**a**) M0, (**b**) M2.9, (**c**) M4.6, and (**d**) M9.8 in 1MH2SO4 solution.

#### *3.6. Energy-Dispersive X-ray Spectroscopy (EDS) Analysis*

Figure 10 shows the results of EDS analysis of the as-received and heat-treated maraging-steel samples in 3.5% NaCl solution. The EDS was completed on the yellow boxed portion as given in the SEM images in Figure 8. The EDS analysis data are given in Table 6 in wt%. The results showed that the Mo peak increased from samples M0 to M9.8, with peaks for Ti, C, Ni, and Fe also observed. The chloride peak was higher in the M9.8 sample relative to others, which is likely related to its decreased corrosion resistance and the associated increased number of chloride ions on the M9.8 surface.

**Figure 10.** EDS spectra of the maraging steel after immersion in 3.5% NaCl solution.


**Table 6.** EDS analysis data in 3.5% NaCl solution.

Figure 11 shows the results of EDS analysis of the as-received and heat-treated maraging-steel samples in 1 M H2SO4 solution. The EDS was completed on the yellow boxed portion as given in the SEM images in Figure 10. The EDS analysis data are given in Table 7 in wt%. Similarly, peaks for Ti, C, Fe, and Ni were observed following their precipitation as carbides. Additionally, we observed an increase in the intensity of the Mo peak from samples M0–M9.8, with peaks for sulfur and oxygen ions also observed as the surface reacted with the SO4 ions present in the solution. Given the unfavorability of sulfur ions on pitting resistance, we found greater corrosive effects on the surface of the maraging-steel sample in H2SO4 solution.

**Figure 11.** EDS spectra of the maraging steel after immersion in 1.0 M H2SO4 solution.



#### *3.7. Raman Spectroscopy Analysis*

Raman spectroscopy analyses of the corroded surfaces of the samples following potentiodynamic polarization are shown in Figures 12 and 13, for both solutions. Standard normal variate (SNV) model is an effective procedure to make the output data more comparable. In this method, the spectrum mean subtraction and standard spectrum deviation procedure is used. As long as the original scale of the spectra is not interesting, this is an efficient way of removing constant baseline effects and scaling differences from

spectra. Previous studies reported that increases in the proportion of α-FeOOH and gamma \* (total mass of γ-FeOOH, β-FeOOH, and magnetite) in the corrosion products sugges<sup>t</sup> a decline in the corrosion rate [38,39]. In the present study, we identified α-FeOOH (280 cm<sup>−</sup>1) as a significant phase of the corrosion product present on a superficial level, and it was more prominent in the M9.8 sample relative to the others. This suggested that a higher level of α-FeOOH in the M9.8 sample promoted an increase in the corrosion rate. Additionally, higher amounts of α-FeOOH and gamma \* (Figure 13) were observed in 1 M H2SO4 solution, indicating that the maraging steel showed a higher degree of corrosion in the presence of H2SO4 solution as compared to the NaCl solution. In the case of sulfate solution, a phase shift occurs at 310 cm<sup>−</sup><sup>1</sup> and a new peak is seen at 1000 cm<sup>−</sup>1. More γ-FeOOH occurs in sulfate solution at 80, 1320, and 1410 cm<sup>−</sup>1. For the same composition, only one peak at 1380 cm<sup>−</sup><sup>1</sup> occurs for the sample in chloride solution. The β-FeOOH phase occurs at 480 cm<sup>−</sup><sup>1</sup> only in the case of sulfate solution. No phase shift occurs for the δ-FeOOH phase, and it shows a peak at 400 cm<sup>−</sup>1. Only an extra peak at 610 cm<sup>−</sup><sup>1</sup> occurs in sulfate solution for δ- FeOOH.

**Figure 12.** Raman spectra of maraging steel with different Mo concentrations in 3.5% NaCl solution.

**Figure 13.** Raman spectra of maraging steel with different Mo concentrations in 1 M H2SO4 solution.
