**3. Results**

## *3.1. Surface Analysis*

The SEM/EDS analysis is used to probe the morphologies of the SunFO coated SS. The addition of the pristine MoS2 in the SunFO significantly impacted the surface morphology. As shown in Figure 2, the SunFO with the MoS2 coating film shows the presence of a large number of MoS2 flakes which averaged an even layer 1–3 μm thick on the entire surface (Figure 2a–d). The EDS data indicated that the surface consisted of Mo, S, Cr, Ni, and Fe corresponding to particles of MoS2 and the surface of the SS (Figure 2e).

**Figure 2.** Surface morphology of sunflower oil (SunFO) with molybdenum disulfide (MoS2) coating on the stainless steel (SS) observed by SEM and EDS: (**a**) 1000×; (**b**) 3000×; (**c**) 5000×; (**d**) 10000×; (**e**) EDS results on the red spot.

Figure 3 shows cross-sectional images of SunFO with the MoS2 coating specimen. The thickness of the coating was verified at about 3–4 μm and shows uniform and dense surface features. According to the EDS mapping results (Figure 3b,c), it was revealed that the MoS2 particle existed in the SunFO coating layer. However, it was difficult to observe the layered structure of the oil, so the SEM analysis was re-conducted on an unpolished condition. In Figure 4, a layered structure was observed, and it was demonstrated that the coating structure of the specimen corresponded to the expectation outlined in Figure 1.

The XRD patterns in Figure 5 show the crystalline nature of the MoS2 and substrate. In previous reports, the diffraction pattern of bulk MoS2 has strong and sharp peaks which correspond to (100), (002), (100), (103), (105), and (110) planes [19]. After SunFO with MoS2 coating, the MoS2 peaks were only detected via the (002) peak corresponding to the angle of 15◦ because the MoS2 was oriented to a layered structure on the SS. The additional diffraction peaks at 44.5◦ and 19.6◦ indicate crystalline iron in the SS substrate and SunFO.

**Figure 3.** Cross sectional images of SunFO with MoS2 coating on the SS observed by SEM and EDS: (**a**) 10000× (Cross-section of the specimen); (**b**) Fe observed by EDS mapping; (**c**) Mo observed by EDS mapping; (**d**) S observed by EDS mapping.

**Figure 4.** SEM images of unpolished SunFO with MoS2 coating on the SS observed by SEM: (**a**) 1000×; (**b**) 3000×; (**c**) 5000×.

**Figure 5.** XRD results of (**a**) Bare SS; (**b**) SunFO coated SS; (**c**) SunFO with MoS2 coated SS.

#### *3.2. Electrochemical Analysis (EIS Tests)*

The impedance spectra were obtained in the form of Nyquist plots of the data from the three-electrode system as shown in Figure 6. The tests were conducted with the following specimens, bare, SunFO coating and SunFO with a MoS2 coating at room temperature. The specimens were tested in a 3.5% NaCl solution for a total of 63 h. The Nyquist plot consisted of a depressed capacitive loop at high frequency values [20,21]. As shown in Figure 6a, the capacitive loop of bare SS increased as time progresses, because the passive film on the surface goes to stable in the solution [22–24]. While, in case of the SunFO coated SS, the loops decreased with immersion time, which was caused by a deterioration of oil coating (Figure 6b) [25,26]. The capacitive loop of SunFO coating with MoS2 on SS had little change during the test period, so it was regarded as a non-time-dependent effect in the solution as shown in Figure 6c.

**Figure 6.** Electrochemical impedance spectroscopy (EIS) results in the form of Nyquist plot in 3.5% NaCl solution; (**a**) Bare SS; (**b**) SunFO coated SS; (**c**) SunFO + MoS2 coated SS.

In Figure 7a, the capacitive loop of bare SS was the smallest initially, however it displayed a reversed trend compared with the plot of the SunFO coated SS at the final time (Figure 7b). The SunFO coating with MoS2 on the surface maintained the loop tendency during the entire immersion time. It can be also verified in the OCP graph seen in Figure 8. The SunFO with MoS2 coating SS revealed very stable potential, which obviously differed from the bare and SunFO coated SS. Moreover, the potential of SunFO coated SS continuously decreased and reversed compared with the potential of bare specimen after about 28 h, which was the same tendency observed in the time-dependent variation of the Nyquist plot.

**Figure 7.** Time-dependent EIS results in the form of Nyquist plot in 3.5% NaCl solution: (**a**) initial stage (3 h); (**b**) final stage (63 h).

**Figure 8.** Open-circuit potential of specimens during the immersion time.

Figure 9 presents the EIS results in the form of Bode phase plots during immersion for 63 h.The Bode phase plot provided more clear description of the electrochemical frequency-dependent behavior than did the Nyquist plot, where the frequency values are implicit [27]. The high frequency spectra detects local surface defects, whereas the low frequency spectra detects the processes within the film and at the metal/film interface, respectively [28,29]. As shown in Figure 9a, in case of the bare SS, the phase angle maximum is slightly increased, and the width of the graph is wider in the low frequency region according to the test time. It means the passive film on the surface goes to more uniform and thicker [30–33]. In the case of SunFO coated SS, there was evidence of variations in surface conditions (Figure 9b). The initial stage of the specimen had two-time constants, but it changed to one-time constant at the final stage [34]. In addition, the Bode plot of SunFO coated SS has a low phase angle maximum and narrow shoulder width. This indicated that the SunFO coating had a poor protective property and has many defects [35–38]. The specimen coated with SunFO and MoS2 did not exhibit strong time-dependent behavior in the graphs, however it had a very wide frequency area as shown in Figure 9c. This is thought to be due the maintenance of a thick and uniform film throughout the entire test period.

Figure 10a shows the equivalent electrical circuits for bare SS, which had a passive film, and was used to analyze the results of the EIS tests (one-time constant circuit) [27]. In this figure, *R*s is the solution resistance, *CPE*1 is the dielectric strength of the film and water absorbed by the film, and *R*film is the electrical resistance resulting from the formation of an ionic conduction path through the pores in the film. The capacitance generated by the metal dissolution reaction and by the electric double layer at the electrolyte/substrate interface is designated by *CPE*2, and *R*ct is the resistance caused by the metal dissolution reaction. In the case of SunFO coated specimens, however, more circuit parameters should be added because the oil coating generates a new layer which forms a two-layered film (two-time constant circuit) [39]. As shown in Figure 10b, *<sup>R</sup>*coating is the SunFO coating resistance and *CPE*3 is the dielectric strength of the SunFO coating. The ZSimpWin program of the defined equivalent circuits was used to fit the EIS data to determine the optimized values for the resistance parameters, which are presented in Table 2. The *R*film and *R*ct of bare SS increased according to the immersion time. These values indicated an increase of the passive film's stability, which was the same tendency identified in the Nyquist and Bode plot. The film and coating capacitance (*C*film, *C*coating) is described by the expression [20,40]:

$$\mathbf{C} = \frac{\varepsilon A \nu}{d} \tag{1}$$

where A is the surface area of specimen, ε is the dielectric constant, and *d* is thickness of the passive film in solution. This equation suggests that a decrease of *Cdl* is related to an increase of passive film's thickness. The *C*film of bare SS decreased with time, therefore this indicated that the passive film on the surface becomes thicker at final stage. In the case of the SunFO coated specimen, there were too many error values when it was applied to a two-time constant circuit after the initial stage. As shown in the Bode plot, two-time constants were displayed only at the initial stage and one-time constant was shown after that. In other words, the SunFO coating on the SS surface became degraded prior to 12 h. For this reason, the simulation was conducted to the point of one-time constant after the initial stage. According to the data shown in Table 2, *R*film and *R*ct decreased according to immersion time, which meant the film had deteriorated. Especially, the *R*film and *C*film rapidly decreased after 36 h which indicates that a thick and porous non-protective layer was generated on the surface [41–43]. The SunFO with MoS2 coated SS showed an obvious two-time constant, which fit well with the two-time circuit. Both values, *<sup>R</sup>*coating and *R*ct, increased according to the test period, and the *<sup>R</sup>*coating value at 63 h was markedly greater than the *R*film of bare SS at the same time, which indicated excellent protective film properties. Moreover, the *R*film value remained very small relative to that of bare SS. It is presumed that the SunFO coating with the MoS2 blocks the electrolyte and oxygen so that the passive film could not form.

**Figure 9.** EIS results in the form of Bode plot in 3.5% NaCl solution: (**a**) Bare SS; (**b**) SunFO coated SS; (**c**) SunFO + MoS2 coated SS.

**Figure 10.** An equivalent circuit used to fit the results of the EIS tests: (**a**) one-time constant circuit; (**b**) two-time constant circuit.


**Table 2.** EIS results of specimens according to the coating materials.

The SunFO is a complex mixture of triacylglycerol consisting of tri-esters of glycerol and fatty acid [44,45]. At high temperature, the unsaturated fatty acid of SunFO undergoes oxidation reactions and cross links with the SS. According to the XRD results (Figure 5), the SunFO layer has crystalline peaks at 19.6◦ which means the liquid states of the fat were crystallized on the SS. According to the expected coating structure (Figure 1), the SunFO coating, which has fatty acid hydrocarbon chains, makes lamellar structures as identified by the subcell concept [9,11]. However, the lamellar structures of the SunFO are aggregated structures that have a number of defects. In the SunFO with MoS2 coating film, the MoS2 has layered structures that, during the coating process on SS, are well layered [8,9,11,46]. The interactions with aggregations of the SunFO lamellar structure and layered MoS2 in coating the films acts as a high ordered layer barrier for the protection of the metals from electrolytes. The defects of both materials act as active centers for molecular adsorption and functionalization, therefore the combination of both layered materials could reduce the defects and build denser film [47]. Additionally, previous studies have reported that MoS2 particles impart a negative e ffect on the corrosion because of their cathodic partial reactions during the corrosion process which leads to the destruction of the surface film [48]. Moreover, the electrochemical potential of MoS2 particle is higher than the 316L stainless steel, so it could generate galvanic cell between steel and MoS2 which accelerates corrosion reaction on the steel surface [49]. However, in case of the MoS2 with the SunFO film, the lamellar structure of the oil acted as an electrical insulating barrier between MoS2 and steel surface, which prevents the galvanic corrosion and improves the corrosion resistance.
