**1. Introduction**

Austenitic stainless steel (SS) has excellent corrosion resistance in various environments, so they are used as materials in multiple applications such as pipelines, pumps, and structural steels in many industries [1]. Among them, 316L SS has good mechanical and welding properties with high corrosion resistance; therefore, it is regarded as one of the most effective materials in the field of corrosion [2,3]. The high corrosion resistance of 316L SS comes from a thin chromium oxide film which has high stability against corrosion [4]. In marine atmospheric environments, however, it often suffers from pitting-type corrosion due to exposure to stagnant seawater and through the deposition of airborne sea salts, which have high humidity and high chloride concentration [5]. When pitting corrosion is initiated, the pit propagates aggressively because the pH within the pit inside turns acidic due to the generation of CrCl3, which is called the autocatalytic mechanism [6]. Moreover, the pitting corrosion of SS could cause stress corrosion cracking (SCC) which results in unpredictable, brittle fracture [7]. For these reasons, corrosion protection of stainless steel in high chloride environments is a major concern in many industries. As such, several corrosion mitigation methods are being studied and developed in various fields.

Alkyd coatings are extensively used for the surface coatings as binders and adhesives. This class of coating is generated from polyols, dibasic and fatty acids or oils by condensation polymerization [8]. Alkyds have attracted significant attention among coating materials because they are lower cost and incur fewer film defects during applications. Recently, vegetable oils have been highlighted as a new effective organic coating that are nontoxic, nondepletable, domestically abundant, nonvolatile and a biodegradable resource [9]. The vegetable oils are triacylglycerols of fatty acids with high degrees of unsaturated sites which can have the ability to polymerize via cross-linking under certain conditions [10]. Polar molecules present in the oils can be absorbed on metal surfaces and form the corresponding metal oxides, which will enhance the stability of passivation and promote adhesion [9]. Sunflower oil (SunFO) coating is reported to be an e ffective inhibitor of corrosion for carbon steel, which is likely due to the lamellar-like layered structures of the organic film [9,11]. However, there are few studies surrounding the corrosion inhibition mechanism of the SunFO coating for SS and methods for synthesizing the SunFO coating using other e ffective materials. Therefore, in this study, the sunflower oil is selected as a base coating and binder for 316L SS.

The two-dimensional (2D) materials have been extensively researched within the context of several applications as coating materials because of their interesting, atomically thin, physical, chemical and electrical properties [12,13]. Among 2D materials, molybdenum disulfide (MoS2) has been widely applied as a lubrication and thin film protection coating materials because of its tribological and corrosion resistance properties [14,15]. Moreover, MoS2 remains stable in various solvents and oxygenated environments, and it can also withstand high temperatures and pressures [16,17]. Currently, however, many surface modifications, such as chemical vapor deposition (CVD), water transfer are being phased out due to their high cost, lengthy processing time, low output and harmful e ffects on environments. This has motivated current research, which is being conducted to improve the stability and corrosion resistance with a mixture consisting of MoS2 particles and several organic coating materials without imparting the toxic e ffects [18].

This study discusses a new coating method to protect 316L SS from the pitting corrosion when exposed to a 3.5% NaCl solution. Scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS) was used to observe the dispersion of MoS2 on the SS surface and cross section. In addition, X-ray diffraction (XRD) measurements were performed to evaluate the coating materials on SS. After that, the electrochemical properties of organic coatings according to MoS2 were evaluated by using open-circuit potential (OCP) measurement and electrochemical impedance spectroscopy (EIS) tests.

#### **2. Materials and Methods**

#### *2.1. Specimens and Solution Preparation*

The chemical composition of the 316L SS is given in Table 1. For the electrochemical tests and the coating procedure, the surface of the specimen was polished with 2000-grit silicon carbide (SiC) paper, rinsed with ethanol and then dried with N2 gas.


**Table 1.** Chemical Composition of 316L stainless steel (wt.%).

Figure 1 shows a schematic diagram of the experimental procedures and expected coating structure. A 1 mg amount of MoS2 particles (commercially purchased, Sigma-Aldrich, St. Louis, MI, USA) was mixed with 10 mL of sunflower oil (commercially purchased) in 20 mL beakers. The mixture was sonicated and then stirred at 1000 rpm at room temperature for 3 h. Then, 100 μL of the mixture was dropped on the SS substrate and heated at 275 ◦C for 10 min on a hot plate and slowly cooled. At this temperature, the triglycerides which is a main composition of sunflower oil undergo polymerization through oxidation [10]. The SunFO with MoS2 coatings were presumed to have a layered oil structure which contained evenly distributed MoS2 particles.

**Figure 1.** Schematic diagram of the experimental procedures and expected coating structure.

To observe the cross section of the coated specimens, the cross section was polished with 2000-grit silicon carbide (SiC) paper. All electrochemical experiments were conducted in a 3.5% NaCl solution at ambient temperature.

## *2.2. Surface Analysis*

The surface morphology and the cross-sectional image of the specimens were observed using SEM/EDS (JSM-7600F, Jeol Ltd., Tokyo, Japan) to verify the dispersion of MoS2 in organic coating. X-ray diffraction (XRD, D8 Advance, Bruker Co., Karlsruhe, Germany) measurements were performed on the specimens to identify the effective bonding within the organic coating, MoS2 and 316L SS. The XRD analysis of the coated specimens was conducted to confirm the crystalline properties of the coating film using Cu Kα radiation (λ = 1.54178 Å) in the 2θ range 0–60◦ at a scan rate of 0.017◦ 2θ.

#### *2.3. Electrochemical Investigation Method*

All electrochemical experiments were performed using a three-electrode system in a 1000 mL Pyrex glass corrosion cells connected to an electrochemical apparatus (VSP 300, Bio-Logic SAS, Seyssinet-Pariset, France). The test specimens were connected to a working electrode, graphite rods were used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference

electrode. The exposed area of the test specimen to the electrolyte was 0.25 cm<sup>2</sup> (0.5 cm × 0.5 cm). An initial open-circuit potential (OCP) was established within 3 h to carry out the entire electrochemical test. The electrochemical impedance spectroscopy (EIS, VSP 300, Bio-Logic SAS, Seyssinet-Pariset, France) was carried out with an amplitude of 20 mV in a frequency range from 100 kHz to 10 mHz. The EIS tests were performed at 3, 12, 36 and 63 h to investigate variations of the coating durability in corrosive media. The impedance plots were interpreted on the basis of equivalent circuits using a suitable fitting procedure through the ZSimpWin software (ZsimpWin 3.20).
