Effect of the Concentration of a Nitrite-Based Inhibitor and Chloride Ions on the Corrosion Behavior of FCD-500 in a Simulated Marine Engine Cooling Water System

Abstract

This study aims to evaluate the corrosion behavior of ductile cast iron (FCD-500) under simulated marine engine operating conditions and to determine the optimal corrosion inhibitor conditions under chloride contamination. Experiments were conducted at 50 °C and 80 °C, with different Cl⁻ concentrations (0–500 ppm) and sodium nitrite (NaNO₂)-based inhibitor concentrations (0; 9000; 15,000; 17,000 ppm). Immersion and electrochemical tests were performed to analyze the microstructural corrosion characteristics and inhibitor efficiency. The results indicated that as the Cl⁻ concentration increased, proportionally higher levels of inhibitor were required to maintain surface stability. The maximum inhibition efficiency, approximately 97.3%, was achieved with an inhibitor concentration of 17,000 ppm, at a Cl⁻ concentration of 100 ppm.

1. Introduction

In internal combustion engines, significant thermal energy is generated during its operation, necessitating an efficient cooling water system (CWS) to prevent overheating and ensure engine reliability. The CW remains in continuous contact with the high-temperature metal surface, facilitating heat dissipation through thermal exchange. The prolonged exposure of the CW to hot metallic components can induce electrochemical reactions, resulting in accelerated corrosion. Over time, the degradation of the CW may lead to the accumulation of corrosive species, such as dissolved oxygen [1], chloride ions [2], and metal cations [3]. As a result, metallic materials in direct contact with the CWS are continuously subjected to corrosive attack. Furthermore, the metallurgical properties of CWS components significantly influence the corrosion behavior.

Ductile cast iron (FCD) is widely used, due to its excellent wear resistance [4], low-temperature impact toughness [5], cost effectiveness, and superior machinability [6]. As a result, FCD is commonly used in key structural parts, such as cylinder blocks and cylinder heads in propulsion systems. Despite its mechanical and economic advantages, FCD is inherently vulnerable to corrosion, due to the presence of spheroidal graphite within its microstructure. These graphite nodules act as cathodic sites in electrochemical interactions with the surrounding metallic matrix, especially under aggressive environmental conditions. Consequently, FCD components in engine CWSs are repeatedly exposed to thermal and chemical stresses. Therefore, corrosion protection is essential to ensure their long-term durability and safe operation. In engine CWSs in marine vessels, inhibitors are employed as a strategy to maintain coolant quality and prevent corrosion. These inhibitors function either by forming a passive film on the metal surface or suppressing electrochemical reactions. Representative inhibitors for steel include nitrite [7], phosphate [8], silicate [9], and organic compounds [10]. These compounds protect metal substrates through mechanisms such as surface adsorption [11], the formation of a passive layer [12], and the suppression of surface reactions. Currently, nitrite-based inhibitors are widely used in commercial merchant vessels to control the corrosion of CWSs.

Nitrite-based inhibitors are considered to be among the most effective inorganic inhibitors for suppressing the corrosion of steel, by promoting oxidation reactions on the metal surface that lead to the formation of a passive film [13]. As shown in Equation (1), nitrite ions (NO₂⁻) oxidize ferrous ions (Fe²⁺) to ferric ions (Fe³⁺), which form iron oxides, such as hematite (Fe₂O₃) or goethite (FeOOH) [13,14]. These oxides serve as a passive film that suppresses charge transfer and dissolution at the surface.

$$F{e}^{2+}+N{O}_2^{-}+2H^{+}\to F{e^3}^{+}+NO+H_{2}O$$

(1)

Nitrite-based inhibitors are characterized by rapid reactivity in regard to forming oxide layers. However, the effect is difficult to sustain over extended periods, as the passive film can be disrupted by changes in environmental conditions [15]. The engine CWS in marine vessels is a closed-loop system. This means that the nitrite-based inhibitor stays inside the system and is not released after use. It is regularly checked and adjusted to keep the concentration at an effective level. To ensure inhibition performance, the nitrite concentration must be maintained above a critical threshold. Therefore, inhibitors are dosed to maintain a stable nitrite concentration through continuous monitoring and intervention. In marine engine systems, seawater is commonly used as a cooling medium in CSWs. Seawater, which contains approximately 19,000 ppm of chloride ions, presents a highly corrosive environment [16]. Unintentional seawater penetration into the CW is a frequent issue on ships, leading to a sharp increase in CW salinity. Furthermore, evaporation and poor maintenance can result in chloride accumulation exceeding acceptable thresholds over time. Although CW management is generally performed in accordance with manufacturer-recommended concentration limits and control ranges, it remains limited in its ability to proactively detect or respond to unexpected contamination. Existing systems are limited in their ability to proactively detect or respond to unexpected contamination, such as seawater intrusion or inhibitor depletion. Moreover, previous studies have mainly been conducted under standard conditions, with a focus on evaluating the fundamental inhibition performance of such systems [17,18]. A few studies [19,20,21,22] have addressed the corrosion behavior under varying concentrations of inhibitors and chloride ions. In particular, the combined effects of these variables under conditions exceeding critical thresholds remain insufficiently explored.

The use of FCD-500 ductile cast iron in a CWS offers excellent mechanical strength and cost effectiveness; however, its susceptibility to corrosion in the presence of high-salinity seawater has been identified as a major issue, prompting ongoing research to address this challenge [23,24]. Previous studies have examined the corrosion behavior of ductile cast iron in soils with varying NaCl concentrations [25], or under specific conditions such as in nuclear power plants, where tests were conducted at a low NaCl concentration of 1.6 ppm and at a temperature of 25 °C [26]. Some studies have also investigated the effectiveness of corrosion inhibitors, including organic (triethanolamine phosphate) and inorganic (sodium hexametaphosphate) compounds [27]. However, no research has been found that considers the environmental conditions specific to marine engine applications and that investigates the role of nitrite-based additives, accordingly. Unlike land-based systems, marine engine cylinder liners (FCD-500) are exposed to complex environmental conditions, including the intrusion of seawater into the cooling water (with chloride concentrations ranging from 0 to 500 ppm Cl⁻) and elevated cooling water temperatures of around 80 °C [28]. Therefore, studying the corrosion behavior and effectiveness of corrosion inhibitors under these conditions can provide a robust academic foundation for understanding the role of such inhibitors in marine cooling systems. Furthermore, it is expected to serve as a practical study that proposes effective management strategies for the operation of corrosion inhibitor-containing coolants to prevent corrosion in marine engines.

This study aims to evaluate the corrosion behavior of FCD-500 in CW conditions, simulated according to the case of an engine on a marine merchant vessel. The experiments are designed to examine the effects of varying inhibitor dosage and chloride ion levels. In addition, high chloride ion concentration levels that may occur in marine vessels were considered. Moreover, temperature is one of the key factors influencing corrosion and inhibitor performance in CWSs. Therefore, this study considers the temperature dependence of a commercial nitrite-based inhibitor, by evaluating the corrosion behavior at two representative CW temperatures commonly operated in marine vessel engines, namely 50 and 80 °C. This study is expected to contribute to the improvement of corrosion control strategies for CWSs in marine propulsion applications.

2. Materials and Methods

2.1. Specimen and Corrosion Inhibitor

The specimen is composed of ferritic ductile cast iron (FCD-500, supplied by HANSBAR^®, Incheon, Republic of Korea), and its mechanical properties and chemical composition, as presented in

Table 1. Mechanical properties of FCD-500 specimen.

FCD-500	Y.S (N/mm2)	T.S (N/mm2)	E.L (%)	Hardness (HB) 10/3000
	389	563	7	192~201

and

Table 2. Chemical composition of FCD-500 specimen.

FCD-500	C	Si	Mn	P	S	Mg	Sn
	2.90~ 3.90	2.20~ 3.40	Max. 0.70	Max. 0.10	Max. 0.02	Max. 0.065	Max. 0.10
	3.57	2.75	0.21	0.047	0.006	0.035	-

, are based on the mill test certificate provided by the supplier. Figure 1 shows the microstructure of the FCD-500 specimen, etched with a 4% Nital solution. The microstructure consists of graphite nodules, ferrite, and pearlite, which are commonly found in ductile cast iron.

A commercial corrosion inhibitor was employed in this study. This product is primarily used in the engine CWS of merchant vessels to prevent corrosion. It contains sodium nitrite (NaNO₂) as the primary active ingredient, functioning as an anodic inhibitor. Aqueous solutions with controlled nitrite concentrations were prepared by adding measured amounts of the inhibitor to distilled water.

Table 3. Nitrite concentration and pH according to the inhibitor dosage, with reference to the manufacturer’s control standard.

Dosage per 500 mL D.I Water	Nitrite Concentration, ppm	pH	Manufacturer’s Recommendation
Uninhibited	0	6.4	-
1 mL	3000	10.4	Lower Control Limit
2 mL	5000	10.6
3 mL	7000	10.8
4 mL	9000	11.0	Within Control Limits
5 mL	11,000	11.1
6 mL	13,000	11.2
7 mL	15,000	11.2
8 mL	17,000	11.3	Upper Control Limit

shows the nitrite concentration and pH of each test solution, according to the inhibitor dosage. In this study, the inhibitor dosages were chosen based on the manufacturer’s recommendations. Similarly, engine manufacturers limit the concentration of chloride ions in coolant to less than 100 ppm. Therefore, the chloride ion range in this study was set to represent three representative conditions: chloride free (0 ppm), the manufacturer’s recommended threshold (100 ppm), and elevated levels (200 and 500 ppm) that may occur in actual marine vessel operations. The Nitrite concentration was quantified using a titration kit supplied by the manufacturer. The chloride ion concentration was controlled by adding sodium chloride (NaCl). To simulate the actual operating conditions of marine engines, two temperature levels were employed: 50 °C to represent the preheating temperature during engine downtime, and 80 °C to simulate the maximum temperature during full load.

2.2. Immersion Test of FCD-500 Coupons

The FCD-500 immersion coupons (20 mm $\times$ 20 mm $\times$ 10 mm) were mechanically polished using SiC abrasive paper up to #600 grit to achieve a uniform surface finish. Subsequently, the specimens were immersed in test solutions containing various dosages of the corrosion inhibitor, ranging from 0 to 17,000 ppm. The immersion tests were conducted at controlled temperatures of 50 °C and 80 °C to evaluate the temperature dependence of the inhibitors over a 24 h period. After the completion of the immersion test, the specimens were ultrasonically cleaned in ethanol to remove any corrosion products from their surface, then dried in a dry oven for at least 12 h. To determine the corrosion rate, each specimen was weighed before and after immersion, using an analytical balance, with a precision of 0.1 mg. The immersion tests were conducted three times for each condition, and the weight loss was represented by the average value.

2.3. Surface Morphologies

The surface morphology of the specimen was analyzed using a SEC SNE-4500M Plus scanning electron microscope (SEM, Daejeon, Republic of Korea, SEC), operated at an accelerating voltage of 20 kV.

2.4. Electrochemical Test

Electrochemical tests were performed on the FCD-500 specimens, which were used as working electrodes in the test solutions. The working electrode with a surface area of 1 cm² was placed in a three-electrode corrosion cell connected to a potentiostat (VSP, Seyssinet-Pariset, France, Bio-Logic). A silver/silver chloride electrode (Ag/AgCl sat. KCl) and a graphite rod were used as reference and counter electrodes, respectively. The solution was naturally aerated and stirred continuously, using a magnetic stirrer, at 80 rpm, to maintain uniform mass transport conditions. Prior to the electrochemical test, the system was allowed to stabilize at open-circuit potential (OCP) for 1 h. In the potentiodynamic polarization test, all the samples were polarized at a scan rate of 0.5 mV/s, within a potential range of −250 to +250 mV versus the OCP. Tafel extrapolation was used to calculate the corrosion potential (E_corr.) and corrosion current density (i_corr.) from the polarization curves. The EIS test was conducted in 10 mV amplitude signal peak-to-peak corrosion potential conditions, over a frequency range from 50 mHz to 100 kHz. The EIS parameters were fitted using the EC-Lab^® software (Ver. 11.36).

3. Results and Discussion

3.1. Corrosion Inhibition According to the Nitrite Dosage

Figure 2 shows the surface morphologies of the test coupons after immersion in the chloride-free solutions containing various inhibitor dosages (3000–17,000 ppm) at 50 °C and 80 °C for 24 h. In regard to the uninhibited condition, general corrosion and visible rust were observed. This trend was more pronounced at 80 °C. At lower inhibitor dosages (3000–5000 ppm), corrosion was partially inhibited; however, localized rust (highlighted in the red box) was observed on the surface, suggesting incomplete inhibition of the corrosion damage. At higher inhibitor dosages above 5000 ppm, the coupons exhibited corrosion-free surfaces at both temperatures, demonstrating excellent inhibition performance against general and localized corrosion. These results indicate that an inhibitor dosage of approximately 5000 ppm is required to achieve effective passivation in the absence of a chloride-induced attack.

Figure 3 presents the surface morphologies of the test coupons after immersion in solutions containing 100 ppm chloride and various inhibitor dosages (3000–17,000 ppm) at 50 °C and 80 °C for 24 h. The corrosion patterns were similar to those shown in Figure 2 under chloride-free conditions. The corrosion morphology shifted from uniform corrosion in regard to the uninhibited condition to localized corrosion upon dosing using the inhibitor, indicating a change in the surface passivation behavior. Under 100 ppm Cl⁻, surface protection was achieved at both temperatures when the inhibitor dosage exceeded 13,000 ppm. This suggests that the presence of Cl⁻ deteriorates the effectiveness of the inhibitor and destabilizes the passive film. Furthermore, as the Cl⁻ level increases, higher inhibitor dosages are required to achieve and maintain surface stability against corrosion.

Figure 4 shows the weight loss of the immersion test coupons as a function of the inhibitor dosage and chloride ions (0 ppm, 100 ppm). Weight loss serves as an indicator of the extent of metal corrosion, with lower values indicating more effective inhibition. The graph clearly illustrates the overall trends in weight loss, showing a decrease with an increasing inhibitor dosage and an increase in the presence of chloride ions and at elevated temperatures. Specifically, when subject to the solution condition containing 100 ppm of chloride ions at 80 °C (yellow line), the weight loss was measured to be 6.7 mg in the absence of an inhibitor. However, even with the addition of 3000–7000 ppm of inhibitor, a greater weight loss was observed of 13 mg, 9.9 mg, and 6.3 mg, respectively. These results suggest that the range of low dosages was insufficient to prevent chloride-induced corrosion. Although localized corrosion affects only a small surface area, it can induce a significantly greater mass loss due to accelerated anodic dissolution and a deep penetration depth at active sites. An insufficient inhibitor dosage may lead to increased localized surface activity, indicating the necessity of exceeding the threshold concentration to achieve corrosion protection. In this study, when the inhibitor concentration exceeded 13,000 ppm, all the test conditions showed stable corrosion inhibition, with weight loss consistently maintained below 1.0 mg.

In actual field conditions, corrosion develops over serval years; SEM provides a practical approach to estimating long-term corrosion behavior from short-term experimental data. Figure 5 presents SEM images of immersion test specimens, aimed at evaluating the microstructural corrosion under selected solution conditions. The specimens were selected at one dosage level above the inhibitor dosages, where no visual corrosion was observed in Figure 2 and Figure 3, enabling micro-scale observations of early-stage localized corrosion damage. In all the samples, polishing marks were clearly visible, indicating that the inhibition performance was maintained across the bulk surface.

In the specimens subject to 3000 ppm (Figure 5a) and 5000 ppm (Figure 5b) at 80 °C, micro-scale corrosion was observed at the matrix surrounding the graphite nodules. The FCD-500 consists of spheroidal graphite nodules embedded in a ferritic–pearlitic metallic matrix. In corrosive environments, this structure behaves as a micro-galvanic cell. In such cells, graphite serves as a cathodic site, making the surrounding matrix susceptible to localized anodic dissolution. As shown in Figure 5a, localized corrosion preferentially initiates in the ferrite phase surrounding the graphite nodules, subsequently propagating outward from these regions. Under conditions containing 100 ppm of chloride ions at 50 °C, the specimen exposed to 11,000 ppm of inhibitor (Figure 5c) exhibited the formation of micro-scale pits. Notably, increasing the inhibitor dosage to 13,000 ppm (Figure 5d) led to a significant reduction in the occurrence and severity of such pits, demonstrating enhanced corrosion inhibition performance at higher dosage levels. This finding suggests that an inhibition dosage of at least 13,000 ppm is required to effectively inhibit the corrosion of FCD-500 material.

3.2. Corrosion Behavior Under Simulated Marine CWS Conditions

In marine engines, the failure to properly manage the CW system can lead to a rapid increase in the chloride ion concentration, caused by either a depletion of corrosion inhibitor additives or seawater penetration. In this study, four representative CW conditions were defined under the optimal engine operating temperature (80 °C) to evaluate the corrosion behavior of FCD-500 in relation to varying inhibitor concentrations and levels of chloride ions. The four conditions are as follows:

• Condition 1: well-maintained CW, with optimal additives and a negligible chloride level (Cl⁻ free);
• Condition 2: adequate inhibitor concentration maintained, but chloride concentration has reached the recommended limit (Cl⁻ 100 ppm);
• Condition 3: inhibitor concentration maintained, but the chloride concentration has exceeded the recommended limit due to insufficient management (Cl⁻ 200 ppm);
• Condition 4: inhibitor concentration maintained, but with seawater intrusion, equivalent to 2% of the total coolant volume (Cl⁻ 500 ppm).

Figure 6 presents the weight loss from the immersion coupon tests conducted under different chloride ion concentrations (0, 100, 200, and 500 ppm) and inhibitor dosages (11,000–17,000 ppm) at 80 °C. Under 500 ppm of Cl⁻, the average weight loss reached 6.8 mg, which is attributed to severe localized corrosion. As the inhibitor dosage increased, the weight loss was significantly reduced; however, visible rust was still observed. Under all the other conditions, the weight loss was less than 1 mg, and corrosion evidence was not observed. These results indicate that when the chloride concentration exceeds the threshold limit of 100 ppm, an adequate inhibitor dosage (11,000–17,000) can still effectively inhibit corrosion. Nevertheless, at high chloride concentrations (above 500 ppm), such as due to seawater penetration, visible rust on the surface was observed, even at 17,000 ppm, which exceeds the manufacturer’s recommended dosage.

Figure 7 presents the potentiodynamic polarization curves of FCD-500 in solutions containing various concentrations of inhibitors and chloride ions (Cl⁻) at 80 °C. As shown in Figure 7a, the FCD-500 in the uninhibited conditions exhibits a typical Tafel behavior in both the anodic and cathodic regions, characterized by a gradual increase in the current density with increasing potential [29]. This indicates that the dissolution process is dominated by activation-controlled kinetics. Under this condition, the E_corr. and i_corr. were measured to be −660 mV and 13 μA/cm², respectively. In contrast, the corrosion current density increased markedly with the addition of chloride ions. This effect is attributed to the specific adsorption of Cl⁻ on the metal surface, which weakens the passive film and facilitates charge transfer [30]. Figure 7b presents the polarization curves of FCD-500 in the presence of 9000 ppm of corrosion inhibitor, with varying chloride ion concentrations. In all conditions involving the inhibitor, a well-defined passive region was observed in the range from 10⁻⁶ to 10⁻⁵ A/cm², indicating the formation of a stable passive film on the metal surface. Furthermore, with the addition of the anodic inhibitor, NaNO₂, the corrosion potential was observed at approximately −240 to −270 mV, suggesting that nitrite ions contribute to the enhancement of the passive layer. However, at 500 ppm of Cl⁻, a sharp increase in the anodic current was observed as the potential increased, suggesting the breakdown of the passive film and the initiation of localized corrosion. This breakdown behavior highlights the deterioration of the passive film in chloride-rich environments, particularly when additional oxidizing species are present, such as dissolved oxygen and an acidic agent. In Figure 7c, at an inhibitor dosage of 15,000 ppm, the polarization behavior remained stable up to 200 ppm Cl⁻. However, at 500 ppm, a marked increase in the current density was observed in the anodic polarization region, accompanied by hysteresis loops, indicative of the repeated breakdown and re-passivation of the passive film. Also, the corrosion potential shifted in the active direction, suggesting degradation of the passive film and increased susceptibility to localized corrosion due to chloride ions. This behavior suggests that, under conditions of rapid seawater penetration, the inhibition performance of a nitrite-based CWS becomes unstable when the chloride concentration exceeds 500 ppm at an inhibitor dosage of 15,000 ppm. Figure 7d shows the polarization curves of FCD-500 in the presence of 17,000 ppm, with varying chloride ion concentrations. The corrosion potential exhibited minimal variation, with a slight shift toward the cathodic direction, indicating the stability of the passive film, even at chloride ion concentrations of up to 500 ppm. Although minor electrochemical variations were observed at certain chloride concentrations, no significant changes in the corrosion behavior were identified.

Figure 8 presents the i_corr. as a function of the chloride ion concentration (0–500 ppm) and inhibitor dosage (0, 9000, 15,000, and 17,000 ppm). In the uninhibited condition (gray bars), the i_corr. increased sharply as the chloride ion concentration increased, reaching 174.1 μA/cm² at 500 ppm. This clearly demonstrates that chloride ions accelerate corrosion by promoting anodic dissolution. At 9000 ppm (pink bars), a substantial reduction in the i_corr. was observed (2.9–5.1 μA/cm²), with only minor variations across increasing Cl⁻ levels. Further increases in the inhibitor dosage to 15,000 ppm (orange) and 17,000 ppm (green) maintained the i_corr. below 6.42 μA/cm² across all the Cl⁻ concentrations, demonstrating excellent corrosion inhibition performance.

3.3. EIS Analysis of Inhibition Performance

EIS is a reliable electrochemical technique, widely used to quantitatively evaluate the interfacial resistance between metals and corrosive media. Figure 9 presents the impedance response of FCD-500 in a 100 ppm Cl⁻ solution at 80 °C, with different inhibitor concentrations. As shown in the Nyquist plot, the blank condition exhibited a linear impedance response, indicating that no protective film was formed on the FCD-500 surface and that corrosion proceeded directly at the metal–solution interface. In contrast, as the inhibitor concentration increased, the impedance spectra exhibited a more evident capacitive semicircle, along with an overall increase in diameter. This trend reflects an increase in the charge transfer resistance with higher inhibitor concentrations [31], suggesting enhanced corrosion protection and the formation of a more stable passive film on the FCD-500 surface [32]. In the Bode plot shown in Figure 9b, the blank condition exhibited a consistently low phase angle over a wide range of frequencies, with a broad and ill-defined peak. This implies a weak capacitive response and an insufficient passive layer at the interface [27]. In contrast, under inhibitor-added conditions, a distinct single peak was observed in the mid-frequency region, accompanied by a sharp increase in the phase angle. Moreover, as the inhibitor concentration increased, the maximum phase angle gradually increased, reaching its highest value at 17,000 ppm. The higher phase angle with an increasing inhibitor concentration suggests the formation of a more stable and compact passive film on the metal surface, which effectively suppresses charge transfer at the metal–solution interface and confirms the improved corrosion inhibition performance.

Figure 10 shows the equivalent circuit model using the fitted EIS data, and the detailed parameters are provided in

Table 4. EIS parameters and inhibition efficiency for the FCD-500 in 100 ppm Cl⁻.

Inhibitor Dosage, ppm	Rs, Ω	Yo × 10−4, S·sn	n	Rct, Ω	IE, %	x2  ×  10−3
Blank (0)	528.1	25.81	0.86	991.6	0	1.08
7000	182.8	6.44	0.59	3492	71.6	0.3
9000	161.0	0.65	0.88	30,280	96.7	1.04
15,000	99.75	0.62	0.9	35,990	97.2	0.33
17,000	95.87	0.71	0.88	36,100	97.3	0.61

. Here, R_s represents the uncompensated solution resistance, and R_ct is the charge transfer resistance. Q denotes the constant phase element (CPE), which is used in place of the double-layer capacitance (C_dl) to account for the non-ideal capacitive behavior of the working electrode. Moreover, χ² was used to evaluate the goodness of fit, Y_o represents the magnitude of the pseudo-capacitance, and n indicates the deviation from the ideal capacitive behavior.

The R_s was measured to be 528.1 Ω under the blank condition and decreased progressively with an increasing inhibitor dosage, reaching 95.87 Ω at 17,000 ppm [33]. This trend is attributed to the increase in solution conductivity, as the nitrate-based inhibitor acts as an electrolyte in the solution [34,35]. Under the blank condition, the R_ct was 991.6 Ω, indicating relatively high corrosion resistance. However, the Y₀ was measured at 25.81 $\times$ 10⁻⁴ S·sⁿ, suggesting the occurrence of active charge exchange at the interface between the solution and the metal surface and significant metal dissolution in the area with no protective film. And, then, Y₀ decreased sharply in the range of 0.62~0.71 $\times$ 10⁻⁴ S·sⁿ with the increasing inhibitor dosage above 9000 ppm, indicating a reduction in the interfacial capacitance and an enhancement of the passive layer [36,37].

At 7000 ppm, the R_ct increased to 3492 Ω, representing an enhancement compared to the blank condition. This result implies that the inhibitor contributed to the formation of a passive film on the metal surface. As the inhibitor concentration increased, the R_ct gradually increased as the passive film was enhanced. The IE calculated from R_ct (Equation (2)) increased consistently with the inhibitor dosage and reached its highest value of 97.3% at 17,000 ppm.

$$IE\%=\left[{\displaystyle \frac{R_{ct\left(inh\right) }-R_{ct\left(blank\right)}}{R_{ct\left(inh\right)}}}\right] \times 100$$

(2)

4. Conclusions

This study evaluated the corrosion behavior of FCD-500 in simulated cooling water conditions in a marine engine, focusing on the combined effect of the nitrite-based inhibitor dosage and the Cl⁻ level. The results obtained from the immersion coupon test and electrochemical analysis were as follows:

• In the absence of Cl⁻, general and localized corrosion were effectively inhibited at both 50 °C and 80 °C when the inhibitor dosage exceeded 5000 ppm in the immersion coupon test;
• In 100 ppm of Cl⁻, the inhibitor required a concentration above 13,000 ppm to ensure comparable corrosion inhibition performance;
• FCD-500 showed uniform corrosion under uninhibited conditions, whereas insufficient inhibitor concentrations led to localized corrosion. It was primarily observed around the matrix near the graphite nodules;
• With a high level of Cl⁻ (500 ppm), visible rust was observed even at 17,000 ppm, although the corrosion current density remained below 6.42 μA/cm² in the electrochemical reaction;
• Electrochemical impedance spectroscopy (EIS) confirmed the formation of a stable passive film with an increasing inhibitor concentration. The highest inhibition efficiency, calculated from the EIS parameter, was approximately 97.3% at 17,000 ppm inhibitor dosage under 100 ppm of Cl⁻;
• Overall, the inhibition performance was well-maintained within the inhibitor concentration range from 11,000 to 17,000 ppm under chloride ion concentrations below 200 ppm. However, in high-chloride environments (500 ppm), the inhibition performance became unstable or declined.