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  
[PERSON]  
1 Department of Marine Engineering, Graduate School, Mokpo National Maritime University, Mokpo 58628, Republic of Korea; [EMAIL_ADDRESS]  
[PERSON]  
2 Division of Cadet Training, Mokpo National Maritime University, Mokpo 5  
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. \[Fe^{2+}+NO_{2}^{-}+2H^{+}\to Fe^{3+}+NO+H_{2}O \tag{1}\]  
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].