Question 41 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is A. **Explanation for A (Corrosion inhibition):** Nitrite-based chemical treatments (often sodium nitrite) are classic and highly effective anodic inhibitors used in closed-loop, recirculating freshwater cooling systems, such as those found on marine diesel engines. In these systems, oxygen dissolved in the water naturally causes oxidation (rust and corrosion) of metal components (like cylinder liners, heat exchangers, and pump housings). Nitrites work by reacting with the metal surface to form a thin, stable, protective film (an oxide layer, typically gamma iron oxide) that passivates the anode sites, thereby preventing the electrochemical process necessary for corrosion to occur. This function is crucial for maintaining the integrity and heat transfer efficiency of the engine components. **Why other options are incorrect:** * **B) Biological growth inhibition:** While cooling systems can suffer from biological fouling, nitrites are specifically added for corrosion control. Biocides (such as isothiazolinones or glutaraldehyde) are typically used for biological growth inhibition. * **C) Freezing point depression:** Freezing point depression is achieved by adding glycols (ethylene or propylene glycol) to the cooling water, especially when the vessel might operate in extremely cold environments or during lay-up. Nitrites do not significantly alter the freezing point. * **D) Boiling point elevation:** Although any dissolved solute can slightly elevate the boiling point (a colligative property), this effect is minimal with typical nitrite concentrations. Boiling point elevation in engine cooling systems is primarily achieved by pressurizing the system, not by the chemical treatment itself.