Question 15 MODE01 - Chief MODU Engineer

The Correct Answer is A **Explanation for Option A (Correct Answer):** Option A, "Vanadium, sodium and sulfur in the fuel," correctly identifies the primary cause of high-temperature corrosion, specifically the degradation resulting in "wire drawing" on exhaust valves and metal reduction on cylinder heads. This type of degradation is often referred to as 'hot corrosion' or 'fluxing corrosion.' 1. **Vanadium and Sodium:** When heavy fuel oils are burned, vanadium (V) and sodium (Na) impurities form molten ash compounds, notably sodium vanadate ($\text{Na}_2\text{O} \cdot \text{V}_2\text{O}_5$ or similar complexes), at the operating temperatures of the combustion space (typically above $550-600^\circ\text{C}$). 2. **Fluxing Action:** These molten vanadates act as a flux, dissolving and stripping the protective oxide layer (e.g., chromium oxide or nickel oxide) that normally forms on high-temperature alloys used in exhaust valves and cylinder heads. Once the protective layer is removed, the underlying metal is exposed and rapidly oxidized and corroded. 3. **Sulfur:** Sulfur (S) further exacerbates this process, often forming sodium sulfate ($\text{Na}_2\text{SO}_4$) which can combine with the vanadates, lowering the melting point of the ash deposits and increasing the severity of the corrosive attack. The resulting loss of material is what manifests as "wire drawing" (grooves or score marks) on the sealing surfaces of exhaust valves and seats. **Explanation for Incorrect Options:** * **B) Excessive combustion temperatures:** While extremely high temperatures are required for hot corrosion to occur, temperature alone is insufficient to cause the specific chemical degradation described by "wire drawing" without the presence of corrosive agents like vanadium and sodium salts. Excessive temperature generally leads to thermal fatigue or creep failure, not the chemical fluxing observed in wire drawing. * **C) Improper fuel injection/combustion in cylinders:** Improper combustion (e.g., post-combustion or afterburning) typically leads to thermal overloading, carbon deposits, or mechanical wear (abrasion) due to blow-by, but it does not introduce the corrosive chemical compounds necessary for high-temperature fluxing corrosion described by "wire drawing" marks. * **D) Water contamination of the fuel:** Water contamination primarily causes issues related to poor atomization, injector damage, microbial growth (in storage), and potentially hydraulic shock or cavitation. While water can contribute to low-temperature acid corrosion (dew point corrosion) in cooler exhaust sections, it does not cause the severe high-temperature chemical corrosion ("wire drawing") seen on components within the immediate combustion space.