What is cold corrosion and hot corrosion in marine diesel engines?

flowchart TD
    A[Fuel sulphur] -->|Combustion| B[SO₂ → SO₃]
    B -->|+ H₂O| C[H₂SO₄ vapour]
    C -->|Condenses at dew point| D[Cold corrosion on liner]
    E[Fuel ash: Na, V, S] -->|Molten salts| F[Hot corrosion on valves/turbine]
    D --> G[Neutralised by high-TBN lube]
    F --> H[Prevented by temp control/additives]

• Cold corrosion: Sulphuric acid from fuel sulphur (SO₂ → SO₃ → H₂SO₄) condenses at the acid dew point on cylinder liners, causing corrosive wear
• Hot corrosion: High-temperature attack by sodium, vanadium and sulphur contamination in fuel on exhaust valves, turbocharger blades and boiler tubes
• Cold corrosion control: Keep liner surface above acid dew point; use high-TBN cylinder lubricants matched to fuel sulphur
• Hot corrosion control: Limit metal temperatures to avoid molten-salt range; use fuel additives; maintain correct combustion
• Mechanism: Electrochemical for cold corrosion; chemical molten-salt fluxing for hot corrosion
• As Second Engineer: Monitor liner temperatures, TBN trends, fuel sulphur, exhaust temperatures; adjust cylinder oil feed rate and combustion parameters; record in engine logbook and report to Chief Engineer