Understanding Hot And Cold Corrosion In Marine Engines (Part III)

COLD CORROSION

Sulphur is another element which is a naturally found in crude oil. Its level is indicated by the content of sulphur found in the residual fuel stream obtained during the process of crude oil refining.

The current sulphur content in the fuel varies from 0.1% to 3.5 % under MARPOL Annex VI. The new global cap of 0.50% m/m will apply on and after 1 January 2020 or 1 January 2025, depending on the outcome of MEPC review.

Sulphur in the fuel acts as a natural EP (Extreme Pressure) additive, providing inherent lubricity in the fuel passing through the injectors and pumps.

With plenty of oxygen available in the combustion chamber, the Sulphur is converted to SO2 and it further combines with oxygen to form SO3 Sulphur trioxide.

When SO3 comes in contact with water or water vapour present in the scavenge air, it will react and form H2SO4.

If the engine is running inefficiently at low RPM, the liner temperature is on the lower side and below the dew point of sulphuric acid and water (120-160 deg C). Corrosive mixtures will condense on the linear walls causing cold corrosion of cylinder liner.

In low sulphur fuels, late or slow combustion will increase the thermal load on cylinder components, leading to overheating, lubrication problems and cold corrosion.