Cold corrosion in two-stroke engines
The sulphuric acid dew point mechanism, liner wall temperature control, and why the low-load risk inverted from acid attack to over-lubrication after the 2020 sulphur cap.
Cold corrosion is the attack on a cylinder liner running surface by sulphuric acid condensing out of the combustion gases when the liner wall temperature falls below the acid dew point. It became a recognised problem when engines began running at sustained low load, because low load means a cooler liner.
Before 1 January 2020, on 2.5 to 3.5 percent sulphur heavy fuel oil, the mechanism drove sharply raised liner wear rates and produced a family of countermeasures: high base number cylinder oils, jacket cooling water bypass, load-dependent cylinder liner cooling and rating-dependent liners, the cooling arrangements raising liner wall temperature by about 12 degrees C.
On 0.50 percent sulphur fuel the acid load is roughly a fifth of that, and the position inverted. The engine is now described by its maker as excessively protected against corrosion at low fuel sulphur, and the governing low-load risk has become over-lubrication and its consequences: ash and lacquer deposits, scavenge-space fouling and scavenge fires. The surviving cold-end risk migrated downstream to the exhaust gas boiler, whose gas temperature must be held above the sulphur condensation level. See cylinder lubrication and feed rate and slow steaming and engine cleanliness .