HOT VS COLD CORROSION

Hot corrosion(vanadium and sodium)

• Hot corrosion is basically vanadium corrosion. Vanadium is the undesirable impurity in the fuel which is naturally occurring in marine fuel in soluble form
• When both vanadium and sodium are present in Na: Va of 1:3 ratio vanadium pentoxide is formed, a hard component whose melting point is 630deg.c.
• This component eats into the metal surface, leaving the surface exposes to corrosion.

Practical ways to restrict

• Vanadium cannot be separated by onboard centrifuges or purifier
• By using special fuel additive like ash limiters
• By maintaining exhaust temperature below a melting point less than 400 deg cels
• Special Sterlite coating on exhaust valve seats
• Regular inspection of the exhaust manifold and systems
• Vanadium can be restricted by limiting the content in the fuel by checking the BDN and confirming with the help of lab analysis

What is cold corrosion

• Sulphur is another element found in marine fuel
• Cold corrosion is the abnormal corrosion that occurs, when there is a drop in engine temperature(due to low loads) and temperature fall below the dew point 120-160 deg celsius and sulphur products sulphur trioxide (So3)reacts with condense water to form H2So4
• Sulphuric acid forms on the liner walls in an engine cylinder and corrodes the liner surface.

Effects of cold corrosion

• This abnormal corrosion then creates excessive wear of the liner material.
• Sticking of piston ring and piston grooves
• Decrease in engine life
• Cold corrosion is at its most serious in the newer engine designs. for part-load or low-load operation (also known  ‘slow steaming.
•  Some modified engines become mildly corrosive whereas others may be more seriously affected.

Practical ways to restrict

• Increasing scavenge temperature
• Using modern lubrication methods like alpha lubricator system and pulse lubricating system
• Using appropriate cylinder oil (TBN number)
• Scape down analysis of the scavenge space oil and employing condition monitoring system onboard to in TBN and iron wear
• Other modifications may include; turbocharger cut-out, variable turbocharger nozzle rings fitted, exhaust gas by-pass valve fitted, and engine tuning changes. 

After 2020 Global sulphur cap

• the IMO has decided that the global fuel sulphur limit of 0.50% will enter into force in 2020.
• This requirement is in addition to the 0.10% sulphur limit in the North American, US Caribbean, North Sea and Baltic Sulphur Emission Control Areas (SECAs). 
• Vessels that have exhaust gas cleaning systems installed will be allowed to continue using high-sulphur fuel oil (HSFO).