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

Why cold corrosion becomes an issue for newer engines?

New energy efficient marine engines are imposing severe operating conditions with ultra-long strokes and higher pressure while burning a low sulphur fuel. Adoption of slow steaming operation has also lead to an extremely cold corrosive situation in the engine.

Another important reason is that new marine engines are designed to comply with tier III NOx regulations and EEDI guidelines. To meet these new regulations, engine cylinders must operate under increased pressure and reduced operating temperatures (reduce NOx emission), thus creating conditions below dew point to allow water to condense on the cylinder linear walls. This then combines with sulphur from the combustion process to form H2SO4 which leads to cold corrosion.

EGR also brings acidic components into the air mixture and impacts temperature in the combustion chamber.

Thermal stress and pressure constant which can lead to the risk of corrosion are more severe in long stroke engine.

Engines of Older ships are often modified to run at low load operation. They are additionally installed with systems like variable turbo charger nozzle rings fitted, exhaust gas bypass valve fitted, jacket cooling bypass etc. to perform slow steaming. The older engines are provided with modification to run at low load but no additional modification is done to tackle cold corrosion.

The ultra low slow steaming engines operate at up to 10% of its full load, which again results in low temperatures in the combustion chamber. Once the temperature falls below the dew point, it will lead to cold corrosion.

Effects of Cold Corrosion:
- Sticking up of ring grooves
- Sticking of piston ring
- Decreased operational life of cylinder line

How to manage cold corrosion? 
Whilst it is very difficult to avoid cold corrosion, there are a number of steps you can take which can help to tackle the issue:

1. Identify how serious the problem is – MAN recommends the use of a ‘Sweep Test’ and Wärtsilä have a similar ‘Quick Test’. ExxonMobil has a team of field engineers who can advise on how to effectively conduct these tests.

2. Implementing a Cylinder Condition Monitoring programme to monitor the performance of the engine and cylinder oil is crucial. This programme should monitor the parameters of iron wear and the residual Total Base Number (TBN) in scrape down oil.

- It will enable the correct cylinder oil to be selected. ExxonMobil can offer cylinder oils to help address cold corrosion.

- It will enable the feed rate to be optimised to suit the engine, operating conditions and the cylinder oil in use.