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Cold corrosion

Updated 11 Oct 2019

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It is not only controlling SOx emissions that has created problems in the tribology of modern engines. Addressing the twin effects of controlling NOx and meeting the EEDI requirements has led to a new phenomenon – cold corrosion.

Cold corrosion is when sulphuric acid forms on the liner walls in an engine cylinder and corrodes the liner surface. This abnormal corrosion then creates excessive wear of the liner material. In order to comply with NOx and EEDI regulations, engine makers have needed to increase pressure and reduce operating temperatures, which has been done by way of lower rotations per minute, longer strokes and increased scavenge and combustion pressures.

This creates conditions below the dew point that allows water to condense on the cylinder liner walls. This then combines with sulphur from the combustion process to form sulphuric acid, which leads to cylinder liner wear, termed cold corrosion. The NOx and EEDI rules only apply to newer engines but older engines can also suffer from the problem if they are modified, as many have been, to allow for slow steaming when engine loads below the original design parameters become routine rather than exceptions.

Among the modifications are turbocharger cut-out, retrofit of variable turbo charger nozzle rings, exhaust gas by-pass valve fitted and engine tuning changes. Some modified engines become mildly corrosive whereas others may be more seriously affected. The problem can be exacerbated because, in order to avoid cold corrosion occurring, a simple solution is to keep coolant temperatures elevated. However, this is precisely what engineers on board have tried to avoid in the past and it is counterintuitive to years of training and experience.

Another mistake is over-lubricating in order to reduce acidity within the cylinder liner. To meet the ideal acidity level, it is not unknown for lubricant use to quadruple, which is expensive and creates other problems. It is therefore advisable to monitor feed rate and seek advice.

Electronic lubrication systems can reduce cylinder oil consumption but, as an open loop system, it does not provide feedback on the impact of such reduction and, sensibly, a safety buffer is often applied. Without a reliable feedback system to accurately monitor the effect on the engine, changing feed rates based solely on OEM’s recommendations could increase associated wear caused by under-lubrication and seriously harm the engine.

To penetrate the lubrication safety buffer, safely achieve the true optimum feed rate and realise maximum savings, offline or online tools are available to closely monitor lubrication conditions. In order to overcome the problem, most engine makers have issued advice on measures to identify the severity of the problem and what can be done to counteract it. MAN

Energy Solutions recommends the use of a ‘Sweep Test’ and Wartsila has a similar ‘Quick Test’. In addition, most lubricant manufacturers have developed test kits and can offer more advice and assistance on conducting tests.

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