ExxonMobil Interview: Understanding and Dealing with Cold Corrosion

by Ship & Bunker News Team
Tuesday July 29, 2014

The engines in many of today's vessels, both new and old, suffer from cold corrosion.

This relatively new problem can dramatically reduce the life of an engine's cylinder liners, and has been brought about by changes in legislation impacting both marine fuel sulfur levels and engine design, along with unprecedented market conditions that have forced operators to change the way they operate their fleet.

Given the prohibitive cost of replacing cylinder liners, monitoring wear has gone from being something operators considered best practice to what is now an essential requirement. And in the case of some newer engines, it's a mandatory requirement.

Iain White, field marketing manager at ExxonMobil Marine Fuels & Lubricants, recently talked to Ship & Bunker to explain the history behind cold corrosion, and to discuss the company's recently enhanced MobilGard Cylinder Condition Monitoring (CCM) Program that helps operators understand what is happening inside their engines and take an informed view on how to mitigate the issue.

Why Do Marine Engines Now Suffer From Cold Corrosion?

"As we were developing our cylinder oils, we were focused on the changing sulfur content of marine fuel. Globally that limit dropped from a maximum of 4.5 percent to 3.5 percent, and Emissions Control Areas (ECAs) were introduced where the sulfur level is currently at a maximum of 1.0 percent. From January 1, 2015 that drops to 0.10 percent," said White.

At the same time engine manufactures were focusing on the legislation that affected them. Firstly there was the Tier 2 requirement that reduced NOx, then the 2013 Energy Efficiency Design Index (EEDI) requirement to focus on fuel consumption.

"To meet these requirements, engines have increased their stroke, while propellor size has grown 20% larger."

White explained that the new engines burn almost the same amount of fuel as before but are running at 20 percent lower revs, so fuel per stroke has increased, firing pressures have increased significantly, and combustion temperatures have lowered.

"The change in operating conditions within the cylinder liner to a cooler temperature that is below the dew point, along with higher pressure, means that whereas before sulphur in the engine was in a gas form as SOx, now it combines with the condensation to form liquid sulphuric acid," he said.

Market conditions are also exacerbating the problem, White noted, particularly for container ships where slow steaming has become the norm.

"Every new engine has a problem, but now those older vessels that are slow steaming are being modified to make them more fuel efficient, and that makes what was a perfectly fine engine into one that is corrosive," he said.

"In an extreme case, liners that would have lasted 10 years can last less than 1,000 hours."

Dealing with Cylinder Wear

To correctly deal with cylinder wear, operators need to use a higher BN cylinder oil, such as Mobilgard 5100. Given that different engines have varying levels of cold corrosion, White said that determining the best course of action should be done on a case by case basis.

A service such as ExxonMobil's MobilGard Cylinder Condition Monitoring (CCM) Programme, first launched in 2002, will help operators determine the best base number (BN) cylinder oil and feed rate to use.

The service has recently been enhanced to include a new test specifically to identity levels of cold corrosion.

"The expanded tests are an improvement over just measuring total iron in the samples. Total iron tells you there's a problem, but not what it is," said White.

Operators using the programme are now sent three test kits to use on board.

Test 1 measures residual total BN (TBN), which is an indicator of how much of the oil's alkalinity has been used to neutralise the acids formed during combustion.

Test 2 measures small particles of iron, indicating the level of abrasive wear.

Test 3, the newest test, monitors the level of iron salts and is an indicator of liner wear due to cold corrosion.

"Operators can send us the reading, and within 72 hours we can send a reportĀ  for that engine," said White.

ExxonMobil says it has nearly 200,000 sample results that it uses as the basis for its recommendations for feed rates and cylinder oil BN selection, and today it has more than 350 vessels using its MobilGard CCM programme.

MAN Sweep test

As part of its MobilGard CCM, ExxonMobil has developed a tool to help operators identify the correct BN cylinder oil and feed rate by performing a Sweep Test which is recommended by MAN for their engines.

In this test, the feed rate is reduced over a set period of time and samples are taken periodically for analysis.

Graphing the results shows the correlation between the feed rate, the residual TBN, and the wear indicators.

Why is Feed Rate Important?

It's easy to see how too little lubrication for an engine can lead to excess abrasive wear, but it is perhaps less obvious that over lubricating can be equally problematic.

"We've found that when you over lubricate - when your feed rate is too high - you get deposit build up that can then cause wear. Using more cylinder oil than necessary is also costly," said White.

"Most operators over lubricate, and from our experience that can be by as much as 20 to 50%."

Why is Selecting the Right BN Important?

The base number (BN) of a lubricant is the milligrams of Potassium Hydroxide it contains per gram of oil, which tells you how much reserve of alkalinity is available to neutralise the acidic by-products of combustion.

Until recently cylinder oil BN was in the range of 40 to 70 BN.

While there has been a trend to use fuels that are lower in sulfur that produce lower amounts of acidic by-products, the change in engine conditions resulting in cold corrosion have led to the development of 100 BN lubricants.

First introduced in November last year, MobilGard 5100 is Exxonmobil's 100 BN cylinder oil that it says is specifically designed to protect against cold corrosion in two-stroke marine diesel engines operating on heavy fuel oil.

White said it was important to note that using a cylinder lubricant with a base number that is too high can also cause issues, as similarly to over lubricating, this can lead to deposit build up that can also lead to wear.

Future Requirements

From January 1, 2015, vessels operating within ECAs will have to use a marine fuel with a sulfur content not exceeding 0.10 percent by weight, a change that will once again alter acidity conditions within marine engines.

"Switching from high sulfur to 1.0% sulfur bunkers as vessels do now, they can keep using the same cylinder oil for about two weeks. But when switching to 0.10 percent distillate fuels a change of cylinder oil needs to be made sooner, and will depend upon the specific OEM guidelines" said White.

"Those vessels will need two carry different kinds of lube."

ExxonMobil says that in September it intends to launch a new low BN lubricant specifically for use with 0.10 percent fuel sulfur fuels.