Cold corrosion: what it is, and what actually fixes it

Cold corrosion became a widely discussed problem in the 2010s, and the timing was not a coincidence. Two things happened at once: fuel prices pushed operators into slow steaming, and a generation of long-stroke, high-efficiency two-stroke engines came into service. Both changes lowered cylinder liner wall temperatures. The chemistry that follows had always been there — the conditions for it had simply become common.

The acid dew point is the whole mechanism

Burning a sulphur-bearing fuel produces sulphur dioxide, and a portion of that oxidises to sulphur trioxide. Sulphur trioxide combines readily with water vapour in the combustion gas. Whether the result stays as vapour or condenses as liquid sulphuric acid depends on one thing: whether the surface it meets is above or below the acid dew point.

That dew point is not a fixed number. It rises with the sulphur content of the fuel and with the water vapour content of the gas. On a high-sulphur fuel the dew point sits high enough that condensation can occur on a liner surface that feels perfectly normal by every other measure.

So the failure is a meeting of two conditions. Acid has to be present, which the fuel decides. And the liner wall has to be cool enough for it to condense, which the engine and its operation decide. Remove either condition and there is no corrosive wear.

What pushes liner temperature down

Four things account for most cases, and only the last is about the lubricant at all.

• Part load operation. Less fuel burned means less heat into the liner. Sustained slow steaming is the single most common contributor.
• Cooling that does not follow load. A jacket water system tuned for full load will overcool at 40 % load unless it is controlled to compensate. Many engines have load-dependent cooling for exactly this reason.
• Cold seawater. The same cooling arrangement removes considerably more heat in the North Atlantic in winter than in the Gulf.
• Engine design. Modern long-stroke engines extract more work from the same fuel, which by definition leaves less heat behind — including in the liner.

This is why cold corrosion frequently appears across a fleet rather than on one vessel. A change in commercial operating pattern changes the load profile on every ship at once.

Recognising it, and ruling out the alternatives

High iron in the scrape-down drain is the headline signal, but iron on its own does not identify the mechanism. Several quite different failures raise iron, and they call for opposite responses.

Physical inspection adds the confirmation. Corrosive wear tends to concentrate in the upper part of the liner, where the gas is hottest and the acid load highest, and it removes the honing marks — a polished, mirror-like surface where the cross-hatch should be is characteristic. On engines where cylinder oil is delivered through quills at fixed positions, wear can develop in a lobed pattern around the liner circumference, heavier between the injection points where the alkaline reserve arrives last and thinnest.

One sample proves nothing either way. A trend across three or more drains taken on the same fuel, load and feed rate is what separates a measurement from a diagnosis.

Why more base number is not the whole answer

Raising the base number, or raising the feed rate, does work. Both increase the alkaline reserve arriving at the liner, and both will reduce corrosive wear. That is a real intervention, not a placebo.

But it is treating the consequence. The cause is that the liner surface is below the acid dew point, and that condition is still there. It has costs of its own: a higher base number on a fuel that does not demand it leaves unused detergent behind as ash, and a higher feed rate is a direct, permanent increase in consumption.

The temperature side of the problem is usually the cheaper fix and is often already available in the engine. Load-dependent jacket cooling control, correct cooling water temperature setpoints, and following the engine builder's part-load guidance address the condition rather than compensating for it. On engines fitted with them, the builder's own anti-cold-corrosion measures should be the first place to look, not the last.

A working sequence

If liner wear is rising and cold corrosion is the suspicion, this order avoids chasing the wrong variable:

• Establish the trend. Three consecutive drain analyses with iron and residual base number, on a known fuel and feed rate.
• Rule out the alternatives using the table above — particularly water and cat fines, which are separate problems with separate fixes.
• Check the operating profile. How much time at what load, and what the jacket cooling is doing across that range.
• Confirm the current feed rate against the engine builder's stated minimum for that engine, load and fuel sulphur.
• Only then consider a change of grade — and change one variable at a time, or the next three analyses will not tell you which one worked.

If you want the drain results read against the grade and feed rate you are running, send them with the engine details and the current bunker delivery note. That is enough to say something specific rather than general.

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