Container Sweat: Moisture and Corrosion in Sea Freight

One of the most common reasons a sea shipment arrives damaged is not impact but moisture. The container was sealed, no rain got in, the packaging looks intact — and yet the metal parts have rusted, the board has gone soft and the labels have lifted. The mechanism behind that picture is container sweat, known at sea as container rain.

How Container Rain Forms

Air holds a certain amount of water vapour depending on its temperature. Warm air holds more; as it cools its capacity falls, and at a certain point the excess condenses into droplets. That point is the dew point.

A container heats under the sun by day and cools quickly at night. On a route crossing the tropics that cycle repeats every day for weeks. Because the roof and walls cool faster than the air inside, condensation forms there first, and the droplets that gather eventually fall onto the cargo. The rain falls from the inside.

There is a second form of condensation. If the cargo itself is colder than the air around it, moisture collects directly on the product's surface. On a shipment loaded in a cold port and carried into a warm region, the damage comes not from water dripping off the roof but from the thin film of moisture forming on the goods.

What decides the severity is therefore not the length of the route so much as the number of climate zones crossed. A container that spends the same time in one climate produces far less condensation than one that heats and cools several times — a short route that changes climate is riskier than a long one that does not.

The Moisture Usually Arrives With the Cargo

This is the counter-intuitive part: the moisture generally does not get in from outside, it was loaded along with the goods. Wooden pallets and crates, corrugated boxes and the product itself all carry a certain moisture content. Once the doors close, that moisture stays inside and joins every cooling cycle.

The practical consequence is direct. A pallet that stood in the rain, boxes stored outdoors, or freshly cut timber that has not fully dried all raise the shipment's moisture load. ISPM-15 heat treatment is a plant-health measure and does not guarantee dry timber — a wet but properly stamped crate clears customs and rusts the cargo inside it.

Desiccant Alone Is Not the Answer

Desiccants bind moisture from the air, but their capacity is finite and they stop working once saturated. Set against a cargo that keeps releasing moisture for the whole voyage, an under-specified desiccant only buys the first few days.

Where it sits, and what it works with, matter as much as how much of it there is. If the load is wrapped in a barrier film, the desiccant goes inside that closed volume and does meaningful work there; loose in an open container it is fighting a far larger mass of air. The correct approach is layered: reduce the source of moisture, create a small volume with a barrier, put the desiccant in that volume, and protect the metal surfaces.

Corrosion-inhibiting coatings and VCI packaging materials are used on metal parts as well; they delay corrosion where moisture does reach the surface. They still do not replace reducing the moisture in the first place.

Materials: What to Use and What to Avoid

Closed-cell materials have a clear advantage here. XPE and PE foam do not absorb water, hold their volume and carry no moisture to the product. Open-cell polyurethane sponge does retain it: placed under a metal part in a damp container it holds moisture right at the contact face and can cause staining and corrosion. Direct contact between open-cell material and metal is a choice to avoid in sea freight.

Corrugated board loses strength as it takes up moisture, so the stacking calculation for a sea container cannot be the one used for a dry warehouse. Using PP twin-wall sheet as a tier layer is worth considering, since it does the same job without being affected by damp.

Stretch film cuts both ways. It restrains the load and keeps external moisture off, but it can equally trap the moisture the load brought with it. Wrapping a damp pallet tightly means keeping that moisture around the goods for weeks.

Labels and printing suffer too. Under condensation, adhesive labels can lift and ink can run, making the cargo harder to identify on arrival. Where the shipping marks matter, a moisture-resistant label belongs on the packaging specification rather than being treated as an afterthought.

Discipline at Loading

The moment of loading decides as much as the packaging. Wet pallets and damp boxes do not go into a container, and cargo that has stood in the rain is dried before it is loaded. Leaving a gap between the goods and the container wall keeps condensation forming on that wall from touching the packaging directly, and isolating the load from the floor does the same job.

The container's own condition deserves a check too: door seals, holes in the roof and moisture left by the previous cargo all set the starting condition of the voyage. Where the container has ventilation openings, they need to be clear.

The right approach is not buying one product but designing the shipment's moisture scenario from the start. Route, transit time, the product's material and the structure of the packaging together determine the layers required — send us those details and we can settle the solution with you.