Container rain on long sea voyages: protecting buckles, seals and strapped loads

Container shipping in mid-2021 is slow. When Yantian port in Shenzhen introduced strict quarantine measures in late May, Reuters reported more than 23,000 export containers waiting and more than 40 ships at anchor, with Maersk expecting congestion and delays to last at least 14 days. Longer waits at terminals and at sea mean more days and nights inside a closed steel box, and more chances for moisture to condense.

That matters twice for packaging components. Cartons of buckles, seals and clips travel as cargo in their own right, and once fitted, the same components hold other cargo together on long routes. This guide explains how container rain forms and what to specify, pack and check.

How container rain forms

The CTU Code, the joint IMO, ILO and UNECE code of practice, explains the mechanism in its annex on condensation. A closed container quickly develops its own internal climate, and its humidity is set by the water held in the cargo, its packaging and the dunnage. The Code estimates that dry dunnage and pallets, at 12–15% moisture content, will bring the air inside to around 60–75% relative humidity once the doors are shut.

As the box warms during the day, moisture leaves the cargo and packaging and collects in the warm air near the roof. When the temperature falls in the late afternoon, the air at the roof reaches its dew point and water forms in drops that fall onto the cargo. The Code notes that daily temperature swings for containers stowed on top of a deck stow can exceed 25 °C, while positions in the hold vary far less. Repeat that cycle for weeks and the amount of free water can be considerable.

A second mechanism, cargo sweat, happens at the end of a voyage. If cold cargo arrives somewhere warm and humid and is unpacked at once, moisture from the air condenses on the cold surfaces. The Code describes this as particularly harmful to metal products. The GDV Container Handbook gives the same advice: let sealed cargo adjust to the destination temperature before the doors are opened.

What humidity does to metal parts and their cartons

The CTU Code sets the corrosion threshold for ferrous metals at a relative humidity of 40% or more. With dry dunnage and packing alone bringing the air to about 60–75%, a closed container on a long route will usually sit above that threshold.

For zinc-coated parts, the specific risk is wet storage stain. The American Galvanizers Association describes it as a bulky white or grey deposit that forms when newly galvanized articles are packed closely and stored or shipped in damp, poorly ventilated conditions; most cases are light. The AGA’s guidance concerns hot-dip galvanized steel, and the zinc coating on a formed wire buckle is a different product, but the principle of tight packing plus trapped moisture applies to any zinc surface.

Cartons suffer too. The Code notes that ordinary cardboard can lose up to 75% of its stability as humidity rises from 40% to 95%. A collapsed carton of buckles is a handling problem; a collapsed carton under strap means a strapped stack that has lost its tension.

Packing components for a long, slow route

• Keep components dry before packing. Do not pack cartons stored in a damp area or loaded in rain.
• Use dry pallets and dunnage. They are a major moisture source; ask whether pallets are kiln dried.
• Pair moisture barriers with desiccants. For corrosion-sensitive metal goods the Code recommends desiccants combined with sealed plastic wrapping. A liner without enough desiccant can simply trap moisture inside.
• Protect the top of the load. The Code suggests a fleece cover over very sensitive cargo to catch falling drops.
• Plan the unpacking. Agree with the receiver when to open the container, based on the temperature change along the route.

Choosing finish for components that travel

The same conditions affect buckles and seals already fitted to a load. A galvanized buckle carries a zinc coating; a phosphated buckle has a dark conversion finish. Which suits a given route is a specification question, and the name of a finish is not proof of performance on a particular voyage. When you review a finish, describe the route:

Route factor	Why it matters	Information to give the supplier
Voyage length and transhipment	More daily temperature cycles	Typical transit time and expected dwell
Climate change along the route	Container sweat and cargo sweat	Origin and destination climates, season
Stowage	Deck positions see larger swings	Whether deck stowage is likely
Storage on arrival	Outdoor storage extends exposure	Indoor or outdoor, and for how long
Other cargo	Timber, fibres and some goods release moisture	What else shares the container

The finish comparison guide sets out how to frame that conversation, and the Australia and New Zealand guide covers evidence packs for long routes.

Inspect after arrival, not only before dispatch

On arrival, record the condition of the cartons, any white or red staining on components, and whether strapped loads are still tight. Photographs taken as the container is opened help separate transport moisture from storage moisture later. The receiving checklist covers the component side.

Specify components for long routes

If your route is long or prone to delay, include the voyage length, climate and storage plan in the enquiry. Browse galvanized wire buckles, phosphated wire buckles and steel strapping seals, then send the route details through the enquiry page. Packing for a specific order is agreed with the quotation.