Stopping rust in an export crate

Why steel rusts in a crate

A crate that crosses an ocean sees large swings in temperature. Warm humid air in the day cools at night, and water condenses on the coldest surface, which is often bare metal inside the crate. Wood adds to it by holding moisture and releasing it slowly. The result is rust on exactly the surfaces that are machined, ground or finished and cannot afford a pitted face.

Painted surfaces survive better, but the exposed ones, such as spindles, shafts, ways and bores, are where damage appears. Coating them with a protective oil helps, but oil is messy and has to be removed on arrival. A sealed wrap that protects without a coating is the usual alternative.

How VCI works

VCI stands for volatile corrosion inhibitor. The chemical is impregnated into paper or film, and it slowly evaporates into the enclosed air. The vapour settles on metal surfaces and forms a thin protective layer, so rust has no foothold even where the wrap does not touch the part.

VCI works inside an enclosed space. An open wrap or a torn bag lets the vapour escape and lets moist air in. That is why VCI is paired with a barrier bag and why heat sealing matters: the protection depends on the air staying contained and the volume being small enough for the vapour to reach everything.

Barrier bags and heat sealing

A barrier bag is a multilayer film with very low moisture transmission, made to enclose a load and be sealed closed. Sealing is done with a heat sealer that welds the film edges, which gives a continuous seam. Tape and folds leak, and a leaking bag is a bag with a desiccant pack doing nothing useful.

The bag must also be protected from the crate. Sharp corners, nail points and splinters puncture film. The builder pads the contact points, keeps fasteners out of the bag's path, and for heavier loads uses a bag of heavier gauge or lays a protective liner inside the lumber.

Steps for a machine going by sea

The sequence below is typical for machine tools and cast parts. A packer follows the specification for the load, so treat it as a guide to what happens, not a recipe.

1. 01
   Clean the metal

   Remove dust, chips and any fingerprints from bare surfaces. Moisture and salt on the metal go under the wrap with the load.

2. 02
   Wrap with VCI

   Wrap exposed steel in VCI paper or cover the load with VCI film, lapping seams and covering all bare faces.

3. 03
   Position the desiccant

   Place desiccant units where air collects, not touching the metal, and hang them in the bag in breathable pouches.

4. 04
   Bag and seal

   Draw the barrier bag over the load, remove excess air, and heat-seal the seam continuously.

5. 05
   Protect the bag

   Pad the points where blocking and bracing meet the bag, and keep the bag clear of fasteners.

6. 06
   Close the crate

   Complete the crate, with the humidity indicator visible if the specification includes one.

What the crate builder needs to know

Tell us which surfaces are bare metal, how long the load is expected to be in transit and in storage, and whether it will be opened on arrival or stored in the crate. A load that sits in a port yard for weeks needs more protection than one that moves straight to a plant.

Also say whether the load can be sealed whole or must ship in parts. A bag sized for a single complete machine is different from several bags for subassemblies, and the crate has to leave room for the bag. Bags need extra clearance, and that clearance is set by the load, not as a standard.

Storage, inspection and unpacking

Protection does not end when the lid is nailed down. Crates held in a port yard, a bonded warehouse or beside a plant door for weeks face the same condensation cycles as a sea voyage. Store them off the ground on their skids, under cover where possible, with the keep-dry side up and the humidity card visible through its window.

On arrival, open the crate slowly. Cut the bag along a seam, never across the load, and look at bare metal under good light before wiping anything. Fine surface bloom, discolouration or pitting should be photographed and reported to the carrier and the packer at once. Keep the wrap, the bag and the cards, since they show how the load was protected.