A polypropylene container does not absorb moisture and does not lose strength when it is wet. That is genuinely useful and it is routinely misunderstood as protection for the contents. It is not. The container sheds water to the inside of the box rather than holding it, the enclosed space keeps it there, and a box of steel components in a closed plastic container is a box of steel components in a small humid room.
What fibreboard did that plastic does not
A corrugated fibreboard box absorbs water into its walls, and while that destroys the box, it also removes moisture from the immediate environment of the parts. A plastic container has no such capacity. Every gram of water that enters or forms inside stays in the air, on the parts, or in the dunnage. Where a load has been switched from fibreboard to plastic without changing anything else, corrosion sometimes gets worse rather than better, and this is why.
The enclosed space is the problem
A closed container is a small volume with a limited amount of air and no exchange. Any moisture introduced – by the parts, by the dunnage, by condensation during a temperature cycle – raises the humidity of that small volume quickly, and there is nothing to bring it back down. The same parts on an open shelf in the same building would not corrode, because the building’s air volume is effectively infinite by comparison.
Where the water comes from
Parts arriving from a wet process carry a film. Parts moved from a cold area into a warm one collect condensation on every surface. Dunnage holds moisture from a previous trip. And the air inside the box at closing carries whatever the building had. None of these are large individually, and in a closed two-hundred-litre volume they are enough to take the humidity to saturation. The routes are set out in where moisture gets into a closed box.
Time is the multiplier
Corrosion is slow, so a short trip is usually harmless and a long one is not. That is why the problem appears on storage and on long sea transits and not on an internal lane. Where the same container and the same parts are fine on a two-day domestic route and rusted on a five-week voyage, the difference is duration, not the container.
What actually prevents it
Dry the parts before packing, which is the most effective single measure and usually the cheapest. Add desiccant sized for the enclosure and the duration. Add VCI where the geometry traps moisture. And reduce the duration the load spends closed and warm. Four measures, and the first is the one most often skipped because the parts look dry.
When it is worth treating
A machined steel component going into a customer’s assembly is worth treating; a mild steel bracket in a non-critical position may not be. The decision should be made per part rather than per fleet, and it should be based on what a corroded part costs – which for a component that stops a line is far more than its own value. Where a fleet carries mixed contents, the treatment should follow the most sensitive part rather than the average one.
What to check on arrival
Whether the corrosion is on upward-facing surfaces, which suggests condensation, or is general, which suggests sustained humidity. Whether it is worse at the top of the load, which suggests the roof dripping, or evenly distributed, which suggests humidity. Those two observations separate the mechanisms and they cost nothing to make. The sea-transit version is described in container rain.
A note on responsibility
Where a container is supplied for a customer’s parts, the supplier is often asked whether the container caused the corrosion. It did not – the container is not a corrosion source and polypropylene contributes no moisture, salts or corrosives. The cause is the moisture in the system, and being able to say that clearly, with the mechanism explained, is worth more in that conversation than any assertion that the box is fine. The moisture side of the same lane is covered in moisture and condensation on a long sea leg.