Desiccant and vapour-phase corrosion inhibitors are often treated as interchangeable, and they are not. One removes water from the air; the other protects a metal surface from the water that is there. Where a corrosion problem persists despite using one of them, the usual reason is that the other was the one needed – or that both were needed and only one was used.
What desiccant does
A desiccant adsorbs water vapour and lowers the relative humidity inside an enclosure. Below roughly fifty per cent relative humidity, most corrosion processes slow dramatically, and below forty they nearly stop. That is the mechanism: desiccant does not protect metal, it creates an environment in which corrosion is slow. Where the enclosure leaks, the desiccant is fighting a continuous inflow and it will be exhausted quickly.
What VCI does
A vapour-phase inhibitor releases a compound that deposits on metal surfaces and interferes with the corrosion reaction. It works in the presence of humidity rather than by removing it, and it protects surfaces the desiccant cannot reach – inside a bore, under a head, in a thread. Where parts have geometry that traps moisture, VCI reaches places desiccant cannot affect.
Why they are often both needed
Desiccant lowers the humidity and VCI covers what is left. Where the enclosure is genuinely sealed, desiccant alone is usually enough. Where it is not – and a reusable container with a lid is not – humidity will rise again as soon as the desiccant is exhausted, and VCI provides a second line. The combination costs a little more per trip and it is the difference between protection that lasts a week and protection that lasts a voyage.
Sizing desiccant for a reusable container
The requirement is the water already in the enclosure – in the air, on the contents, in the dunnage – plus what will enter over the period. The second term dominates for a container that is opened or that breathes. Where desiccant is sized for the air volume alone, it is typically under-sized by a factor of several for a long transit, and it will be saturated well before arrival.
Where to put it
High in the enclosure, because the vapour it needs to absorb moves slowly and cold dense air sits at the bottom. Away from the contents, so it does not contact parts directly or leave residue. And where it can be checked or replaced without unpacking, because a saturated desiccant looks identical to a fresh one unless it is an indicating type.
Compatibility with the container and the contents
Some VCI formulations are not suitable for certain metals – a VCI intended for steel can accelerate corrosion on some non-ferrous alloys, and the wrong one on copper or its alloys is a real risk. Some are not accepted for food-contact applications. Where VCI is used, the formulation has to be matched to the metals present, and that is worth stating in the specification rather than assuming a general-purpose product will do.
Cost per trip, and whether it is worth it
A desiccant sachet is cheap, and replacing it every trip is a consumable cost on every container in the fleet. On a large fleet that is a real annual number, and it is worth comparing against the cost of the corrosion it prevents. Where the parts are cheap and the corrosion is cosmetic, the answer may be to stop treating them. Where a single corroded component stops a customer’s line, the consumable cost is trivial by comparison.
Checking whether it works
Put a humidity indicator in with the load and read it on arrival. Below forty per cent means the desiccant was sufficient and the enclosure was tight enough; above sixty means one of the two was not true. Where the indicator reads high despite adequate desiccant, the enclosure is leaking, and the fix is in the lid rather than in the sachet. The wider picture on moisture routes is in where moisture gets into a closed box.