What actually rusts a part in transit
A part does not rust because the air is humid. It rusts because water condenses on it. A carton loaded in a warm plant at the end of a summer day, put in a trailer that cools overnight and then sits on a dock in the morning sun goes through two or three dew-point crossings before it is delivered. Each time the metal is colder than the air around it, water forms on the surface. Steel with a few microns of condensate on it and a little oxygen in the air starts to oxidize within hours.
Cortec published a note on this in the summer that is worth reading now, because fall is when shops that got away with it in July start seeing claims. The temperature swings get bigger in September and October, not smaller, and export shipments are still crossing the equator.
Why more desiccant is the wrong answer
The traditional response is to throw a desiccant bag in the box. Desiccant absorbs moisture from the air, which lowers the humidity inside the package and delays condensation. Two things go wrong with it.
First, desiccant is passive. It does nothing to the metal surface. Until the humidity is pulled down it offers no protection, and the first hour after packing, when the part is still warm and the air is still saturated, is when the surface is most exposed.
Second, desiccant saturates. A one-unit bag holds a fixed number of grams of water and then it is done. A carton with a cardboard divider, a wood pallet or a paper wrap inside it carries its own water into the package, and a leaky liner lets more in on every temperature cycle. Once the desiccant is full, the package is back to where it started, usually somewhere mid-ocean. Cortec’s phrase for it is that throwing more desiccant at the problem, hoping it will be enough, is no longer the go-to option.
Desicorr: absorb the moisture and protect the surface
Desicorr VpCI pouches combine both mechanisms in one unit. The pouch contains desiccant, so it pulls moisture out of the air the same way a conventional bag does. It also contains a vapor phase corrosion inhibitor that releases through a breathable membrane, fills the air space and forms a molecular layer on every metal surface in the package. The VpCI layer protects the part from the moment the box is closed, before the desiccant has lowered the humidity, and it keeps protecting after the desiccant saturates. The layer dissipates when the package is opened, so there is nothing to clean off.
The result is a package that does not depend on staying dry. If the liner leaks or the box sits in a humid warehouse for a month, the part is still covered.
Sizing by package volume
Desicorr is dosed by the air volume of the package, not by the weight of the part. Cortec publishes the coverage and absorption figures for each size:
| Pouch size | Protects (enclosed volume) | Moisture absorption | Typical use |
|---|---|---|---|
| 1/6 unit | 1 cubic foot | 1.32 g | Small cartons, individual bagged parts, kits |
| 1 unit | 5 cubic feet | 9.9 g | Standard shipping cartons, tote bins, small crates |
| 4 unit | 20 cubic feet | 39.6 g | Pallet boxes, Gaylords, equipment crates |
| 8 unit | 40 cubic feet | 79.2 g | Large crates, shrink-wrapped equipment, containers |
A few practical rules. Measure the inside of the container, not the part. Round up. Split the dose across two pouches at opposite ends of a long crate rather than putting one large pouch in a corner. And for anything that will be opened for inspection and resealed, add a pouch at reseal, because the first one has been working on the open air.
The 1/6 and 1 unit pouches come in a windowed design with indicator spheres that change color when the desiccant is exhausted, which lets a receiving inspector confirm the package was still working on arrival. The larger sizes are non-windowed with reinforced seals so beads cannot escape into the package. The Desicorr NW version is the non-windowed pouch in the small sizes.
Where it fits with the rest of the package
Desicorr is an addition to VpCI packaging, not a replacement for it. For most metal parts the base package is a VpCI-126 bag or liner, which supplies inhibitor from the film itself. Desicorr goes in on top of that for humid lanes, export, long warehouse dwell or any shipment where the liner might be compromised. For dry domestic ground freight a BioPad emitter is usually enough and costs less.
The one thing no pouch fixes is a part that goes in wet. A part washed in a Build-All or Magido aqueous washer and packed while still damp will carry more water into the box than a one-unit pouch can absorb. Dry the part first. The washer’s blow-off or a few minutes on a rack is the cheapest moisture control in the whole routine.
Which products
- Desicorr VpCI pouch, 1/6 unit, 300 per carton (windowed)
- Desicorr VpCI pouch, 1 unit, 300 per drum (windowed)
- Desicorr NW VpCI pouch, 1/6 unit and 1 unit (non-windowed)
- 4 unit and 8 unit pouches for crates and containers, quoted on request
- VpCI-126 Blue bags and liners as the base package
- BioPad emitters for dry domestic lanes
See the VpCI product selection guide for matching the base film to the metal.
Talk to us
If you have had a rust claim this summer, send us the carton size, the lane and a photo of the part, and we will size the package. Contact us.
Source: Cortec Corporation published product data.
