Of all the controls available for corrosion in transit, drying the parts before they go into the container is the most effective and the least used. It is cheap, it needs no consumables, and it addresses the largest single source of moisture in the system. The reason it is skipped is that parts which look dry usually are not, and nobody measures.
Looking dry is not being dry
A part that has been through a water-based wash, or that has sat in a humid building overnight, carries a surface film. It may be invisible – a few grams across a whole load – and in a closed container that is more than enough to saturate the internal air. Where a load is packed from a wet process without a drying step, the moisture is already in the enclosure before it is closed.
How much water is actually there
A single component may carry a fraction of a gram. A container holding two hundred of them carries tens of grams. To saturate two hundred litres of air at room temperature takes a few grams. The arithmetic is unforgiving: a load that looks dry can contain several times the water needed to take the enclosure to saturation, and it will.
Where the drying happens
At the end of the washing process, which is the most common place to add it and the easiest to justify. In a staging area with air movement, which works for parts that arrive damp rather than wet. Or as part of the packing operation, with an air blow-off at the bench. All three cost far less than a desiccant programme sized to deal with the water instead.
Compressed air, and its limitations
An air blow-off removes standing water effectively and does not remove a thin adsorbed film, which needs either heat or time. Where parts are blown off and then packed immediately, the film remains. Where they are blown off and allowed to stand in a warm, dry area for an hour, it goes. The difference matters, and it is why some drying steps appear not to work.
Coolant and cutting fluid residues
A machined part carries cutting fluid, which is not water but holds water and often contains chlorides or other aggressive species. Where parts go into a container straight from a machine, the fluid is a corrosion contributor in its own right and it defeats a desiccant by continuously releasing moisture. Cleaning before packing, or at least draining thoroughly, is part of the same control.
What to measure
Weigh a sample of parts before and after drying, or use a humidity indicator in a closed container with a normal load and read it after an hour. Either test shows whether the drying step is doing anything. Where a desiccant programme is in place and the indicator still reads high, the parts are the source, and more desiccant is the wrong response. The sizing logic is in desiccant and VCI inside a reusable box.
Cost comparison
A drying step is a one-off equipment cost and a small energy cost. A desiccant programme sized to compensate for wet parts is a perpetual consumable cost, on every container, for as long as the lane runs. Over three years the difference is usually an order of magnitude, and the drying route also works better, because it removes the water rather than competing for it.
What to write into the process
A drying method, a dwell time, and a check. The check can be as simple as a visual inspection for standing water plus a periodic humidity indicator test. Where those three are written down, the control survives shift changes and new staff; where they are not, it lasts as long as whoever introduced it.