A lane that runs near the coast, or a facility within a few kilometres of the sea, carries salt in the air and on surfaces. Salt is not itself corrosive to most metals in the way that people assume. What it does is far more effective: it holds water on the surface and keeps the surface wet at humidity levels where it would otherwise be dry. That is the mechanism, and understanding it changes what the countermeasures should be.
Deliquescence, and why it matters
Salt absorbs moisture from the air above a certain relative humidity and dissolves in it, forming a concentrated solution on the surface. For sodium chloride that threshold is around seventy-five per cent at room temperature. Below it, the salt is dry and largely harmless. Above it, the surface stays wet indefinitely, because the salt keeps drawing moisture from the air to replace what evaporates. A surface that would dry in a normal building never dries in a salty one.
What that does to the humidity threshold
In clean conditions, steel corrodes slowly below about fifty per cent relative humidity. With salt present, the surface is wet at seventy-five per cent and the corrosion rate at that humidity is many times higher. The practical consequence is that the humidity target for a salty environment has to be set lower, and a desiccant quantity that is adequate inland is not adequate on a coastal lane.
Where the salt comes from
From the air at a coastal facility, deposited on everything including the inside of containers stored outdoors. From handling – hands, gloves, packaging that has been outside. From sea spray on a vessel or a dock. And from the load itself, where parts have been anywhere near the coast before packing. A container does not generate salt; it accumulates it from every one of those routes.
Washing, and whether it helps
Rinsing salt off with fresh water works, and it has to be followed by drying, because a rinse that leaves the surface wet is worse than the salt was. Where parts are washed, the drying step is the one that matters, and where it is skipped the result is often a corrosion problem that appears worse than before. A rinse is only half a treatment.
What the container contributes
Very little in terms of salt, and that is worth stating precisely: a polypropylene container contains no chloride and does not leach any. It is not a source. It is, however, a collector – a container stored outdoors near the coast accumulates deposited salt on every surface including the inside, and then encloses the parts with it. Where containers are stored outdoors on a coastal site, storing them closed or under cover matters more than any in-box treatment.
Handling the humidity target
Where a coastal lane has a corrosion problem, the first response should be to lower the target humidity rather than to add more of the same treatment. That usually means more desiccant, a tighter enclosure, and a shorter period closed. The sizing logic and the interaction between the two treatments are covered in desiccant and VCI inside a reusable box.
What to measure
Surface chloride is measurable with a simple test kit, and humidity inside a closed container is measurable with an indicator. Both are cheap. Measuring the chloride level on parts at the point of packing tells you whether the problem is being introduced before the box is closed – which is where it usually is – and measuring the internal humidity tells you whether the treatment is working.
Practical summary
Store containers closed and covered near the coast. Dry parts thoroughly before packing, especially after any washing. Set the humidity target lower than you would inland. And treat the enclosure as the variable, because the salt will always be there.