When a machine fits in a container, corrosion protection is a packaging problem. When it does not, it becomes a weather problem. Out-of-gauge cargo — oversized weldments, structural steel, wind turbine components, large skids and frames — goes breakbulk on the top deck, where it spends weeks in salt spray, UV and nightly condensation with nothing between it and the Atlantic but whatever you put there before it left your yard.
Cortec® published a field approach to this problem in October, built around railway bogie frames: heavy, wide, full of bare machined surfaces and structural voids, impossible to containerize. It layers three kinds of protection, because no one of them covers the whole part. That logic is worth walking through, because most export rust claims we see come from doing one of the three and assuming it covered the other two.
Why on-deck freight fails differently than crated freight
A crated part inside a container sees a fairly stable enclosure. It may get hot, it may condense, but it is not being hit with chloride-laden water. On-deck cargo is. Salt spray carries chloride directly onto bare steel, and chloride does not evaporate off when the sun comes out — it stays on the surface and keeps pulling moisture out of the air long after the deck is dry.
The second difference is geometry. Large fabrications have internal voids: box sections, tube runs, bolt pockets, double walls. Water gets in through drain holes and seams and does not come back out. You can coat every outside surface perfectly and still find rust blooming from inside a box section on arrival.
The third difference is time under cover. A tarp handles rain. It does not handle months of UV, and it does nothing about the air trapped underneath, which on a humid loading day is the air that will condense on the part every night of the crossing.
The three layers, and what each one is actually for
1. Temporary coating on exposed metal
Unpainted machined faces, flange faces, shafts and weld areas carry the highest risk, because there is no paint film doing anything for them. A temporary coating such as Cortec VpCI®-368 forms a tough barrier film that will not wash off in rain or spray but comes off later with pressure washing and an alkaline cleaner. Painted surfaces are not automatically safe either, especially where the paint is chipped or the edges are thin; a non-tacky water-based coating such as VpCI®-391 adds a layer that is inconspicuous enough to leave on the equipment permanently if nobody wants to bother removing it.
Both are removable by design, so tell the receiving plant which coating is on there and how to take it off — a line on the packing list prevents a lot of phone calls.
2. Vapor phase inhibitors in the void spaces
Voids are the layer most exporters skip, because you cannot see the problem and cannot brush a coating into it. The answer is to put the inhibitor in as a vapor. Fogging the internals with a VpCI powder such as VpCI®-609, or with a fogging fluid such as VpCI®-339, fills the cavity with inhibitor that diffuses and adsorbs onto every interior surface, including the ones a nozzle never reached.
The one step that decides whether this works: cap the openings. The inhibitor has to stay in the enclosure to build and hold its molecular layer. An uncapped drain hole vents the protection and leaves you with an untreated void, which is why a void-space job is as much about plugs and tape as it is about chemistry.
3. A cover that resists UV, not just water
A heavy-duty VpCI shrink film such as MilCorr® does two things a tarp does not: it resists UV for multi-year outdoor exposure, and it releases vapor phase inhibitor into the air space under the film, so the enclosure protects the part rather than just hiding it. It can also be fitted with custom access doors — which means a surveyor or customs inspector can look at the cargo without cutting the film open and destroying the protection on day three of a thirty-day voyage.
Which products to look at
- Cortec® VpCI®-368 wax-like temporary coating, 5 gallon pail — outdoor-service barrier for bare metal, removable by pressure washing
- Cortec® VpCI®-391 water-based non-tacky temporary coating, 5 gallon pail — inconspicuous top layer over painted surfaces
- Cortec® VpCI®-609 water-soluble VpCI powder, 50 lb — void-space and enclosure protection
- Cortec® VpCI®-339 fogging fluid, 5 gallon pail — fogging option for internals and hollow structures
- Cortec® MilCorr® VpCI® shrink film, 20′ × 210′ × 10 mil — UV-resistant heavy-duty cover with inhibitor built in
- Cortec® EcoWeave® woven VpCI sheeting, 48″ × 500′ — lighter-duty woven covering, cut and sewn to shape, where shrink film is more than the job needs
For the cover, our guide on how to shrink wrap equipment with MilCorr® VpCI® film covers film sizing, seaming and access doors. For coating selection, dry times, film thickness and removal, the Cortec® coatings application guides put the per-product data side by side.
The step before all three layers
Every one of these layers assumes a clean, dry surface. Chloride, mill scale, old rust and shop oil sit between the inhibitor and the metal, and the protection performs to whatever is underneath it rather than to the data sheet. On large fabrications that means abrasive blasting, then coating promptly, before the blasted steel flash rusts in the same humidity you are shipping through. Shops doing this at volume work inside a blast room so prep and coating happen in one controlled space on one shift. Smaller machined components in the same shipment are better cleaned in an aqueous parts washer, which gets the shop oils off before the temporary coating goes on.
If you are quoting an export job and want help matching the coating, the void treatment and the cover to the cargo and the route, get in touch. We stock the Cortec® line in Illinois and can work from your drawings and crate dimensions.
