Guide
How do you specify VCI packaging so machined steel parts do not rust in ocean freight?
VCI packaging is bags, film or paper loaded with a volatile corrosion inhibitor: a chemical that vaporizes from the carrier, fills a closed package and coats bare metal with a molecule-thin film. It protects machined steel parts in ocean freight only when it is part of a written preservation spec: clean, dry parts, the right rust preventive, a closed VCI enclosure close to the metal, desiccant sized for the barrier, and dry outer packing.
By Saola Manufacturing engineering team · Published · Updated
The problem buyers actually face is a crate of shafts, brackets or housings that left the supplier bright and arrived after several weeks at sea with orange bloom on the top layer, rust rings around the bores, or brown spots where the parts touched the carton. The usual first reaction is to order “VCI bags” next time. That helps, but a VCI bag thrown into a damp wooden crate with an undersized desiccant pack still fails. This guide shows how to tell rust that formed in transit from rust that was already on the parts, how to diagnose a rusty shipment before rejecting it, which preservation method fits which shipment, and the purchase order wording that turns “pack for export” into something a supplier cannot misread.
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How do you tell rust that formed in the container from rust that left the plant on the parts?
Transit rust follows the package: it is worst on the top layer, near torn bag seams, under water stains and where parts touch wood or cardboard. Process rust follows the part: it sits on the same features of every part, often under the oil film, in coolant drying marks or in fingerprint shapes.
The distinction decides who owns the problem. Rust that formed in the box points to the packaging spec, the container or the voyage. Rust that was already there points to cleaning and preservation after machining, and no packaging upgrade fixes it. Before anyone argues about it, open several boxes from different positions in the shipment and record where the rust is: which layer, which face of the part, and what the part was touching.
Three quick observations separate most cases. First, look at the pattern across boxes: if only the boxes from one side of the container or the top of each crate are affected, the water came from above or from one wall. Second, look under the preservative: if the rust sits under an intact, undisturbed oil film, it was most likely there when the oil went on. Third, compare with any pre-shipment photos or a retained sample from the same lot. Bright parts in the photos and rust on arrival point to transit; a dull or spotted surface already visible in the photos points to the process.
What makes machined steel parts rust inside a closed shipping container?
Moisture that is already inside the box, not sea water, is the usual cause of transit rust: water in wooden crates, pallets and cartons evaporates in the heat of the day and condenses on the container roof and on cold metal at night. Missing or thin rust preventive, a leaky container and salt or sweat residues on the parts make it worse.
The German Insurance Association’s Container Handbook states that relatively large quantities of sweat in closed containers are always attributable to the cargo or its packaging, or a wooden container floor, and that this water vapor condenses on walls and a ceiling cooled by nocturnal radiation. The daily swing is large: its container climate page reports that at about 25 °C outside, the air inside a brown-painted container rises to approximately 50 °C. Warm air carries moisture out of wood and paper; when the roof cools, it rains back down. That is “container rain,” and it explains why the top layer suffers first. The same handbook’s loss prevention section puts air-dry lumber at a 12 to 15 % water content and freshly cut lumber at around 20 %, and its worked example dries pallets from 15 % to about 10 % to avoid sweat. A crate built from fresh timber carries its own supply of water into the box.
Ranked by how often they show up on machined steel parts, the causes are: wet or freshly cut wood in crates and pallets; parts packed without a continuous rust preventive film; VCI bags left open, torn or too far from the metal; desiccant that was undersized or placed in a package that was never a vapor barrier; container damage letting rain or spray in; and residues on the parts themselves, such as water-based coolant, fingerprints or salt.
| What you see on arrival | Most likely origin | Confirming check |
|---|---|---|
| Rust on the top layer or top faces only, drip marks, water-stained cartons | Transit: container rain from moisture inside the packing | Wood moisture reading on crate and pallet; humidity indicator color; desiccant saturated |
| Rust concentrated on one side of the shipment, wet floor or wall marks, daylight through the box | Transit: container leak or damage | Container inspection report or survey photos; light test of the empty container |
| Rust lines where parts touched wood, cardboard or each other | Transit: contact with damp, hygroscopic packing; no barrier between part and crate | Rust matches the contact outline; packing material damp to the touch |
| Rust near an open or torn VCI bag, good parts deep inside the same bag | Transit: VCI enclosure not closed or damaged in handling | Bag closure and seams; rust density versus distance from the opening |
| Same spots on the same feature of every part, under an intact oil film | Process: rust formed before preservation | Wipe the oil off one spot; compare with pre-shipment photos or a retained sample |
| Rust in bores, threads or pockets with tide marks | Process: water-based coolant or wash water left to dry in recesses | Residue in recesses; same pattern on parts from different boxes |
| Rust in fingerprint shapes | Process or handling: sweat salts from bare hands after cleaning | Visible ridge pattern under magnification |
How do you diagnose a rusty shipment at receiving before you reject it?
Quarantine the lot, record the evidence the packaging still holds, then sort the damage by surface: cosmetic bloom on non-functional faces, light rust on functional features, and pitting. The packaging evidence disappears once the boxes are opened and dried, so capture it first.
- Photograph before unpacking. Record the container or shipment number, the outside of the crates, water stains, tears and the position of each box in the load.
- Read the indicators. Note the humidity indicator card color and whether the desiccant bags are swollen or wet. A saturated desiccant means the package took on more water than it was sized for.
- Check the wood. A pin-type moisture meter on the crate and pallet tells you whether the packing itself brought the water. A reading in the high teens or above points to fresh or undried timber. The German Insurance Association’s Container Handbook says the water content of lumber inside a sealed package must be greatly reduced, because it releases water vapor inside the barrier.
- Map the rust. Use the table above: top layer and contact lines point to transit; the same feature on every part points to the process.
- Wipe and magnify. Clean a few spots with a solvent and a soft cloth. Light bloom that wipes off and leaves a smooth surface is cosmetic; pits that remain mean the base metal is damaged.
- Grade by function. Pitting on a bearing seat, sealing face, gauge thread or ground diameter usually means rejection. Light bloom on a non-functional face may be cleaned and re-preserved under a written deviation.
- Notify early. Tell the supplier, the forwarder and, if cargo insurance applies, the insurer before rework, and keep the samples and photos for the corrective action report.
Which preservation method fits which shipment: rust preventive oil, VCI, or a sealed bag with desiccant?
For bare machined steel crossing an ocean, the robust default is a thin rust preventive film plus a closed VCI bag, with dry or treated outer packing. When the parts are high value, the voyage is long or storage follows, add a heat-sealed water-vapor barrier with sized desiccant and a humidity indicator.
Each method works on a different principle, and each has a failure mode. A contact preventive is a film you can see; the corrosion preventive compounds in MIL-PRF-16173E (Amendment 2, 2017) range from hard film through soft film, water-displacing soft film and a transparent non-tacky film to a steam-removable film. Grade 3, the water-displacing soft film, suits parts that come off an aqueous wash and may still hold traces of water. The film fails where it is wiped off by handling or never reaches a blind hole.
VCI works through the air in the package. The Transport Information Service of the German Insurance Association explains in its VCI method section that the inhibitor deposits on metal surfaces as a monomolecular layer, that the packaging space should be sealed so the molecules cannot escape, that the carrier should be no further than 30 cm from the metal, that surfaces should be as clean as possible, and that VCI products from different manufacturers are not compatible with each other. The same section notes that no step is needed to remove VCI protection from the goods, which is why it suits parts that go straight to assembly.
The desiccant method dries the air instead. The TIS desiccant method section says it works only when the contents are enclosed in a barrier layer, that its effective range starts at less than 40 % relative humidity, that the number of desiccant units is calculated per DIN 55474, and that aluminum composite film performs far better than PE film on water vapor permeability. A desiccant bag dropped into an ordinary polyethylene VCI bag is fighting moisture that keeps diffusing in; it may be saturated long before the container is opened.
| Shipment situation | Minimum preservation | What usually goes wrong |
|---|---|---|
| Short, dry route; parts oiled for machining anyway; used soon after arrival | Clean, dry parts; light rust preventive film; closed VCI bag or VCI paper wrap | Coolant left in recesses; bag left open |
| Ocean freight through tropical ports, bare machined steel, normal value | Water-displacing soft-film preventive; closed VCI bag per part layer; layer separators; kiln-dried wood or plastic pallet | Damp crate wood; parts touching carton; VCI paper far from inner parts |
| High-value precision parts, long voyage or storage after arrival | Preventive film or VCI inside a heat-sealed aluminum-composite barrier bag; desiccant calculated for the barrier and duration; humidity indicator card visible | Barrier punctured by sharp edges; desiccant sized by guesswork; hygroscopic packing left inside the barrier |
| Parts that must stay dry of oil (going straight to painting, bonding or cleanroom) | Dry VCI film or paper only, compatible with the next process; closed enclosure; dry outer packing | Oil applied by habit; VCI chemistry not checked against the coating or bonding process |
| Mixed metals in one box (steel with zinc-plated, copper-alloy or aluminum parts) | Multi-metal VCI confirmed by its maker for every metal in the box | Ferrous-only VCI used on mixed loads |
Coated parts change the question. Zinc plating, black oxide and phosphate give different levels of protection, and black oxide and phosphate rely heavily on a supplementary oil or wax for rust resistance; the phosphate versus black oxide guide covers that choice. Whatever the finish, the packing still has to keep condensed water off the parts.
What should the purchase order and drawing say so the next shipment does not rust?
Write preservation and packing as numbered requirements on the purchase order, or on a packaging spec the order references, not as “export packing.” Name the cleaning state, the preventive and its grade, the VCI form and closure, the barrier and desiccant rule, the outer packing material and the evidence the supplier sends before shipment.
A clause the supplier can follow and you can inspect against might read:
- Cleaning. “PARTS CLEAN AND DRY BEFORE PRESERVATION. NO COOLANT OR WASH WATER IN BORES, THREADS OR POCKETS. HANDLE WITH GLOVES AFTER CLEANING.” Where cleanliness limits matter, reference them as described in the technical cleanliness guide.
- Preventive. “APPLY CORROSION PREVENTIVE PER MIL-PRF-16173, GRADE 3, OR APPROVED EQUAL. FULL COVERAGE INCLUDING BORES AND THREADS.” If oil is not allowed, say “NO OIL; DRY VCI ONLY.”
- VCI. “PACK EACH LAYER IN A CLOSED VCI BAG SUITABLE FOR STEEL, ONE VCI MANUFACTURER PER PACKAGE. BAG CLOSED BY HEAT SEAL OR TAPE. NO PART TO TOUCH WOOD OR CARDBOARD.”
- Barrier and desiccant (when required). “HEAT-SEALED WATER-VAPOR BARRIER BAG WITH DESICCANT QUANTITY CALCULATED PER DIN 55474 FOR THE STATED TRANSIT AND STORAGE PERIOD. HUMIDITY INDICATOR CARD VISIBLE THROUGH THE PACKAGE OR AT THE TOP.”
- Outer packing. “KILN-DRIED WOOD, MOISTURE CONTENT NOT ABOVE [__] %, OR PLASTIC PALLET; CRATE LINED WITH A MOISTURE BARRIER; NO FRESHLY CUT LUMBER.” Fill in the limit you will check at receiving. Wood packing for international shipment also has to meet the importing country’s wood packaging rules (ISPM 15 marking), but that mark is a pest treatment, not a moisture limit, so state the moisture limit separately.
- Evidence. “SEND PHOTOS OF PARTS BEFORE PRESERVATION, OF THE CLOSED INNER PACKAGE AND OF THE PACKED CRATE BEFORE SHIPMENT.” Those photos settle the transit-versus-process question in minutes.
- Receiving. “BUYER WILL REJECT PITTING ON FUNCTIONAL SURFACES; LIGHT BLOOM ON NON-FUNCTIONAL SURFACES SUBJECT TO DEVIATION.” Pair it with the sampling plan in the receiving inspection guide.
For general commercial shipments, the US reference point is ASTM D3951, Standard Practice for Commercial Packaging (2018, reapproved 2023), which covers preservation, packing and marking for shipment by any mode. Citing it on the order sets a floor; the clauses above add what ocean freight to a humid port actually needs.
Who does what on a Saola order for steel parts shipped by sea?
Machined steel parts are made by CNC turning and CNC milling at our lead CNC plant in Vietnam. Finishing is done by partner plants we manage, and the quote names the production site and the country of origin.
Preservation and packing are treated as part of the specification, not as an afterthought. When the RFQ or purchase order carries a preservation clause like the one above, the quote confirms each item: which preventive or VCI form is used, whether a barrier bag and desiccant are included, what the outer packing is, and which pre-shipment photos come with the lot. Items that cannot be met as written are flagged in the quote rather than changed silently.
Parts that also need plating, black oxide or another finish from the surface finishing menu go through a partner plant we manage, and the quote says where that step happens. If a shipment arrives with rust, the photos, indicator readings and part samples you collect at receiving are what lets the root cause be traced to the step that failed. Send the drawing and the packing requirement through the quote form, or ask for a preservation clause to be proposed for your part and route.
Frequently asked
Does VCI packaging work if the bag is left open?
Poorly. The inhibitor works by saturating the air in the enclosure, and an open or torn bag lets it escape while damp container air gets in. Close the bag by heat seal, tape or fold, and keep the VCI carrier close to every part.
Can I put desiccant inside a VCI bag?
Yes, and many shippers do, but the desiccant only controls humidity for long inside a real water-vapor barrier. In an ordinary polyethylene bag moisture keeps diffusing in, so size the desiccant for the barrier film and the duration rather than by habit, and keep the bags from touching the parts: the GDV Container Handbook warns that salt-based desiccants can cause corrosion once saturated.
Is VCI paper or a VCI bag better for machined parts?
A closed VCI bag is easier to keep sealed and protects every part in it; VCI paper works for wrapping individual parts or interleaving layers but leaves gaps if the wrap opens. Many ocean shipments use both: paper between layers and a closed bag around the load.
Do VCI products protect zinc-plated, brass or aluminum parts?
Only if the product is formulated for those metals. Some VCI products are made for ferrous metals only, and products from different manufacturers should not be mixed in one package. Ask the VCI maker to confirm compatibility with every metal and coating in the box.
Should rusty parts be cleaned and accepted or rejected?
It depends on where the rust is and how deep it goes. Light bloom that wipes off a non-functional face can often be cleaned and re-preserved under a written deviation. Pitting on sealing faces, bearing seats, threads or ground diameters usually means rejection, and the lot should get a corrective action that names whether the cause was transit or process.
Sources
- 4.4.3 The VCI method (Volatile Corrosion Inhibitor), Transport Information Service, German Insurance Association (GDV), accessed October 2026
- 4.4.4 The desiccant method, Transport Information Service, German Insurance Association (GDV), accessed October 2026
- Container Handbook 10.3.3, Water vapor in the container, German Insurance Association (GDV), accessed October 2026
- Container Handbook 10.3.7, Loss prevention measures for avoiding moisture damage in container transport, German Insurance Association (GDV), accessed October 2026
- Container climate, Transport Information Service, German Insurance Association (GDV), accessed October 2026
- MIL-PRF-16173E Interim Amendment 2, Corrosion Preventive Compound, Solvent Cutback, Cold-Application, US Department of Defense, 2017
- MIL-PRF-3420H, Wrapping Materials, Volatile Corrosion Inhibitor Treated, Opaque, US Department of Defense, 2008
- ASTM D3951-18(2023), Standard Practice for Commercial Packaging, ASTM International, 2023