During a 20-to-40-day sea voyage, container humidity often reaches 85%–95%, and the diurnal temperature difference can hit 30°C. At night, condensation forms “container rain” that drips onto the cargo; combined with salt spray penetration, rusting of bare steel parts upon arrival is almost inevitable. Rust prevention during transport is not merely a matter of applying a coat of oil;
it involves a complete process chain: Cleaning → Drying → VCI Wrapping → Sealing → Desiccant → Verification.
I.
Know the Enemy: Three Sources of Corrosion
First is salt spray: Marine air contains high levels of sodium chloride (chloride ion concentration in seawater is approximately 19,000 ppm). This adheres to metal surfaces, forming an electrolyte film that accelerates electrochemical corrosion. Second is condensation (“container sweat”): High-temperature, high-humidity air trapped inside the container during the day cools against the steel walls at night, causing water droplets to fall onto the cargo—a cycle that repeats throughout the journey. Third is high humidity: Relative humidity inside the container often exceeds 85%. Corrosion rates for steel parts accelerate significantly above 60% humidity; exceeding 80% is akin to storing steel in “invisible rain.”
Hardware structural parts (such as stamped parts, fasteners, and castings) have large exposed surface areas and are particularly vulnerable.
II.
Process 1: Cleaning and Drying—The Foundation
Thoroughly remove cutting fluids, sweat stains, salts, grease, and machining residues from the workpiece surfaces. Blind holes, threads, counterbores, and recessed cavities must all be blown dry. Wait for the workpiece temperature and humidity to stabilize before packaging to ensure no moisture is trapped inside the package.
Experience: 90% of rust-prevention failures stem from “packaging without proper cleaning”—residual electrolytes on the surface continue to cause corrosion within the sealed environment.
III.
Process 2: VCI (Volatile Corrosion Inhibitor) Packaging
The core principle of VCI paper, bags, and films is the continuous release of anti-corrosion molecules within a sealed space to form a monomolecular protective layer on the metal surface. This protects areas inaccessible to coatings—such as internal cavities and grooves—and simultaneously safeguards steel, aluminum, and cast iron components. Key operational points: VCI materials must be placed close to the metal surface;
ensure a tight seal to prevent vapor loss (do not use ordinary plastic bags as a barrier between the VCI material and the metal); wrap items individually before boxing; and use VCI paper to separate stacked parts. Select VCI types based on the required protection duration (6–12 months is common for ocean freight; long-term solutions can last up to 24 months).
For polished parts like brass, use an interleaving paper to prevent the VCI film from adhering to the surface and causing oxidation.
IV.
Process 3: Desiccants and Sealed Environment
Place sufficient desiccant inside the box (silica gel or mineral desiccant, quantified based on the box’s internal volume); for long-haul shipments, place specialized container desiccant bags inside the shipping container to absorb ambient moisture; use high-barrier aluminum foil composite bags for the outer layer to enhance moisture resistance.
The container loading stage is equally critical: load under dry conditions, ensure no standing water inside the container, and verify that door seals are intact—if loading occurs after rain or if there is standing water on the container floor, all previous rust-prevention efforts will be wasted.
V.
Verification and Acceptance
Before mass shipment, conduct trial packaging using actual components and perform salt spray and damp-heat cycling tests to verify that the rust-prevention solution meets standards (marine environments are harsher than static laboratory conditions, requiring dynamic temperature and humidity cycling tests). After shipment, require an inspection of the goods upon arrival at the destination port to check for corrosion and provide feedback, thereby establishing a closed-loop process for continuous rust-prevention improvement.
Industry experts offer a reminder: effective rust prevention during transport relies on a combination of “low-cost materials and rigorous procedures.” While VCI bags and desiccants are inexpensive, failure to strictly execute the cleaning, sealing, and container-loading steps will still result in rust upon arrival;
codifying these steps into Standard Operating Procedures (SOPs) and posting them at the packaging line is far more effective than simply choosing expensive materials.
