Moisture protection for sea freight involves more than just tossing in a couple of desiccant packets: humidity inside containers often exceeds 85%, diurnal temperature fluctuations cause condensation, and wooden pallets and cardboard boxes themselves can generate moisture. Effective moisture-proof packaging requires a four-tier system—barrier layer + inner liner + desiccant + verification—with precise dosage calculations for the desiccant.
This article outlines the moisture-proof packaging requirements and calculation methods for metal structural components.
I.
Moisture Protection Objectives and Structure
Objective: Maintain the relative humidity surrounding the cargo below the safety threshold for metals (60% RH), or 40% RH for precision parts.
Four-tier structure: Outer barrier layer (film-coated waterproof cartons or aluminum foil composite bags to block external moisture); middle moisture-proof inner liner (sealed packaging to prevent dripping condensation); internal desiccant (to absorb moisture from the air trapped inside); supplemented by a humidity indicator card (for immediate visual verification of humidity levels upon opening).
Additional measures at the container loading stage include ensuring a dry container interior, controlling the moisture content of wooden packaging, and hanging desiccant bags from the container ceiling to absorb ambient moisture.
II. How to Calculate Desiccant Dosage
A combination of empirical formulas and reference values is used: based on volume, ocean shipments typically require 0.8–1.2 kg/CBM (use the lower limit for short 15–20 day routes, the mid-to-upper range for standard 25–35 day routes, and increase the amount for hot, humid equatorial routes); based on container type, a standard 20ft container requires 6–8 kg (approx. six 1 kg calcium chloride hanging bags), while extreme humidity conditions require 10–12 kg; a 40ft container requires 10–14 kg (or 6–8 hanging bags).
A rigorous approach follows the standard formula: n = (V × b + Σmi × ai) / c (container internal volume moisture + moisture released by packaging materials, divided by the absorption capacity of a single bag), with a 20% safety margin included.
Desiccant quality is verified against standards such as MIL-D-3464E—where one standard unit absorbs ≥6 g of moisture at 25°C and 40% RH.
III.
Don’t Overlook “Moisture Released by Packaging”
Wooden pallets, solid wood crates, and cardboard boxes contain inherent moisture that is slowly released as vapor during the voyage—a common pitfall in moisture-proofing plans. Control the moisture content of wooden packaging before export (ISPM 15 heat treatment incidentally reduces moisture content) and minimize the use of moisture-absorbing cushioning materials inside the container. For long voyages, 40ft containers, or shipments with many wooden pallets, increase the desiccant dosage to the upper limit of the reference chart.
Example: A standard 40ft container load requires 6–8 kg of hanging bags, whereas loads containing wood or hygroscopic goods require 10–12 kg.
IV. Container Loading and Verification Requirements
Loading: Load during dry weather; ensure the container is free of standing water and door seals are intact; keep cargo away from the condensation zone near the ceiling (leave headspace or elevate the cargo); place desiccant bags inside after loading (hung from the interior ceiling). Verification: Place humidity indicator cards in every carton;
upon arrival, check the card color to assess moisture exposure; for large shipments, first validate the plan using simulated heat-and-humidity testing (replicating the route’s temperature and humidity cycles); use indicator card data during customer complaint reviews to pinpoint whether the issue occurred during packaging or loading.
Affix “Moisture-Proof” warning labels (umbrella or raindrop icons) to outer cartons to alert personnel during handling and storage.
V.
Grading Recommendations
General hardware (galvanized parts, rough-machined parts): Film-lined cartons plus internal desiccant (0.8 kg/CBM) are sufficient. Precision hardware (bearings, precision stamped parts): Aluminum foil bags + VCI + desiccant + humidity indicator cards; maintain internal relative humidity (RH) below 40%. Ultra-long routes or routes to tropical/Middle East regions: Full-container desiccant bags + increased dosage + pre-loading moisture content checks.
Industry experts advise: Moisture-proof packaging costs only a fraction of the total cargo value, whereas a single claim for rust caused by condensation can be dozens of times higher—allocate resources based on the shipping route, spending on dosage and verification rather than on materials that merely “look premium.”
