Galvanized coil surfaces can develop white rust, the powdery zinc corrosion product, when moisture condenses inside packaging during sea transport or long storage. Desiccants are the first line of defense against this: they absorb the water vapour that enters the package and keep the microclimate dry around the coil. Choosing the right desiccant is a matter of matching four criteria to the transit and storage plan, and this guide explains how to do it without over- or under-protecting the cargo.
Core Selection Criteria
Four criteria matter. First, strong and long-lasting moisture absorption: the desiccant must pull humidity down quickly and keep the package below about 60% relative humidity for the whole journey; absorption capacity depends on temperature, so the product must perform at the ambient range of the route. Second, chemical neutrality: the zinc coating is attacked by acids and alkalis, so the desiccant must not release corrosive gases or liquids. Third, safety and cleanliness: the sachets must be robust and dust-free so that breakage cannot contaminate the coil surface or violate transport rules. Fourth, correct dosage: calculated from the package volume, typically 50-100 g per cubic meter, adjusted for transit time and humidity.
Desiccant Types Compared
| Type | Strength | Best use | Limitations |
|---|---|---|---|
| Silica gel | Fast absorption, chemically neutral, dust-free grades available | Short-term storage of 1-3 months, dry areas, high-cleanliness coils such as appliance grade | Limited total capacity for very humid long voyages |
| Calcium chloride | Very high capacity, keeps absorbing even at high humidity | Sea freight, rainy season, storage beyond 3 months | Can form a solution; must be sealed against leakage onto the zinc |
| Montmorillonite | Low cost, neutral | Cost-sensitive short-distance transport in dry areas | Lower capacity than silica gel and calcium chloride |
| Molecular sieve | Stable absorption at high temperature | Summer transport, high-temperature containers, applications needing stable performance | Higher cost |
Selection by Storage and Transport Scenario
Match the desiccant to the journey. For short-distance transport under one week in a dry season, silica gel is sufficient. For long sea freight or rainy-season deliveries, solidified calcium chloride is the workhorse because it keeps absorbing at high humidity. For storage beyond six months, combine silica gel and calcium chloride to balance immediate absorption with long-term capacity. For high-temperature summer routes, molecular sieve maintains stable performance where other types lose efficiency. The surface treatment of the coil also guides the choice: passivated galvanized coils tolerate calcium chloride, while unpassivated coils or coils with a thin passivation layer should use neutral silica gel to remove any corrosion risk.
Handling and Dosage Rules
Desiccants must never touch the zinc surface directly. Calcium chloride in particular is slightly acidic and can corrode the coil if a sachet leaks, so it must be packed in sealed, abrasion-resistant pouches and positioned away from the coil face where possible. Dosage is not fixed: calculate from package volume, transit time and expected humidity, increasing the 50-100 g per cubic meter baseline for long voyages, rainy seasons and tropical routes. The package must be effectively sealed; a desiccant in an open, ventilated package cannot control humidity. Finally, choose products that meet recognized environmental and safety requirements such as RoHS and REACH so that the packaging itself does not contaminate the galvanized surface.
Combining Desiccants with the Rest of the Package
Desiccants work as one component of a moisture-protection system. The coil should first receive rust-preventive oil or VCI paper, then a moisture-barrier wrap, then desiccants, edge protection and banding, all inside a structurally sound skid or crate. The barrier wrap keeps liquid water out and slows vapour entry, while the desiccant handles the vapour that still penetrates. Each layer covers what the others miss, and specifying all of them together gives reliable protection for galvanized coils on the longest routes.
Frequently Asked Questions
Q1. How much desiccant do I need per cubic meter of package?
Typically 50-100 g per cubic meter as a baseline. Increase the dose for long sea freight, rainy-season deliveries and tropical routes, and verify the calculation with the desiccant supplier, who will ask for package volume, transit time and expected humidity.
Q2. Can calcium chloride desiccant touch the galvanized surface?
No. Calcium chloride can form a slightly acidic solution when it absorbs moisture, and direct contact can stain or corrode the zinc. It must be kept in sealed, abrasion-resistant pouches and placed away from the coil face.
Q3. Silica gel or calcium chloride for a 30-day sea shipment?
Calcium chloride is usually the right choice for long sea freight because its capacity is far higher and it keeps absorbing at the high humidity found inside containers. Silica gel alone may be exhausted before arrival on humid routes.
Q4. Do desiccants work if the package is not sealed?
No. A desiccant controls humidity only inside a sealed space. If the wrap is torn or the package is ventilated, moisture enters faster than the desiccant can absorb it, and protection fails.
Q5. What humidity should the package maintain?
The target is an internal relative humidity below about 60%, which is the practical threshold to prevent condensation and wet-storage stain on galvanized surfaces. Lower humidity, around 40-50%, is used for long storage and high-value coils.
Q6. Are desiccants enough to protect galvanized coils?
No. They are one part of the system. Combine them with VCI paper or rust-preventive oil, a moisture-barrier wrap, edge protection and proper stowage away from container walls, where condensation is worst.