Many flanges pass factory inspection but arrive at the warehouse rusted after ocean shipping and storage. This “in-package corrosion” results from失控 management of temperature, humidity, and condensation during storage. Establishing a scientific packaging and environmental control system is key to preserving flange value.
Corrosion Mechanism: Why Closed Packages Still Rust
Even in sealed packaging, if the relative humidity inside exceeds 60% and temperature fluctuations cause condensation, corrosion will still occur. Moisture in the air forms a thin liquid film on the flange surface; oxygen diffuses through the film, causing electrochemical corrosion. Residual cutting fluid, sweat, or cleaning agents inside the package accelerate this process.
Final Treatment Before Packaging
- Thorough drying: After the final cleaning step, use hot air drying or oven drying at 80°C for at least 30 minutes. Confirm no visible moisture on the flange surface.
- Rust preventive oil application: Spray or dip apply removable RPO, ensuring uniform film thickness covering all surfaces (including bolt holes and sealing face). For stainless steel flanges, use chlorine-free, sulfur-free rust preventives.
- VCI technology: Place volatile corrosion inhibitors inside the package; their molecules adsorb onto the metal surface forming a monomolecular protective layer. VCI paper should be placed directly against the flange, not loosely inserted.
Selection of Packaging Materials
- Inner layer: Wax-free, acid-free VCI paper or polyethylene film. Never use newspapers,普通 cardboard, or chlorine-containing plastics.
- Outer layer: Heavy-duty polyethylene bag or aluminum-plastic composite film, providing a moisture barrier. Heat seal or tie tightly, and预留 space for desiccant.
- Desiccant: Silica gel or molecular sieve, quantity calculated based on package volume and expected storage period. Typically use 200g desiccant per cubic meter of package volume. Desiccant should be pre-dried and placed in breathable pouches.
Warehouse Environmental Control
- Temperature: Maintain 15–25°C, avoid direct sunlight and heat sources.
- Relative humidity: Keep below 40%. Equip warehouse with dehumidifiers, record temperature and humidity daily.
- Condensation prevention: Avoid sudden temperature changes. Flanges moved from cold storage should equilibrate at room temperature for 2 hours before opening; otherwise, surface condensation occurs instantly.
- Stacking: Place flanges on pallets at least 150mm above floor, maintain 500mm clearance from walls for air circulation and inspection.
Package Validity and Re-Inspection
Different rust prevention schemes have different validities:
- RPO only: 6 months (indoor), 3 months (outdoor/ocean).
- RPO + VCI + desiccant + aluminum-plastic bag: 12–24 months.
After validity expires, sample open packages; if rust spots appear, reprocess.
Packaging Requirements in Procurement Technical Documents
Explicitly specify:
- Rust prevention method and materials (grades, types).
- Package construction (layers, sealing, desiccant quantity).
- Package appearance requirements (intact, clear labeling).
- Validity period and storage conditions.
All data are sourced from publicly available sources and are provided for learning, communication, and reference purposes only. If there are any errors, please contact for correction. Please make your own judgment, this website assumes no responsibility.