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How to avoid corrosion and green spots caused by condensation salt spray during storage and shipping
source：www.cnlichao.net | Release time：2026年10月13日1、 Mechanism of Green Spot Corrosion
The temperature difference between day and night at sea is large, with containers experiencing high temperatures during the day and low temperatures at night. The dew point of the air inside the container is higher than the surface temperature of the steel coil, and water vapor condenses between the steel strip layers and end faces; The chloride ions in the sea mist adhere to the surface of stainless steel with the condensed water film, penetrate the passivation film, form pitting corrosion, and accumulate corrosion products, showing green spots. The surface oxide layer of hot-rolled coils is loose, making them more prone to this type of corrosion compared to 2B cold-rolled coils; In addition, carbon steel powder adhering to the surface of the board can induce electrochemical corrosion, accelerate rusting, and generate green spots.
2、 Factory multi-layer moisture-proof and rust proof packaging
1. Inner layer: VCI vapor phase rust proof paper wrapped in a whole roll, covering the outer periphery and two end faces, releasing vapor phase corrosion inhibitors, and isolating water vapor and salt spray; Polishing mirror rolls requires additional surface protection film to avoid scratching the passivation film.
2. Middle layer: Thickened PE waterproof film is used for overall sealing, and a circular protective plate (end cover) is installed on the end face. The inner ring of the core should also be wrapped, with a focus on sealing the end face gaps and sealing the overlapping areas with adhesive tape.
3. Edge protection: Install corner and edge protection on the inner and outer rings to prevent the steel straps from crushing the moisture-proof packaging.
4. Desiccant: Place sufficient calcium chloride/montmorillonite desiccant in the core and packaging cavity; Calculate the usage based on the length of the voyage and container capacity; Desiccant must be placed after the steel coil has completely cooled down, and steel coils with moisture cannot be packaged.
5. Strap: Use steel straps with gaskets to avoid directly crushing the packaging film.
Attention: The packaging materials (wooden pallets, paper, protective boards) themselves must be dry. Wet auxiliary materials sealed inside the packaging will directly condense inside the sealed packaging.
3、 Container loading protection (export shipping)
1. Check the container before loading: inspect the top, side walls, and floor for any signs of water leakage, and check if the container floor is damp; Container high-capacity desiccant strips can be placed in advance.
2. Steel coil placement: Priority should be given to vertical placement (with the eyes facing upwards) to reduce the risk of water entering the gaps between steel strips; The steel coils are cushioned and isolated, firmly fixed to prevent shipping bumps and displacement, and to prevent tearing of the outer packaging.
3. Control the temperature difference during container loading: Do not directly seal high-temperature steel coils into the container; The temperature of the steel coil should be as close as possible to the ambient temperature inside the box to avoid cooling and condensation after sealing the cabinet.
4. In box isolation: moisture-proof film is laid on the inner wall of the container to isolate the condensation and dripping of the container wall onto the steel coil; It is strictly prohibited to mix with ordinary carbon steel, chemical, and salt containing goods in the same container.
4、 Warehouse storage protection (before/after arrival)
1. Warehouse conditions: Indoor sealed warehouse, relative humidity control ≤ 60%, and try to control the temperature difference between day and night within 10 ℃; Continuous operation of dehumidification equipment during the rainy season and in coastal warehouses; Outdoor stacking is strictly prohibited.
2. Stacking standards: Use steel pallets or IPPC fumigation dried wooden pallets, with a height of ≥ 15cm above the ground and ≥ 30cm above the wall, away from external walls, windows, and condensation pipes to prevent moisture and condensation on the wall.
3. Partition storage: Stainless steel coils should be stored in a separate area and should not be mixed with carbon steel materials to prevent carbon steel powder from falling onto the surface of the board and causing iron ion pollution, which can lead to pitting and green spots.
4. Packaging management: Do not remove moisture-proof packaging before use; If the outer packaging is damaged, immediately pack and replace the desiccant in a dry environment.
5. Regular inspection: Regularly check whether the packaging is damaged and whether there are signs of moisture; Once the packaging is damaged, it should be dealt with promptly.
5、 Taboos on process control
1. During hoisting, transportation, and port stacking, avoid exposure to rain and seawater spray; Unpacking and packaging operations are prohibited on rainy days.
2. Do not touch the surface of stainless steel with bare hands, as salt residue on your hands can leave a source of chloride ion pollution.
3. Polish high surface grade stainless steel such as 8K and BA, with a thinner passivation film and greater sensitivity to chloride ions. The packaging protection level needs to be upgraded.
4. The chloride ion resistance of 400 series ferritic stainless steel is weaker than that of 304 and 316L, and the marine protection standards are higher.
6、 Emergency response after arrival
Upon opening the package upon arrival, moisture was found inside and water stains were found on the end faces
1. Do not directly tear off all packaging, transfer to a dry warehouse, and let it dry naturally and slowly through ventilation;
2. Check whether there are yellow spots or green spots on the end face and interlayer; Minor floating rust can be professionally passivated and cleaned; Once deep pitting corrosion forms, it cannot be easily repaired.
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