1.How does the type of rust-preventive oil determine its rust-preventive period?
Short-term rust preventative oil (inter-process rust prevention): The rust prevention period may only be a few days to a few weeks, mainly used for temporary protection during the production process.
Medium- to long-term rust preventative oil: This is the most commonly used type for cold-rolled coils. Under standard storage conditions, the rust prevention period is typically 6-12 months.
Long-term rust preventative oil/hard film rust preventative oil: It can form a thicker or harder film, and the rust prevention period can reach 12-24 months or more. It is suitable for sea transport or long-term outdoor storage.
2.How does the storage environment affect the rust-proof period?
Humidity: Air humidity is the primary enemy of steel rusting. A dry environment with relative humidity below 50% greatly extends the rust-preventive effect; while humid environments and coastal areas severely shorten the rust-preventive period.
Temperature: High temperatures accelerate oil film oxidation and evaporation, while low temperatures may cause the oil film to crack. A temperature-controlled, cool warehouse is ideal.
Contaminants: Acidic gases (such as sulfur dioxide), salt spray, and dust in the air can penetrate or damage the oil film, inducing corrosion.
Ventilation and Packaging: Good indoor storage, or tight packaging with rust-proof paper or plastic film lining, can significantly extend the rust-preventive period. Open-air storage results in the shortest rust-preventive period.
3.How does coating quality affect the rust prevention period?
Oil film uniformity: Whether the oil film completely and evenly covers the entire surface of the steel coil, especially the edges.
Oil film thickness: Too thin a film will provide insufficient protection, while too thick a film may result in waste and make subsequent cleaning difficult. The recommended oil application amount (e.g., g/m²) must be achieved according to the product instructions.
Steel plate cleanliness: Before applying oil, the steel plate surface must be clean and dry, free of fingerprints, watermarks, dust, and other contaminants; otherwise, the rust-preventive effect will be directly affected.
4.What are some general industry references and suggestions?
Standard Storage Period: For cold-rolled coils stored in standard industrial warehouses (not temperature and humidity controlled, but rain and moisture protected) using high-quality medium- to long-term rust-preventive oil, manufacturers and traders typically promise a 6-month "factory rust-prevention guarantee." This is a relatively common and conservative period.
Ideal Storage Period: In well-controlled temperature and humidity warehouses with appropriate packaging, a 12-month rust-prevention period is realistic and common.
Overdue Handling: If the storage time exceeds the expected rust-prevention period, it should not be automatically assumed that the material has rusted. The following must be performed:
Uncoiling Inspection: Carefully inspect the surface, especially the edges and underlying layers.
Assess the Degree of Rust: If only slight surface rust is present, it can usually be treated with subsequent pickling or light polishing without affecting further processing. Severe rust may result in losses.
Additional Rust-Preventive Measures: For coils that need to be stored further, consider recoating with the same type of rust-preventive oil.
5.What are the important points to note?
"Shelf life" is not an expiration date: Even after the recommended rust prevention period, it doesn't mean the steel will rust immediately, only that the risk increases significantly. It's a timeframe for performance assurance.
A technical agreement is mandatory: When purchasing cold-rolled coils, the following should be clearly stated in the contract or technical agreement:
Type of rust-preventive oil (brand or standard number).
Amount of oil applied (grams of oil applied per unit area).
The recommended rust prevention guarantee period under agreed storage conditions (e.g., indoor storage, rust prevention period no less than 6 months).
First-in, first-out (FIFO) principle: Warehouse management must strictly adhere to the FIFO principle to prevent some coils from rusting due to prolonged storage.