How to protect the galvanized coating of SPCC-GI during processing?
The galvanized coating of SPCC-GI (hot-dip galvanized cold-rolled steel sheet) is the core guarantee of its corrosion resistance. Damage to the coating during processing (such as scratches, peeling, and oxidation) will directly affect its rust resistance. Therefore, protective measures must be implemented in three stages: pre-processing preparation, process optimization, and post-processing. The details are as follows:
1. Pre-processing Protection Measures
Raw Material Storage and Handling
Avoid Stacking Damage: During storage, separate each layer of steel sheets with wooden or rubber mats to prevent friction and scratches on the coating caused by gravity. Stacking height should be kept to a minimum (usually ≤ 1.5 meters) to prevent deformation of the underlying steel sheets.
Protecting Against Moisture and Dust: Although the galvanized coating is corrosion-resistant, prolonged exposure to high humidity may cause white rust (zinc hydroxide). Store in a dry and ventilated warehouse, covered with waterproof canvas if necessary. Avoid contact with oil, dirt, acids, and bases to prevent chemical corrosion of the coating. Handling Tool Protection: When using a forklift or lifting equipment, rubber or nylon pads should be used on areas that come into contact with the steel plate. Do not use metal hooks to directly hook the edge of the steel plate to prevent the coating from being scratched by sharp objects.
Pre-Processing Cleaning
If there is dust or oil on the steel plate surface, wipe it with a neutral detergent (such as soapy water), then rinse and dry it with clean water to prevent impurities from embedding into the coating during processing or causing mold wear, which could indirectly damage the coating.
II. Process Optimization During Processing (Core Step)
Stamping Process Protection
Die Design and Lubrication:
Die cutting edges should be polished smooth (roughness Ra ≤ 0.8μm) to prevent sharp edges from scratching the coating. The clearance between the die and punch should be slightly larger than that of ordinary cold-rolled steel sheets (typically 5%-10%) to reduce extrusion friction. Special lubricants for galvanized sheets (such as mineral oil or water-soluble lubricants containing extreme pressure additives) must be used to prevent dry friction from causing wear of the coating and high temperatures (heat generated by stamping) from causing oxidation and discoloration of the coating. Acidic or alkaline lubricants are prohibited to prevent corrosion of the galvanized coating.
Stamping Process Parameters:
Reduced stamping speed (especially for deep-drawn parts with complex shapes) to minimize the friction time between the material and the die. Controlled stamping temperature: If heating is required (such as during hot stamping), keep it below the melting point of zinc (419°C) to prevent melting and loss of the coating.
Shearing and Bending
Shearing tools: Use high-speed steel or carbide cutting tools, maintaining sharp edges to prevent tool dullness from tearing the coating. During shearing, the tool clearance should match the sheet thickness (typically 5%-10% of the sheet thickness) to minimize extrusion and peeling of the coating. Bending Protection:
The bending radius should be ≥ 1.5 times the plate thickness (this can be reduced for thinner steel plates) to avoid excessively small radiuses that could cause the coating to tensilely fracture (the zinc layer has a lower ductility than the base material and is prone to cracking due to excessive bending).
Pre-lubricant can be applied to the bends, or a polytetrafluoroethylene (PTFE) film can be applied to the contact area between the die and the steel plate to reduce friction damage.
Welding Process Protection
Welding Method Selection: Spot welding (resistance welding) is preferred to avoid direct damage to the zinc coating caused by the high-temperature arc generated by arc welding (such as stick arc welding). If arc welding is required, use specialized galvanized steel wire (containing deoxidizing elements such as silicon and manganese) and control the welding current (slightly lower than that for ordinary steel plates) to minimize zinc evaporation and coating loss.
Weld Spot Treatment: After spot welding, the zinc layer around the weld may oxidize (blacken). Use a stainless steel wire brush to gently remove the oxidation layer. Apply a high-zinc repair paint (such as cold spray zinc) to repair the coating damage. Handling and Positioning
When transferring steel plates during processing, wear clean cotton gloves to prevent sweat (containing salt) from corroding the coating. Do not drag the steel plates; lift them to prevent scratches from rubbing against the work surface.
The contact areas of positioning tooling should be made of soft materials such as rubber and nylon to prevent direct compression of the coating by the metal tooling.
III. Post-Processing Repair and Protection
Damage Detection and Repair
After processing, inspect the coating surface. If minor scratches are found (without exposing the substrate), repair them with cold-spray zinc paint or zinc paste with a zinc content of ≥95%. If scratches expose the underlying black substrate, clean the damaged area first, then apply a double coat of primer (such as epoxy primer) and cold-spray zinc paint to prevent rust from corroding the substrate.
For large areas of coating loss (such as areas with severe stamping deformation), assess whether it will affect serviceability and, if necessary, replace the steel plate to prevent the spread of rust. Storage and Transportation Protection
After processing, semi-finished products should be wrapped with kraft paper or VCI paper to prevent the coating from being scratched or contaminated by leaving them exposed. If they need to be stored for an extended period, a thin layer of anti-rust oil can be sprayed on (this can be cleaned and removed before assembly).
During transportation, separate components with partitions to prevent friction and collision, and lay rubber mats on the floor of the vehicle to protect against moisture.
Summary of Key Principles
The key to protecting the galvanized layer of SPCC-GI lies in **"reducing friction, avoiding high temperatures, and isolating corrosive media"**:
Control the storage environment before processing to prevent premature damage to the coating;
Optimize molds, lubrication, and process parameters during processing to minimize mechanical damage and high-temperature oxidation;
Promptly repair damage after processing and implement protective measures to ensure the integrity of the coating.
These measures maximize the rust-proofing properties of the galvanized layer and extend the service life of SPCC-GI products.