A typical steel plate preservation machine comprises integrated subsystems that work in tandem to deliver consistent, high-quality protection:
1. Surface Preparation Module
Shot Blasting Unit:
Function: Removes rust, mill scale, and contaminants using high-velocity steel shot or grit, achieving cleanliness grades like Sa2.5 (ISO 8501-1) and roughness profiles of 50–100 microns.
Design: Multi-wheel centrifugal blasting systems with overhead and undercarriage wheels ensure double-sided cleaning of plates up to 5 meters wide.
Dust Collection System: High-efficiency baghouse filters capture 99% of airborne particles, maintaining a clean environment and complying with EPA standards for air quality.
2. Coating Application Module
Spray Painting Systems:
Airless Sprayers: Deliver high-viscosity coatings (e.g., epoxy, polyurethane) at pressures up to 3,000 psi, ensuring uniform coverage and thickness (50–300 microns).
Robotic Arms: Equipped with anti-drip nozzles and 3D vision sensors to adapt to plate contours, reducing overspray by 30% compared to manual application.
Roller Coating Units: Used for thin films or primers on smooth surfaces, offering precision control for coatings as thin as 20 microns.
Hot Melt Coating Systems: Apply molten materials (e.g., polyethylene) for heavy-duty corrosion protection in pipelines, with cooling tunnels ensuring rapid solidification.
3. Curing and Drying Module
Conveyorized Ovens:
Temperature Control: Pre-heating zones (40–60°C) dry blasted surfaces, while post-application ovens (80–200°C) accelerate curing of thermoset coatings.
Infrared (IR) and UV Curing: High-intensity IR lamps or UV arrays reduce curing time from hours to minutes, ideal for high-throughput lines.
Humidity and Ventilation Systems: Maintain optimal conditions (50–70% RH, <3°C dew point) to prevent moisture-induced coating defects like blistering.
4. Quality Control Module
Automated Thickness Gauges: Non-destructive electromagnetic sensors measure wet and dry film thickness in real time, with tolerance limits of ±5%.
Visual Inspection Cameras: AI-powered systems use machine learning to detect coating defects (e.g., runs, holidays) and flag plates for rework.
Holiday Detectors: High-voltage testers identify pinholes or discontinuities in conductive coatings, ensuring 100% coverage.
Operational Processes and Technical Standards
1. Pre-Coating Preparation
Degreasing: Solvent-based or alkaline washes remove oils and grease, often via spray tunnels or ultrasonic baths.
| Parameter | Typical Range | Impact on Coating Adhesion |
|---|
| Cleanliness Grade (ISO 8501-1) | Sa2.5–Sa3 | - Sa2.5: Near-white metal finish (≤5% residue), suitable for most industrial coatings.
- Sa3: White metal finish (0% residue), critical for marine or high-corrosion environments.
- Reduction in undercoating corrosion risk: Sa3 reduces failure rates by 40–60% compared to Sa2. |
| Surface Roughness (Ry) | 60–100 microns | - Increases mechanical interlocking between substrate and coating.
- 30–50% surface area expansion at 100 microns vs. smooth surface.
- Optimal for epoxy paints (requires 70–90 microns for full adhesion). |
| Surface Profile (ISO 8503-2) | Type 2–3 (Angular/Coarse) | - Type 2: Angular peaks from steel grit, ideal for heavy-duty coatings (e.g., polyurethanes).
- Type 3: Coarse texture from large media, enhances grip for thermal spray coatings.
- Paint retention improvement: Angular profiles reduce peel strength by 25–35% compared to rounded profiles. |
| Blast Media Type | Steel Grit (G25–G60) | - Grit produces sharper peaks vs. shot, increasing mechanical bond strength.
- Stainless steel grit avoids iron contamination on aluminum or SS substrates, preventing galvanic corrosion under coatings. |
| Peak Density (ISO 8503-5) | 50–150 peaks/cm² | Higher density (e.g., 120 peaks/cm²) creates a "micro-mechanical anchor," reducing risk of coating delamination by 30%. |
Cleanliness as a Foundation:
- Contaminants (oil, rust, mill scale) act as barriers; even 1% residue can reduce adhesion by 20%.
- Sa3-grade surfaces achieve 100% contact angle with wet coatings, ensuring complete wetting.
Roughness-Specific Adhesion Models:
- Mechanical Interlocking: Roughness creates valleys where coating materials cure, forming a "keying" effect.
- Thermal Expansion Matching: Textured surfaces accommodate differential expansion between substrate and coating (e.g., steel → paint: ΔCTE = 11 vs. 20 ×10⁻⁶/°C).
Profile Geometry Considerations:
- Avoid 过度粗糙 (Ry>120 microns), which can trap air or coating solvents, leading to blistering.
- For thin films (e.g., 50μm coatings), use Type 1 (smooth) profiles to prevent thickness inconsistencies.
- Pre-Blast Surface Preparation: Degrease with solvent wipes (e.g., acetone) before blasting to remove oil, improving cleanliness grade by 1–2 levels.
- Media Recycling Controls: Use magnetic separators to remove broken media fines (<0.1mm), which can create overly smooth surfaces (Ry <50 microns).
- Real-Time Monitoring: Deploy portable roughness testers (e.g., Mitutoyo SJ-210) to verify Ry values every 2 hours during production runs.
- Coating Application Timing: Apply primer within 4 hours of blasting to avoid flash rust, which reduces adhesion by 15–20%.
| Cleanliness Grade | Roughness (Ry) | Coating Type | Pull-Off Adhesion (MPa) | 5-Year Corrosion Failure Rate |
|---|
| Sa2.5 | 60 microns | Epoxy primer | 8.2 ± 1.1 | 18% |
| Sa3 | 90 microns | Polyurethane | 12.5 ± 0.8 | 3% |
| Sa2.0 (Inadequate) | 50 microns | Acrylic paint | 5.1 ± 0.5 | 45% |
2. Coating Application Techniques
a. Wet Paint Systems
Primers: Zinc-rich epoxies (80–90% zinc content) provide cathodic protection, ideal for offshore structures.
Topcoats: Polyurethane or acrylic coatings offer UV resistance and color stability, with gloss levels of 60–90 GU (Guage Units).
Dry Film Thickness (DFT):
Light Duty: 100–150 microns (e.g., indoor structural steel).
Heavy Duty: 300–500 microns (e.g., submerged pipeline coatings).
b. Powder Coating
Process: Electrostatically charged powder particles adhere to plates, then melt and cure in ovens to form a seamless film (50–200 microns thick).
Advantages: Zero VOC emissions, superior edge coverage, and resistance to chipping.
c. Thermal Spraying
Metallic Coatings: Aluminum or zinc wires are melted and sprayed at 300–500 m/s, creating sacrificial layers (200–500 microns) for extreme corrosion environments.
3. Curing and Post-Treatment
Ambient Curing: Suitable for large plates outdoors, requiring 24–48 hours at 20°C and low humidity.
Forced Curing: Reduces time to 1–4 hours, with temperature profiles tailored to coating chemistry (e.g., epoxy cures at 120°C for 1 hour).
Post-Cure Inspection: Non-conductive coatings are tested for porosity using low-voltage holiday detectors (5–10 kV).
