1. Post-Bending Surface Restoration: Eliminate Damage Caused by Forming
Controlled Mechanical Polishing
Use a soft stainless steel wire brush or fine-grained abrasive belt (grit #240–#320) to polish the bend zone. Avoid high-pressure grinding to prevent further wall thinning.
The goal is to remove scratches and loose oxide debris, while creating a uniform surface roughness (Ra 3–5 μm) to provide nucleation sites for patina growth.
Degreasing and Cleaning
Wipe the entire pipe with a degreasing agent (e.g., isopropyl alcohol or neutral detergent) to remove oil residues from clamping and bending dies. Oil stains block the contact between the steel surface and corrosive media, leading to uneven patina.
Rinse with clean water and dry thoroughly to avoid water spots that cause localized oxidation.
2. Stress Relief Annealing: Core Step to Prevent Stress Corrosion
Standard Annealing Parameters for Weathering Steel Pipes
Temperature: 550–620°C (below the pearlite transformation temperature to avoid changing the steel's ferrite-pearlite microstructure and mechanical properties).
Holding Time: 1–2 hours per 25 mm of pipe wall thickness (ensure uniform heat penetration into the bend zone).
Cooling Method: Slow furnace cooling to ≤200°C, then air cooling to room temperature. Rapid cooling will reintroduce residual stress.
Alternative for On-Site Processing: If furnace annealing is unavailable, use local induction heating for the bend zone only-this saves energy and avoids affecting the entire pipe.
3. Artificial Patina Acceleration: Ensure Uniform Corrosion Protection
Accelerator Selection and Application
Choose a water-based accelerator matching the pipe's alloy composition (containing Cu²⁺, Cl⁻, and SO₄²⁻ ions to mimic natural oxidation).
Apply the accelerator evenly with a low-pressure spray gun, focusing on the bend zone. Ensure no missed areas or dripping (critical for color and thickness uniformity).
Controlled Curing Environment
Place the pipe in a chamber with temperature 20–30°C and relative humidity 70–90%. Rotate the pipe every 12 hours to ensure the bend zone is fully exposed to moisture and oxygen.
After 3–5 days, rinse with clean water to remove residual accelerator ions and dry-this locks in a uniform reddish-brown patina.
4. Long-Term Protection: Seal and Maintain the Patina
Transparent Breathable Sealant Coating
Apply a fluorocarbon-based breathable sealant to the mature patina. The sealant isolates the pipe from uneven environmental exposure (rain runoff, dust) and prevents patina peeling, while retaining the natural rust aesthetic.
Reapply the sealant every 2–3 years for outdoor use in harsh environments.
Regular Inspection and Touch-Up
Check the bend zone annually for localized corrosion or patina damage. For minor defects, clean the area, reapply a small amount of accelerator, and cure for 1–2 days to repair the protective layer.
5. Pre-Bending Optimization: Minimize Damage from the Source
Use warm bending (150–200°C) for thick-walled pipes (>8 mm) or small bending radii (≤3× pipe diameter) to reduce residual stress and surface scratches.
Adopt mandrel bending to avoid pipe wall collapse and excessive thinning of the outer arc.
Use polyurethane-coated dies instead of bare metal dies to prevent surface wear during bending.
