Step 1: Surface Reconditioning – Repair Bending-Induced Damage
Mechanical Polishing
Use a soft stainless steel wire brush or fine-grained abrasive belt (grit #240–#320) to polish the bend zone and straight sections. Avoid high-pressure grinding to prevent further wall thinning.
Target surface roughness: Ra 3–5 μm (creates uniform nucleation sites for patina growth).
Degreasing and Cleaning
Wipe the entire pipe with a neutral degreasing agent or isopropyl alcohol to remove oil residues from clamping and die contact.
Rinse thoroughly with clean water and air-dry completely (no water spots left-these cause uneven oxidation).
Step 2: Stress Relief Annealing – Eliminate Tensile Stress (Critical Step)
Standard Annealing Parameters
Temperature: 550–620°C (below the pearlite transformation temperature-avoids altering the steel's ferrite-pearlite microstructure and mechanical properties).
Holding Time: 1–2 hours per 25 mm of pipe wall thickness (ensures uniform heat penetration into the bend zone).
Cooling Method: Slow furnace cooling to ≤200°C, then air cooling to room temperature (rapid cooling reintroduces residual stress).
On-Site Alternative (No Furnace Available)
Use local induction heating for the bend zone only (heats the target area to 550–600°C) to save energy and avoid affecting the entire pipe.
Step 3: Artificial Patina Acceleration – Form Uniform Protective Oxide Layer
Accelerator Selection and Application
Choose a water-based weathering steel accelerator (contains Cu²⁺, Cl⁻, SO₄²⁻ ions-matches the pipe's alloy composition of Cu:0.2–0.5%, Cr:0.3–1.2%).
Apply evenly with a low-pressure spray gun, focusing on the bend zone. Ensure no missed areas or dripping (key for color uniformity).
Controlled Curing
Place the pipe in an environment 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 air-dry-this locks in a consistent reddish-brown patina.
Step 4: Long-Term Protection – Seal and Maintain Patina
Breathable Sealant Coating
Apply a fluorocarbon-based transparent breathable sealant to the mature patina. The sealant isolates the pipe from rain runoff, dust, and salt spray, while retaining the natural rust aesthetic.
Reapply the sealant every 2–3 years for pipes used in coastal, industrial, or cold regions.
Regular Inspection and Touch-UpInspect the bend zone annually for patina peeling or localized corrosion. For minor defects:
Clean the area with a soft wire brush.
Reapply a small amount of accelerator.
Cure in a humid environment for 1–2 days to repair the protective layer.
