1. Purposes of Pickling Weathering Steel Plates
Pre-processing before coating: If the weathering steel plate needs to be coated with a non-breathable protective layer (e.g., for special industrial applications where patina formation is not desired), pickling can remove surface oxide scales to improve coating adhesion.
Repairing severe surface defects: For plates with thick, loose rust or heavy oxide scales caused by improper storage (e.g., long-term exposure to humid environments before use), pickling can clean the surface to expose a fresh, uniform steel substrate.
Preparation for precision processing: In cases where the weathering steel plate requires subsequent machining (e.g., cutting, bending with tight dimensional tolerances), pickling can eliminate surface irregularities caused by oxidation.
2. Pickling Process and Key Precautions
Pickling solution selection: Use dilute hydrochloric acid (5–10%) or phosphoric acid-based pickling agents instead of strong acids (e.g., concentrated sulfuric acid). Add corrosion inhibitors to the pickling solution to prevent over-corrosion of the steel substrate, which can cause pitting or surface roughness.
Process control:
Immerse the steel plate in the pickling solution at room temperature for 10–20 minutes (adjust time based on oxide scale thickness-avoid prolonged immersion).
After pickling, rinse the plate thoroughly with neutralized water (add sodium hydroxide to adjust pH to 7–8) to remove residual acid, then dry it immediately with hot air to prevent flash rust.
Mandatory post-treatment:
If the plate is to be used in its natural patina state, apply a rust stabilizer or patina accelerator to the pickled surface immediately. This promotes the rapid formation of a dense protective patina-without this step, the fresh steel surface will develop loose, non-protective rust in a short time.
If the plate is to be coated, apply a primer within 4–6 hours of pickling to block air and moisture contact.
3. Risks of Pickling Weathering Steel Plates
Damage to corrosion-resistant alloy distribution: The patina of weathering steel relies on surface enrichment of alloy elements (Cu, Cr, Ni). Pickling can dissolve part of these elements on the surface, reducing the speed and uniformity of patina formation. The pickled surface may take 2–3 times longer to form a stable patina compared to the original surface.
Increased early corrosion risk: Without timely post-treatment, the pickled fresh steel surface is highly susceptible to flash rust, especially in humid environments. This loose rust is not protective and may lead to localized pitting corrosion.
Surface roughness issues: Over-pickling can cause the steel surface to become overly rough, affecting the aesthetic uniformity of the final patina.
