Why Post-Cutting Treatment Is Necessary
Damage to the Protective Patina: Q355NH's surface forms a stable rust patina that resists corrosion, but cutting (flame cutting, plasma cutting, etc.) melts and removes this patina at the edges.
Formation of Vulnerable Layers: Cutting generates a heat-affected zone (HAZ) and oxide scale on the cut surface. These layers are loose, porous, and lack corrosion resistance, making them prone to rapid rusting.
Risk of Localized Corrosion: Unprocessed cut edges will rust faster than the original surface, leading to uneven corrosion, edge pitting, or even spreading to the entire steel plate over time.
Common Post-Cutting Treatment Methods (Sorted by Practicality)
Process: Use an angle grinder with a wire brush or abrasive disc to remove oxide scale, burrs, and molten slag from the cut edge. Then apply a weathering steel-specific rust conversion agent (contains tannic acid, phosphoric acid, etc.).
Effect: The conversion agent reacts with the fresh steel surface to form a dense, corrosion-resistant film that integrates with the existing patina, ensuring uniform protection.
Application: Suitable for most scenarios (architectural facades, outdoor structures, etc.).
Process: Immerse small workpieces or local edges in a 5–10% phosphoric acid solution for 10–15 minutes to remove oxide scale. Rinse thoroughly with water, then neutralize with a weak alkaline solution (e.g., sodium bicarbonate) to avoid residual acid corrosion.
Effect: Achieves a clean, active surface that can quickly form a uniform patina naturally.
Note: Not suitable for large workpieces; requires proper waste disposal to avoid environmental pollution.
Process: After grinding the cut edge, apply a weathering steel primer and topcoat (matching the steel's color) to seal the edge.
Effect: Provides immediate corrosion protection and aesthetic consistency, especially for projects requiring a uniform appearance.
Application: Ideal for visible cut edges in architectural or decorative applications.
Exception: If the cut edges are in a low-humidity, low-pollution environment and not exposed to direct moisture (e.g., internal structural components), and the steel will form a new patina naturally over time, treatment may be omitted.
Risk: Uneven rusting may occur in the short term; not recommended for outdoor or harsh environments.
Key Considerations for Treatment
Cutting Method Impact: Flame/plasma cutting produces thicker oxide scale and a larger HAZ, requiring more thorough grinding; laser cutting has a narrower HAZ and cleaner edges, so light grinding + conversion coating is sufficient.
Timing: Treat cut edges within 24–48 hours of cutting to avoid premature rusting of the fresh surface.
Compatibility: Ensure treatment materials (conversion agents, paints) are designed for weathering steel to avoid chemical reactions that damage the patina.
