1. Color Change Process After Rusting
Initial rust stage (1–3 months): Fresh Q355NH has a metallic gray finish. When exposed to air and moisture, it first develops light orange-brown or reddish-brown loose rust. At this stage, the color is relatively bright and uneven.
Patina formation stage (3–12 months): The loose rust gradually reacts with alloying elements (Cr, Cu, P) in Q355NH to form a dense, stable dark reddish-brown or dark brown patina. This is when the color 明显 darkens-shifting from bright rust to a deep, uniform dark tone.
Stable stage (after 12 months): The patina fully matures into a dark brown or near-black protective layer. It stops further significant color changes and maintains the dark, consistent appearance long-term.
2. Key Reasons for Darkening
Q355NH is weathering steel. Its core corrosion resistance relies on forming a dense patina (composed of iron oxides, chromium oxides, and copper phosphates).
Compared to loose, bright red rust (mainly Fe₂O₃·nH₂O), the mature patina has a higher density and richer alloy composition, which absorbs more light and reflects less-resulting in a darker visual effect.
3. Precautions for Usage
The darkening is a normal sign of patina formation, not a quality defect. It indicates the steel is developing corrosion resistance.
For outdoor applications (e.g., architectural facades, bridges), avoid frequent cleaning of the patina-this will disrupt its formation and delay the darkening/stabilization process.
In harsh environments (coastal salt spray, industrial fumes), the patina may form unevenly, leading to spotty darkening. Pre-treat the surface with a weathering accelerator or apply a protective clear coat if uniform darkening is required.
