1. Initial exposure stage (0–3 months)
The original hot-rolled surface gradually oxidizes; color turns from dark gray steel to light yellow → golden brown → bright orange-red.
The rust layer is loose, thin, and uneven; obvious rust runoff may occur in rain.
Color changes rapidly and is unstable during this period.
2. Medium-term maturation stage (3–18 months)
Under alternating wet-dry cycles, the surface oxide layer gradually densifies.
Color deepens from bright red to reddish brown → dark brown → coffee brown.
Loose rust decreases significantly, and the surface becomes more uniform.
Rust runoff weakens and gradually disappears.
As a high-strength weathering steel, Q500NH forms a denser patina than Q355NH, so the darkening speed is relatively stable.
3. Long-term stable stage (1.5–5 years)
After complete patina formation, the color stabilizes at dark chocolate brown or grayish brown.
The surface is compact and well-bonded to the substrate; no obvious color change occurs under normal atmospheric environment.
This dark, mature color can be maintained for decades.
4. Color changes under special environments
In industrial or acid-rain areas, the patina tends to be darker, grayish-brown due to more sulfide deposition.
In clean rural areas, the final color is slightly lighter, mostly warm dark brown.
In high-humidity, poorly ventilated areas, local black or dark streaks may appear.
In mild coastal areas, the color may become uneven due to salt spray interference.
5. Final stable color characteristics
Q500NH's long-term surface color is darker and more uniform than Q235NH and Q355NH.
It will not return to bright orange or light red; it maintains a deep, mature dark brown tone.
Without coating protection, the color will only slightly adjust with seasonal humidity but will not fade significantly.
