The color of Q355NH weathering steel , and this darkening process is both inevitable and characteristic of all standard weathering steels.
1. Color change in the early rusting stage (1–3 months)
At the beginning of outdoor exposure, Q355NH first turns light yellow, then golden brown, and gradually becomes bright orange-red. The rust layer is loose and thin, so the overall color is bright and light, without obvious darkening.
2. Color darkening in the middle stage (3–12 months)
As time goes on, affected by sunlight, rain, and alternating wet-dry cycles, the surface rust layer becomes denser. The bright orange-red slowly deepens to reddish brown, then to coffee brown. Obvious darkening can be observed visually during this period.
3. Final stable color after complete patina formation (1–3 years)
When a stable protective patina is fully formed, the color of Q355NH will further darken and stabilize at dark chocolate brown or grayish brown. After reaching this stage, the color basically stops changing and maintains a deep, dark tone for a long time.
4. Internal reason for the darkening
The darkening is caused by the transformation of the rust composition: the early loose, bright iron oxides gradually convert into dense, stable goethite ( -FeOOH), which naturally shows a darker color and forms a tight protective layer.
5. Environmental effects on darkening
In industrial, high-humidity, or rainy environments, the darkening process speeds up and the final color is darker. In dry rural areas, the darkening slows down, but the color still becomes significantly deeper than the initial orange stage.
