Q355NHB is a weathering steel whose corrosion resistance improves significantly over time as its protective patina gradually forms and stabilizes. Its corrosion behavior follows a clear, predictable three‑stage pattern.
1. Initial stage (0–1 year)
Surface begins to oxidize and form a loose, light‑colored rust layer.
Corrosion rate is relatively high during this early period.
Protective effect is weak because the patina has not yet fully developed.
The material is still adapting to the atmospheric environment.
2. Intermediate stage (1–3 years)
Denser, darker patina starts to form from alloying elements such as copper, chromium, nickel, and phosphorus.
Corrosion rate decreases noticeably as the protective layer becomes more continuous.
Surface rust becomes more stable and less flaky.
The material's corrosion resistance improves steadily with exposure.
3. Stable stage (3–5 years and beyond)
A tight, firmly bonded, dark patina fully covers the surface.
Corrosion rate drops to a very low, constant level and remains stable for decades.
Further corrosion is effectively slowed and controlled.
Corrosion resistance reaches its maximum and long‑term durability is achieved.
Key trend summary
In the first year: corrosion is relatively fast.
After 1–3 years: corrosion resistance improves continuously.
After 3–5 years: corrosion rate stabilizes at a minimum.
Over decades: the patina continues to self‑repair and maintain protection.
