Besides the obvious strength gap (Q355NH's yield strength ≥355 MPa vs. Q235NH's ≥235 MPa), the two weathering steels also differ significantly in weather resistance, mainly due to optimized alloy composition and rust layer performance:
Alloy element content: Q355NH has higher mandatory levels of weathering-enhancing elements (Cu ≥0.25%, Cr 0.40–0.80%, Ni 0.30–0.70%) than Q235NH (Cu ≥0.20%, lower Cr/Ni). These elements synergistically promote the formation of a denser protective patina.
Rust layer formation: Q355NH's patina stabilizes faster (1.5–2 years in mild environments) and is more compact/adherent; Q235NH takes longer (2–3 years) to form a stable layer, which is looser and prone to local peeling.
Long-term corrosion resistance: Q355NH has a 20–50% lower average corrosion rate (0.02–0.035 mm/year) than Q235NH (0.03–0.05 mm/year), especially in harsh environments (coastal/salty areas) where its higher Ni content better resists chloride-induced pitting.
In conclusion, Q355NH is not just a "higher-strength version" of Q235NH-it is also a more weather-resistant grade due to its optimized alloy composition and refined microstructure. This makes Q355NH more suitable for long-span structures, outdoor facilities (e.g., bridges, billboards, transmission towers), or projects in harsh environments (coastal, industrial areas), where long-term corrosion resistance is critical. Q235NH, by contrast, is more cost-effective for low-stress, mild-environment applications (e.g., simple shelters, non-load-bearing decorative parts) where weather resistance requirements are not strict.
