Q355NH and Q420NH weathering steel have identical atmospheric corrosion resistance-their core corrosion-resistant alloy element ranges (Cu, Cr, P) are fully consistent per GB/T 4171 standards, and the only difference between the two grades is mechanical strength (yield/tensile), which does not impact their ability to form a stable protective patina or resist atmospheric corrosion.
Key Details Confirming Equivalent Corrosion Resistance
Identical Corrosion-Resistant Alloy Formulation
Both grades share the exact same mandatory ranges for the alloy elements that drive weathering performance (GB/T 4171):
Copper (Cu): 0.20–0.50% (initiates and densifies the protective patina)
Chromium (Cr): 0.30–1.20% (enhances patina chemical stability)
Phosphorus (P): 0.07–0.15% (improves patina adhesion and uniformity)
Other alloy elements (Si, Mn) differ only to adjust mechanical strength, not corrosion resistance.
Same Patina Formation & Protective Performance
When exposed to outdoor atmospheres (urban, industrial, rural), both Q355NH and Q420NH form a dense, adherent reddish-brown patina (main component: α-FeOOH) within 6–12 months. This patina acts as an identical physical/chemical barrier against water, oxygen, and corrosive ions, resulting in the same low corrosion rate (≈0.01–0.03 mm/year in industrial atmospheres, 6–10 times lower than ordinary carbon steel).
No Grade-Based Corrosion Resistance Classification
GB/T 4171 classifies weathering steel by corrosion resistance grade (e.g., Grade C for general atmospheric corrosion) and strength grade (355/420 = minimum yield strength in MPa)-the two grades fall into the same corrosion resistance class, with strength being the only differentiator.
Why Strength Does Not Affect Corrosion Resistance
The mechanical strength difference (Q355NH: ≥355 MPa yield strength; Q420NH: ≥420 MPa) is achieved by adjusting manganese (Mn) and silicon (Si) content (Q420NH has slightly higher Mn/Si for solid solution strengthening), not by altering the corrosion-resistant Cu/Cr/P core. These strength-enhancing elements do not interfere with patina formation or protective performance.
