Q345NH is a high‑strength low‑alloy atmospheric corrosion‑resistant steel (weathering steel) per GB/T 4171, with excellent corrosion resistance in normal atmospheric environments, about 2–8 times better than ordinary carbon steel.
1. Corrosion mechanism
It adds Cu, Cr, Ni, P (and trace Nb/Ti) to form a dense, stable, adherent protective rust layer (50–100 μm thick) on the surface after long‑term outdoor exposure.
This layer is mainly composed of α‑FeOOH (goethite), which blocks O₂ and H₂O penetration and slows further corrosion.
Corrosion rate drops sharply after ~144 hours and stabilizes at a low level.
2. Corrosion performance in different environments
Clean rural atmosphere: Very good; annual corrosion rate ≤ 0.01 mm/y; stable rust layer forms quickly; long‑term service without painting.
Industrial atmosphere (SO₂, dust): Good; corrosion rate higher than rural but still 1/4–1/6 of ordinary carbon steel.
Coastal/marine (salt spray): Moderate; salt ions damage rust layer compactness; corrosion accelerates; not ideal for severe marine zones.
Acid rain / high humidity: Acceptable but needs proper drainage; stagnant water causes pitting.
Long‑term immersion: Poor; rust layer fails; not for submerged use.
3. Key influencing factors
Alloy content: Higher Cu/Cr/Ni → better rust layer stability.
Surface treatment: Proper surface preparation accelerates stable rust formation; avoid oil/grease/dust.
Environment: Dry‑wet cycles promote dense rust; stagnant water/salt/acid degrade performance.
Microstructure: Fine grains from controlled rolling improve corrosion uniformity.
4. Comparison with ordinary steel & other weathering steels
vs. Q235 ordinary carbon steel: 2–8× better atmospheric corrosion resistance.
vs. Q355NH: Similar mechanism; Q355NH has higher strength but comparable corrosion resistance.
vs. SPA‑H (Japanese) / A588 (US): Comparable performance; Q345NH is cost‑effective for general applications.
5. Application notes
Best for outdoor structures (bridges, towers, containers, vehicles) in non‑severe corrosive environments.
In coastal/industrial zones, paint or surface passivation is recommended to extend life.
Avoid crevices, water traps, and direct contact with dissimilar metals to prevent galvanic corrosion.
