1. Feasibility: Q355NH's composition is compatible with hot-dip galvanizing
During galvanizing, the steel is immersed in molten zinc (around 450°C). Q355NH's carbon content (≤0.18%) and low impurity content (e.g., S≤0.035%, P≤0.035%) prevent excessive reactions with zinc (such as "overheating brittleness" or uneven zinc layer adhesion).
The alloy elements in Q355NH (e.g., Cr, Ni) even slightly promote the formation of a dense zinc-iron alloy transition layer between the steel substrate and the zinc coating, improving the coating's adhesion.
2. Core advantage: Synergistic improvement of corrosion resistance
Zinc coating protection: The outer pure zinc layer (thickness usually 60-100μm) acts as a "physical barrier" to isolate water, oxygen, and corrosive ions (e.g., Cl⁻ in seawater) from the steel substrate.
Sacrificial anode protection: Even if the zinc coating is slightly damaged (e.g., scratches), zinc will corrode preferentially (since zinc has a lower electrode potential than steel), preventing the Q355NH substrate from rusting.
Compared with ungalvanized Q355NH, the galvanized version can extend its service life by 2-3 times in coastal areas (from 15-20 years to 30-50 years).
3. Key notes for hot-dip galvanizing of Q355NH
Pre-treatment of the substrate: Before galvanizing, thoroughly remove the oxide scale, rust, and oil on the surface of Q355NH (e.g., via pickling or sandblasting). Residual oxide scale will cause "bare spots" on the zinc coating, reducing corrosion resistance.
Control of galvanizing parameters: The immersion time in molten zinc should be controlled at 3-5 minutes (too long may lead to excessive thickening of the zinc-iron alloy layer, reducing toughness); after galvanizing, cool the steel slowly (avoid rapid water cooling) to prevent internal stress in Q355NH.
