Hot-dip galvanizing (HDG) is the most widely used corrosion protection method for structural steel. The process involves immersing prepared steel in molten zinc at approximately 450°C, forming a metallurgically bonded zinc-iron alloy coating.
The Galvanizing Process
The process consists of three critical stages: surface preparation, galvanizing, and inspection.
**Surface Preparation** is the most important factor affecting coating quality. It typically involves: degreasing in an alkaline solution to remove oils and greases, pickling in hydrochloric or sulfuric acid (typically 12–14% HCl at 25–30°C) to remove mill scale and rust, and fluxing in zinc ammonium chloride solution to promote zinc adhesion.
**Galvanizing** involves immersion in molten zinc at 445–465°C. Immersion time depends on material thickness and mass — typically 3–6 minutes for standard structural sections. The zinc reacts with the steel surface to form successive layers of zinc-iron intermetallic compounds (gamma, delta, zeta phases) topped by a layer of pure zinc (eta phase).
Coating Thickness Requirements
EN ISO 1461 specifies minimum coating thickness based on the thickness of the steel article:
- Steel ≥ 6 mm: minimum local thickness 70 µm, minimum mean thickness 85 µm\n- Steel ≥ 3 mm to < 6 mm: minimum local 55 µm, minimum mean 70 µm\n- Steel ≥ 1.5 mm to < 3 mm: minimum local 45 µm, minimum mean 55 µm\n- Steel < 1.5 mm: minimum local 35 µm, minimum mean 45 µm
In practice, HDG coatings typically exceed these minimums significantly. Reactive steels (with silicon content 0.04–0.14% or > 0.25%) may produce excessively thick, matte-grey coatings with reduced adhesion.
Design for Galvanizing
Proper design for galvanizing is essential to prevent distortion, ensure complete drainage, and avoid safety hazards. Key considerations include:
**Vent and drain holes**: All closed hollow sections must have vent holes (minimum 10 mm diameter per EN ISO 14713-2) to allow air and steam to escape and molten zinc to flow freely. Insufficient venting can cause pressure build-up and explosive ejection of zinc.
**Symmetry and balance**: Asymmetric fabrications should be designed with consideration for thermal distortion. Heavy and light sections welded together will heat and cool at different rates.
**Overlapping surfaces**: Avoid overlapping surfaces where possible, as capillary gaps can trap acids from pickling, leading to 'weeping' of corrosive flux residues after galvanizing.
Inspection and Testing
Coating thickness is measured using magnetic (EN ISO 2178) or electromagnetic (EN ISO 2360) gauges. Adhesion is assessed by EN ISO 1461 Annex A — the coating should not flake or peel when struck by a pointed hammer.
Visual appearance should be uniform and free from bare spots, lumps, and rough areas that could interfere with the intended use. Note that aesthetic appearance is not a criterion in EN ISO 1461 — surface roughness and zinc crystals ('spangle') are normal.
The expected service life of HDG in atmospheric exposure can be estimated using EN ISO 9224, with zinc loss rates ranging from 0.1–0.7 µm/year in C1 (very low) to 4.2–8.4 µm/year in C5 (very high) corrosivity categories.