Galvanizing / Zinc Coating

Galvanizing is the process of coating steel with a layer of zinc to protect against corrosion. Hot-dip galvanizing — the most common method in India — submerges fabricated steel parts in molten zinc at 450°C, producing a metallurgically bonded zinc-iron alloy layer typically 80-200 micrometres thick. The Indian standard IS 4759:1996 governs hot-dip galvanizing for structural steel, with zinc coating mass typically specified as 350-610 g/m² depending on application (350 for indoor, 460 for general external, 610 for marine/industrial environments).

Galvanizing's protection mechanism is dual: barrier protection (the zinc layer physically prevents oxygen and moisture reaching steel) and sacrificial protection (zinc corrodes preferentially to steel via electrochemical action — even when the coating is scratched, the surrounding zinc corrodes first). Service life of galvanized steel: 50-70 years in rural environments with minimal pollution; 30-50 years in industrial / urban environments; 20-40 years in coastal salt-air environments. The corrosion rate of zinc is roughly 1-3 micrometres per year in mild rural environments and 5-10 micrometres per year in industrial / marine environments.

Indian applications: (1) Transmission-line towers and steel poles — virtually 100% galvanized per IS 4759, life 30-40 years; (2) Industrial structures in marine / chemical environments — galvanized followed by paint for additional protection; (3) Fasteners — IS 1367 mandates galvanized fasteners for outdoor / corrosive applications; (4) Steel beams and columns in roof of factories and warehouses; (5) Cable-tray and conduit systems. Galvanizing should be specified for any structural steel exposed to weather — paint alone provides 5-10 year service life; galvanizing+paint provides 20+ years cumulative. Critical site execution issue: bolts, nuts, and washers must be galvanized to the same coating mass as the structure; unmatched corrosion rates at fasteners undermine the system's protection.