G90 is a coating designation defined under the American standard ASTM A653 for continuously hot-dip galvanized steel sheet. The letter G indicates a pure zinc coating (galvanized), as distinct from A which denotes a zinc-iron alloy coating (galvannealed). The number 90 represents the coating weight: a G90 coating has a total double-sided zinc weight of 0.90 ounces per square foot (oz/ft²). This corresponds to approximately 0.45 oz/ft² on each side. In metric terms, G90 is equivalent to approximately 275 grams per square metre (g/m²) total, or about 20 micrometres (µm) of zinc per side.

The coating is applied by immersing cold-rolled steel sheet into a molten zinc bath at approximately 450°C (840°F), forming a metallurgically bonded zinc-iron alloy layer. This thick zinc coating provides both a physical barrier and sacrificial protection: even if the coating is scratched, the zinc corrodes first, protecting the underlying steel.

G90 is classified as a heavy-duty coating weight within the ASTM A653 G-series, which ranges from G30 (lightest) to G235 (heaviest). G90 is one of the most commonly specified heavy coatings for demanding outdoor applications. Compared to G40 (0.40 oz/ft², ~120 g/m²), G90 offers approximately three times the corrosion resistance and significantly longer service life. Compared to G60 (0.60 oz/ft²), G90 provides superior protection in coastal, industrial, or persistently wet environments.

G90 offers excellent corrosion resistance. Under standard salt spray testing, it typically exceeds 1,500 hours before red rust appears, making it suitable for applications requiring 20 to 30 years or more of service life with minimal maintenance.

Typical applications for G90 galvanised sheet include:

● Construction: roofing, wall cladding, gutters, downspouts, structural purlins for industrial and commercial buildings

● Infrastructure: highway guardrails, utility poles, transmission towers, road signs, bridge components

● Agriculture: grain silos, livestock equipment, ventilation systems, storage bins

● Automotive: underbody parts, chassis components, frame rails

● General industry: heavy-duty storage tanks, cable trays, electrical enclosures, industrial equipment

● Marine and coastal structures where salt spray exposure is significant