Initial loose rust stage (1–3 months)
A thin, uneven reddish-brown rust layer forms on the planter's surface after outdoor exposure. This rust is loose, easy to flake off when touched or washed by rain, and has almost no corrosion resistance-only a preliminary rust color appearance.
Patina densification stage (6–12 months)
The initial loose rust reacts with oxygen, moisture in the air and the Cu/Cr/Ni alloy elements in SPA-H steel. The rust layer gradually compacts and adheres more tightly to the steel surface, the color becomes more uniform (less patchy), and basic corrosion resistance starts to form, effectively slowing down further rusting of the planter substrate.
Mature protective patina stage (12–24 months)
A fully dense, firm reddish-brown oxide film forms on the planter's surface, tightly bonding with the steel base. This mature patina acts as a natural protective barrier, isolating the inner steel from air, moisture and mild corrosive media, and the corrosion rate of the planter drops to ≤0.03mm/year-achieving the long-term anti-corrosion effect unique to weathering steel.
Key factors affecting the patina formation speed of SPA-H planters
●Faster formation (12 months approx.): Temperate zones with moderate humidity, regular rainfall and mild temperatures (e.g., urban parks, residential gardens), the moist environment accelerates the alloy reaction and patina densification.
●Prolonged formation (24+ months): Dry, cold or arid regions with low air moisture (e.g., northern inland areas), the lack of water slows down the oxidation reaction, leading to a much longer cycle for mature patina formation.
●Unstable formation: High-salt coastal areas (within 1km of the sea) or heavy industrial zones with acid mist/dust, the harsh corrosive environment makes the natural patina hard to compact uniformly, and may even cause pitting corrosion on the planter's surface during formation (auxiliary protection like transparent sealer is recommended here).
●Planter structure impact: Planters with closed bottoms or poor drainage may have prolonged patina formation on damp inner surfaces; well-ventilated, open-structured planters form patina more evenly and quickly overall.
