The natural patina forms in four clear stages over 1–5 years, with predictable color, morphology, and property changes.
1. Initial Oxidation Stage (0–3 months)
Surface reacts with oxygen, moisture, and atmospheric pollutants.
Color: light yellow → orange → bright reddish-orange
Rust is loose, powdery, and easy to rub off.
Rust runoff and staining are obvious.
No protective effect yet.
2. Transition Stage (3–12 months)
Alloy elements (Cu, Cr, Ni, P) begin to enrich the surface layer.
Outer loose rust gradually converts into a more stable oxide.
Color: reddish-orange → reddish-brown → dark brown
Surface becomes less powdery, adhesion improves.
Rust runoff gradually decreases.
3. Maturation Stage (1–3 years)
A dense, tightly adherent mixed oxide layer forms.
Main components: goethite (α-FeOOH), magnetite, and alloy-enriched phases.
The layer becomes compact, impermeable to water and oxygen.
Color: dark brown → chocolate brown / purplish-brown
No more powdery rust; surface is stable and self-protective.
4. Stable Stage (3–5 years and beyond)
Patina is fully mature and self-limiting.
Corrosion rate drops to a very low level.
Color remains deep, uniform dark brown with only slight darkening over time.
The layer is protective, durable, and does not peel or flake.
