1. Normal evolution process (no spalling once stable)
●Early stage (0–6 months)
Surface forms loose, orange-red rust that is powdery and easy to rub off.Light flaking or rust runoff may occur when wet.
●Transition stage (6–24 months)
Rust layer gradually darkens, densifies, and adheres more tightly.Loose rust decreases; almost no more flaking.
●Stable patina stage (2–3 years)
Forms a dense, hard, tightly bonded protective layer (dark brown / gray-brown).
●No peeling, no flaking, no powdering under normal dry‑wet cycles.
This layer is stable for decades.
2. When Corten B rust layer does fall off
These conditions destroy the compact patina:
●Long-term immersion in water or prolonged humidity
Continuous moisture prevents stable oxide formation; rust stays porous and falls off easily.
●Coastal salt spray or heavy industrial pollution (high Cl⁻ / SO₂)
Corrosive ions penetrate and break the protective layer, causing accelerated, blistering, flaking rust.
●Mechanical abrasion, vibration or frequent impact
Physical force can chip or peel the surface layer.
●Poor drainage, crevices, or trapped moisture
Creates an aggressive local environment that leads to voluminous, unstable rust.
●Improper surface condition
Oil, paint residue, scale, or rust-preventive oil left on the surface hinders normal patina formation.
3. Comparison with ordinary carbon steel
●Ordinary steel: rust is always loose and keeps falling off, corroding deeper year by year.
●Corten B: only temporary early flaking; stable layer stops spalling and protects the base metal.
