Early stage (0.5–2 years): When first exposed to the atmosphere, the steel's surface iron oxidizes rapidly to form loose, porous rust (e.g., -FeOOH, Fe₃O₄). This loose rust is but acts as a "reaction medium": it traps Cu and Cr ions (released from the steel matrix) and provides a site for the transformation to dense -FeOOH.
Mature stage (2–5 years): The loose rust gradually compacts and stabilizes into the patina as Cu/Cr ions enrich and reorganize the rust structure. The patina then adheres tightly to the substrate, blocking water/oxygen and slowing corrosion to ~1/10 the rate of ordinary carbon steel.
2. Natural environmental factors "self-clean" excess loose rust
Rainwater washes away flaky, non-adherent loose rust, leaving behind the more stable, semi-compacted rust that contributes to patina growth.
Wind and environmental friction further refine the surface, accelerating the densification of the remaining rust layer.
3. Exceptions: When minimal intervention
: If the surface is covered by thick, sticky pollutants (e.g., industrial soot, oil stains) that seal the loose rust and block oxygen/water exchange, use low-pressure water (<=50 bar) to rinse off the pollutants -avoid scrubbing the rust itself.
: Welds may form thicker loose rust due to surface roughness. A soft-bristle brush (not steel wool) can gently remove flaky rust -this helps the weld integrate into the overall patina.
