Initial Oxidation (Natural Weathering Stage 1: 1–3 Months)
When S355J2W is exposed to the atmosphere, oxygen and moisture react with the steel surface to form a loose, porous layer of iron oxide (FeO(OH))-the "initial rust" that looks similar to the rust on ordinary carbon steel. But unlike carbon steel, S355J2W's alloy elements (especially Cu) begin migrating to the surface immediately: copper reacts with oxygen and moisture to form copper-rich iron oxides, which reduce the solubility of the iron oxide layer and start to compact it.
Patina Maturation (Natural Weathering Stage 2: 3–12 Months)
Over time, the alloy elements continue to interact with the oxide layer:
Chromium forms stable chromium-iron oxides that further densify the layer
Silicon refines the oxide crystals to create a uniform, tight structure
Nickel enhances the layer's resistance to moisture and chloride ions
This transforms the initial loose rust into a dense, adherent, reddish-brown patina (10–20 μm thick). This mature patina acts as a physical barrier, blocking most moisture and oxygen from reaching the underlying steel substrate, slowing the corrosion rate to 1/4–1/10 that of ordinary carbon steel.
Self-Healing Mechanism (Ongoing During Service)
When the patina is damaged (e.g., from scratches, minor impact, or localized corrosion), the self-healing process is triggered:
The fresh steel exposed by the damage immediately starts to oxidize, forming a new layer of initial iron oxide
The alloy elements (Cu, Cr, Si) in the underlying steel quickly migrate to the damaged area, driven by the concentration gradient between the exposed steel and the surrounding mature patina
These elements react with oxygen and moisture to form a new, compacted copper-chromium-rich oxide layer that merges with the existing patina, repairing the damage and re-establishing the protective barrier-usually within 1–4 weeks, depending on humidity and airflow conditions.
