SPA-H is a typical weathering steel that contains specific alloying elements such as copper (Cu), chromium (Cr), nickel (Ni), phosphorus (P). Under the long-term action of the atmosphere (oxygen, moisture, temperature changes, and small amounts of pollutants), it forms a dense, stable, self-protective oxide film on the surface - the natural patina. The whole process is gradual, self-improving, and self-limiting, and can be divided into the following detailed stages:
1. Initial surface oxidation (the early loose rust stage)
When the steel is first exposed to air, rain, and dew, the surface quickly reacts with oxygen and water vapor to form primary iron oxides and hydroxides, mainly lepidocrocite (γ‑FeOOH) and ferrihydrite.
Appearance: light yellow, orange, to bright reddish‑orange.
Structure: loose, porous, poorly adhesive, easy to fall off or be washed away by rain.
Feature: This layer cannot prevent corrosion; instead, it allows water and oxygen to continue penetrating inward, so the rusting process continues.
2. Migration and enrichment of alloy elements
Under the action of alternating wet‑dry cycles and electrochemical reactions, the alloy elements in SPA‑H (Cu, Cr, Ni, P) gradually dissolve in the surface electrolyte and migrate outward to the oxide layer, becoming highly concentrated in the outer rust layer.
These alloy elements change the composition and structure of the oxide:
Cu and P inhibit anodic dissolution and reduce corrosion rate.
Cr and Ni refine oxide grains and enhance stability.
The original loose, porous rust gradually transforms into a finer, more adhesive intermediate layer.
Appearance: color transitions from bright orange‑red to deep reddish‑brown.
3. Densification and stabilization of the patina layer
With continuous wet‑dry cycles, unstable oxides (such as γ‑FeOOH) gradually transform into stable, crystalline α‑FeOOH (goethite).Under the "guidance" of alloy elements, the oxide layer becomes:
Denser and lower in porosity
Strongly bonded to the substrate steel
Low permeability to water, oxygen, and corrosive ionsThis layer no longer peels or powders easily and begins to show real protective performance.
Appearance: uniform dark brown, chocolate brown.
4. Formation of mature self‑limiting patina
Once the dense protective layer is fully formed, it effectively blocks the further entry of water, oxygen, and corrosive media.The corrosion reaction is limited to a very slow rate and will not continue to develop into the interior of the steel. This is the "self‑limiting" characteristic of weathering steel.
The patina no longer changes significantly in structure and color.
Under normal atmospheric environment, this stable patina can remain intact for decades.
SPA‑H can be used bare outdoors for a long time without painting.
