Thin patina layer (<5 μm, immature formation stage)
A patina thinner than 5 μm is in the early formation phase, with a loose, incomplete microstructure and poor adhesion to the steel substrate. It cannot form a continuous physical-chemical barrier, so moisture, oxygen, and mild corrosive media (e.g., dust, low-concentration pollutants) can easily penetrate the layer to reach the steel surface, leading to continuous electrochemical corrosion. At this stage, the steel still has a high corrosion rate, and the patina needs time to grow and densify to exert protective effects.
Optimal thickness range (5–15 μm, fully stabilized stage)
This is the mature, stable state of SPA-H's natural patina. The layer is dense, fine-grained, and tightly bonded to the substrate, with a uniform structure enriched in Cu, Cr, Ni alloy oxides. It can effectively block the infiltration of corrosive media, reduce the steel surface's electrochemical activity, and even exhibit slight self-healing ability for minor damage. In this thickness range, the patina achieves the best balance of density and barrier performance, minimizing the corrosion rate of SPA-H to an extremely low level (far lower than ordinary carbon steel) and providing long-term reliable protection for the steel substrate.
Excessively thick patina layer (>15 μm, overgrown/aged stage)
A patina thicker than 15 μm is usually caused by long-term exposure to harsh environments (e.g., high humidity, mild salt spray) or uneven formation. The overgrown layer will lose its dense microstructure, becoming loose, porous, and even prone to cracking or peeling from the substrate. These defects create channels for corrosive media to penetrate the inner layer and reach the steel surface, and the loose patina may retain moisture, accelerating localized corrosion (e.g., crevice corrosion) under the layer. Additionally, the overly thick layer has poor adhesion, and partial flaking will expose the steel substrate, leading to new rust formation and reduced overall corrosion resistance.
