Steel Sheet Phosphatability via Oxide Layer Control

The patent applies parameter changes by precisely controlling the oxide layer thickness (5-20 nm) and compositional parameters (Fe/P ratio of 3-10, Mn content 0.05-0.5 wt%, Si content 0.01-0.1 wt%) to achieve optimal balance between corrosion resistance and acid reactivity. This allows the steel sheet to form a protective oxide layer that is thick enough for corrosion protection but thin and compositionally optimized to maintain high acid reactivity for smooth phosphate treatment.

The patent applies preliminary action by pre-forming an optimized oxide layer on the steel sheet surface before phosphate treatment. This oxide layer, with controlled thickness and composition (Fe/P ratio 3-10, Mn 0.05-0.5 wt%, Si 0.01-0.1 wt%), is prepared in advance to enhance acid reactivity, ensuring that even when phosphoric acid concentration is reduced for environmental compliance, the phosphate treatment still proceeds smoothly with adequate crystal formation and coverage.

The patent applies self-service by designing a steel sheet surface that self-prepares an optimized oxide layer through controlled oxidation, creating a surface inherently predisposed to good phosphate treatment. The controlled oxide layer (5-20 nm with specific composition) automatically forms to provide high acid reactivity, enabling the material to self-facilitate dense phosphate crystal formation even under varying solution concentrations, thus achieving both corrosion resistance and process robustness.