Long‑Term High‑Temperature Behavior of Al‑Si Coatings
The Al‑Si coating on aluminized steel is not static at high temperatures. It undergoes interdiffusion with the steel substrate, forming an intermetallic layer that determines the product’s service life.

Diffusion & Protective Layer Evolution

• As‑Coated Structure: The coating consists of a thin Fe₂Al₅ intermetallic layer at the interface, an Al‑Si alloy overlay (90% Al, 10% Si). This overlay provides initial corrosion and oxidation protection.

• High‑Temperature Exposure (300‑550°C): Aluminum diffuses further into the steel, growing the intermetallic layer (FeAl, FeAl₂, Fe₂Al₅) and depleting the pure aluminum overlay. Silicon accumulates at the interface, slowing down the diffusion rate by forming a barrier against Fe‑Al interdiffusion.

• Final Stable Phase: After long exposure, the entire coating becomes a dense, oxidation‑resistant Fe‑Al intermetallic layer. This layer is why aluminized steel can protect for thousands of hours without spalling, as it is metallurgically bonded.

• Failure Mode: Eventually, the aluminum reservoir is consumed, and iron oxides (scale) begin to form, indicating end‑of‑life. Higher coating weights (AS120 vs AS80) extend this life proportionally.