The corrosion resistance principle of COR-TEN steel lies in its special alloying elements, which form a dense oxide layer upon exposure to the atmosphere. This robust layer acts as a barrier, isolating COR-TEN from the environment and protecting it from climatic corrosion. When COR-TEN or ordinary steel is exposed to the atmosphere, contacting moisture and oxygen, the following chemical reactions occur:

Chemical Reactions

Initial Oxidation:\(\text{Fe} + 0.5\text{O}_2 + \text{H}_2\text{O} \rightarrow \text{Fe(OH)}_2\)

Further Oxidation:\(2\text{Fe(OH)}_2 + 0.5\text{O}_2 + \text{H}_2\text{O} \rightarrow 2\text{Fe(OH)}_3\)

 

The key difference is that COR-TEN steel forms a protective oxide layer that prevents further reactions, while ordinary steel continues to corrode. Although the exact mechanism of how trace elements like copper, chromium, and phosphorus facilitate this protective layer remains unclear, the following observations provide insights:

Mechanistic Insights

Microstructural Difference:

COR-TEN's base layer hardly forms iron oxyhydroxide (FeOOH, an empirical formula for Fe₂O₃·H₂O). Instead, a dense oxide layer forms between FeOOH and the base metal, blocking corrosion.

Ordinary steel allows FeOOH to form directly on the base layer, causing cracks and spalling.

Amorphous Oxide Layer:

The oxide layer between FeOOH and the base in COR-TEN is radiographically amorphous, enhancing its protective properties.

Elemental Distribution:

Copper, chromium, and phosphorus concentrate in the amorphous layer, while sulfate ions (from sulfur) are uniformly distributed within the oxide layer. In ordinary steel, sulfur accumulates in the outer oxide layer, accelerating corrosion.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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