What is a protective coating for weathering steel?

What is weathering steel?
Definition of Weathering Steel: Steel that has been chemically formulated to form a protective patina layer (similar in appearance to rust) without the need for paint. The dense oxidized copper-green layer of weathering steel isolates it from the atmosphere. If weathering steel is scratched, its patina repairs itself. Not only does weathering steel not prevent further corrosion: it also has a unique aesthetic effect - a warm, dark brown textured appearance that requires little or no maintenance.

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Formation of patina on weathering steels

Once exposed to alternating wet and dry conditions, weathering steel begins to rust just like non-weathering steel. However, over time, the rust layer transforms into a patina layer that changes color from a fresh, newly formed orange-brown to a light or dark brown.

The rate of change and final color of weathering steel depends on the atmospheric conditions and contamination levels at the site. Sulfur-intensive atmospheres accelerate the oxidation process and lead to a deeper patina. Frequent wet and dry cycles also accelerate oxidation. The entire oxidation process of this corrosion-resistant steel usually takes 2-6 years.

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