The Patina Timeline: The Weathering Process of Corten Steel

Weathering Steel Patina

The Patina Timeline: The Weathering Process of Corten Steel

While many select corten, or weathering steel, because of its practical benefits, others select the steel for the appearance of its rusty, orange-brown patina. However, weathering steel is silver in its original state – just like standard steel. The patina develops over time when exposed to weather. 

The Weathering Process

The most practical benefit of weathering steel is its ability to resist corrosion. When exposed to moisture, air, and other elements, most low alloy steels have the tendency to rust. And overtime, the rust layer becomes porous and detaches from the metal surface. 

With weathering steel, the rusting process happens in the same way, but the steel produces a stable rust layer, called a “patina,” which serves as a protective, corrosion-resistant barrier. This naturally-developed protective patina regenerates continuously when exposed to weather and stops further access of oxygen, moisture and pollutants. 

The Patina Timeline

The rate at which the patina forms depends on the environment the steel is in as well as various factors, such as humidity and proximity to bodies of water. 

In our experience, the patina forms quicker when the steel is exposed to more environmental cycles. Likewise, cold-rolled materials (18 GA sheets or thinner) form the patina quicker than hot-rolled materials (16 GA sheets or thicker). 

We documented two samples of corten-equivalent weathering steel over the course of 6 months (December 2019-May 2020) to illustrate how the patina forms and changes color over time. The smaller square is 16 GA hot-rolled sheet and the larger square is 18 GA cold-rolled sheet. 

Weathering steel in its original state

December 20, 2019

Patina development

January 3, 2020

Weathering Steel

January 20, 2020

Corten steel square

March 20, 2020

Weathering Steel sample

April 20, 2020

Corten steel sample

May 20, 2020


These photos were taken at our office in Pelham, Alabama, which is built out of cold-rolled weathering steel. Click here to see how the patina developed during the construction of our office building. 

As mentioned above, the rate at which the patina forms depends on its exposure to outdoor elements. Contact us to learn more about how the patina would develop in specific areas!