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Weathering steel. Image Credit: Shutterstock.com/Paul W Thompson

Weathering Steel: Definition, Composition, Properties, and Uses

Xomety X
By Team Xometry
September 9, 2023
 14 min read
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Weathering steel, also known as corten steel, is a unique type of steel alloy. It has gained popularity due to its distinctive rust-like appearance and exceptional corrosion resistance.

This material contains alloying elements such as copper, chromium, and nickel, which create a protective oxide layer on its surface when exposed to the elements. This layer not only gives weathering steel its characteristic rusted appearance but also acts as a barrier, slowing down further corrosion. Weathering steel offers several advantages, including: longevity, low maintenance requirements, and a range of ASTM specifications tailored to different applications. 

In this article, we'll discuss the characteristics, benefits, drawbacks, applications of weathering steel, and more.

What Is Meant by Weathering Steel?

Weathering steel is a type of high-strength, low-alloy steel that is chemically formulated to develop a protective rust-like oxide layer on its surface. This surface layer, called a patina, provides the metal with a high degree of protection. The patina seals off the metal’s surface from the atmosphere, without the need for additional coating. 

The oxide layer forms due to the addition of alloying elements like: chromium, nickel, copper, and phosphorus, which are mixed with iron and carbon. Copper plays a particularly crucial role in fostering the adhesion of the protective oxide layer to the metal and retarding the process of corrosion. The alloying element adds properties like strength and better corrosion resistance (4–8 times) than plain, low-carbon steels to the metal mixture. Unlike traditional steel, weathering steel's patina acts as a protective barrier. It helps to slow down the progression of corrosion. It extends the life span of the material and reduces the need for frequent maintenance and painting. For this reason, weathering steel finds frequent use in outdoor settings or situations in which conventional steel would be prone to rusting.

What Is the Other Term for Weathering Steel?

Weathering steel is also known by its trademarked name COR-TEN® steel. It is occasionally also written as "corten steel" without a hyphen, as a genericized trademark. It is also sometimes referred to as high-strength, low-alloy (HSLA) steel. 

What Is the Origin of Weathering Steel?

The origins of weathering steels trace back to the 1930s when they were pioneered by the US Steel Corporation. That company’s quest for a steel alloy to fortify ore-carrying hopper carts for the transfer of iron ore and coal to the furnace led to the emergence of weathering steel. This new alloy had to embody heightened strength, durability, and hardness without the need for paint. Interestingly, the alloy demonstrated unexpected corrosion-resistant properties that later found application across diverse industries. US Steel coined the name COR-TEN® to trademark this corrosion-resistant steel alloy. "COR" signifies corrosion resistance, while "TEN" represents tensile strength. Today, while the terms corten and COR-TEN® are used almost interchangeably, however, COR-TEN® remains a trademark of US Steel.

Although initially not intended for architectural purposes, weathering steel found its architectural debut in the 1950s. In 1964, the John Deere Headquarters in Moline, Illinois, stood as a pioneering architectural masterpiece clad in weathering steel.

Saarinen's selection of weathering steel, specifically corten, aimed to imbue the structures with an aged and grounded ambiance. This choice was driven by the anticipation that the metal would gracefully age, acquiring a deep, natural hue as the protective oxide layer matured. This layer presents a rich, earthy brownish-red tone that melds durability with stability.

How Does Weathering Steel Work?

Weathering steel works due to the alloying elements prompting the initial rust layer to adhere more effectively to the steel's surface. This prevents the rust from infiltrating deeper and compromising the steel's strength. While weathering steel does undergo rusting, this occurs solely on its external surface. The rust doesn't permeate the inner layers of the steel once the initial outer rust layer has formed. This outer rust layer acts as a protective shield, safeguarding the steel from further corrosion.

How Is Weathering Steel Made?

Weathering steel is produced through a controlled process involving exposure to natural elements. The manufacturing process involves exposing the steel to natural weathering conditions. This process allows it to develop a protective layer of rust or patina on its surface over time. This patina acts as a barrier against further corrosion. Initially, the steel surface is roughened through sanding or blasting. Over time, repeated wetting and drying cycles encourage the development of a stable oxide layer. Additionally, some accelerated methods involving chemical treatments or industrial-grade hydrogen peroxide are performed. The resultant steel gains corrosion resistance. It creates a distinct surface finish that ranges from orange-red to deep purple-brown. 

How Is Weathering Steel Different From Regular Steel?

The difference between weathering steel and regular steel lies in their composition, treatment, properties, and applications. The unique properties of corten steel, such as its enhanced corrosion resistance and distinctive appearance, are primarily attributed to its composition and exposure to environmental conditions. On the other hand, regular steel is a carbon-containing alloy that lacks the properties of corten steel. It is susceptible to rust and corrosion upon exposure to moisture and oxygen over time. The differences between the two steel types come down to the following factors: properties, combination of elements, corrosion resistance, and applications.

Corten steel undergoes immersion in a solution containing chromium, nickel, manganese, and silicon to prevent rusting and enhance durability. In contrast, regular steel, while robust, lacks this corrosion-resistant treatment. Corten steel plates also boast greater malleability and resistance to impact damage. Regular steel, on the other hand, primarily consists of iron and carbon. This makes it durable but susceptible to corrosion if not treated properly. 

Corten steel is deliberately rusted to achieve an aged appearance. Ordinary steel corrodes due to its chemical composition, which leads to rapid oxidation in humid conditions. Finally, regular steel finds applications in various sectors such as: architecture, light-duty instruments and equipment, machinery parts, automotive, military, and aerospace. Corten steel, particularly Corten A steel plate, is weathering steel designed for outdoor use due to its corrosion-resistant coating—suitable for applications like architectural structural supports.

Is Weathering Steel Better Than D2 Tool Steel?

It depends. Weathering steel and D2 tool steel are distinct materials with different strengths and applications, making a direct comparison of "better" or "worse" challenging. The choice between the two depends on specific application requirements. For example, weathering steel excels in corrosion resistance and aesthetics, while D2 tool steel excels in hardness and wear resistance for cutting and machining tasks. 

Is Weathering Steel a High-Grade Alloy Steel?

No. Weathering steel is not typically classified as high-grade alloy steel in the same sense as some other specialized alloy steels like stainless steel or tool steel. It is a specific type of steel that falls under the category of low-alloy steel.

Weathering steel is composed primarily of iron, but it also contains small amounts of various alloying elements such as: copper, chromium, and nickel. While weathering steel is valued for its corrosion resistance and unique aesthetic qualities, it is not primarily chosen for its mechanical properties like: hardness, toughness, or strength.

To learn more, see our guide on Alloy Steel Properties.

What Are the Different ASTM Specifications for Weathering Steel?

Weathering steel is covered by the following several ASTM specifications, each designed to provide specific corrosion-resistant properties:

1. A847

ASTM A847 pertains to low-alloy, high-strength steel used for cold-formed welded rectangular and square pipe and tubing. It offers superior resistance to atmospheric corrosion compared to carbon steel. A847 is particularly suitable for applications requiring enhanced corrosion resistance and high strength, such as bridges and structures. It features a minimum yield and tensile strength of 50 ksi and 70 ksi, respectively.

2. A709-50W

ASTM A709-50W is a specification standard for structural steel primarily which encompasses high-strength and low-alloy steel structural bars, plates, and shapes. In the majority of environments, the resistance to atmospheric corrosion of this steel significantly surpasses that of carbon steel, regardless of the presence of copper. It mandates an atmospheric corrosion index of 6.00. When appropriately designed and facing atmospheric exposure, A709-50W proves suitable for various applications without paint, including: pedestrian bridges, highway bridge girders, railroad bridges, and overpasses. This specification boasts a minimum yield and tensile strength of 50 ksi and 70 ksi, respectively.

3. A871-65

ASTM A871-65 specifies low-alloy, high-strength structural steel plate with an improved resistance to atmospheric corrosion. It offers minimum yield and tensile strength of 65 ksi and 80 ksi, respectively. A871-65 excels in corrosion resistance compared to carbon steels. It finds applications in areas like power transmission and lighting poles.

4. A242

ASTM A242 is a high-strength, low-alloy structural steel specification with enhanced atmospheric corrosion resistance. Applicable to steel plates up to half an inch in thickness, A242 provides superior atmospheric corrosion resistance compared to carbon steels. A242 boasts a minimum yield and tensile strength of 50 ksi and 70 ksi, respectively.

5. A606-4

ASTM A606-4 applies to both hot- and cold-rolled steel sheets, strips, and coils. This high-strength, low-alloy specification is known for its improved atmospheric corrosion properties. A606-4 can be used for structural and miscellaneous purposes, and can be used unpainted in many applications if properly designed. It features a minimum yield and tensile strength of 50 ksi and 70 ksi, respectively.

6. A588

ASTM A588 is a specification for high-strength, low-alloy structural steel, featuring improved atmospheric corrosion resistance. It encompasses various structural shapes like: angles, channels, beams, plates, and bars. This specification is particularly suitable for welded bridges and structures in which weight reduction, durability, and corrosion resistance are important. With a minimum yield strength and tensile strength of 50 ksi and 70 ksi, respectively, A588's corrosion resistance surpasses that of carbon steels.

What Are the Chemical Compositions of Weathering Steel?

The chemical composition of both Corten A and B weathering steels are given in Tables 1 and 2, respectively: 

Table 1: Corten A Chemical Composition
FeCSiMnPSAlCuCrNi
Fe
Bal.
C
0.15
Si
0.25–0.75
Mn
0.20–0.50
P
0.07–0.15
S
0.03
Al
0.015–0.06
Cu
0.25–0.55
Cr
0.50–1.25
Ni
0.65

Table Credit: https://masteel.co.uk/products/weathering-steel/

Table 2: Corten B Chemical Composition
FeCSiMnPSAlVCuCrNi
Fe
Bal.
C
0.19
Si
0.30–0.65
Mn
0.80–1.25
P
0.035
S
0.03
Al
0.02–0.06
V
0.02–0.10
Cu
0.25–0.40
Cr
0.40–0.65
Ni
0.4

Table Credit: https://masteel.co.uk/products/weathering-steel/

What Is the Carbon Content of Weathering Steel?

Weathering steels have a carbon content below 0.2 wt%. They are enriched with alloying elements such as Cu, Ni, Cr, Si, P, and Mn, which collectively contribute to a total content ranging from 1.00 wt% to 5.00 wt%.

What Are the Properties of Weathering Steel?

The properties of weathering steel are listed in Table 3:

Table 3: Properties of Corten Steel
Corten ACorten B
Thickness strip products (mm)
Corten A
2–12
Corten B
2–13
Thickness plate products (mm)
Corten A
6–12
Corten B
6–40
Yield Strength RelN/mm² (Minimum)
Corten A
345
Corten B
345
Tensile Strength R(m)N/mm² (Minimum)
Corten A
485
Corten B
485
Elongation A(so)% (Minimum)
Corten A
20
Corten B
19
Formability
Corten A
Good
Corten B
Good
Weldability
Corten A
Good
Corten B
Good

Table Credit: https://www.azom.com/article.aspx?ArticleID=12974

What Are the Characteristics of Weathering Steel?

Weathering steel possesses several distinctive characteristics that make it unique and suitable for various applications. These are:

  1. Exceptional resistance to atmospheric corrosion.
  2. Has a distinctive appearance, ranging from shades of orange-brown to deep reddish-brown.
  3. Offers excellent tensile strength and durability.
  4. Has a longer life expectancy than bare, cold-rolled steel.
  5. Can be welded using appropriate methods that allow the rusting process to occur in a manner consistent with the rest of the structure.

Does Weathering Steel Rust?

Yes, weathering steel does rust. However, the rusting process is controlled and serves a protective purpose. Weathering steel forms a layer of rust, known as a patina, on its surface when exposed to the atmosphere. This patina acts as a barrier against further corrosion by protecting the underlying steel from direct contact with moisture and oxygen. The rusting process of weathering steel is different from the rapid and detrimental rusting that occurs in conventional steel. The controlled rusting of weathering steel contributes to its enhanced corrosion resistance and durability in various atmospheric conditions.

Can Weathering Steel Be Welded?

Yes. Weathering steel can be welded using gas shield, spot, or submerged arc welding techniques, which depend on the thickness of the steel. It's important to choose a welding method that enables rusting to take place in a manner consistent with the rest of the structure.

Can Weathering Steel Be Tempered?

No, weathering steel cannot be tempered in the same way as certain other types of steel, like carbon or alloy steel. Tempering is a heat-treatment process that involves reheating steel to a specific temperature and then cooling it down to achieve desired mechanical properties, such as hardness and toughness. Weathering steel develops its unique properties and protective patina through exposure to the atmosphere over time. This process is not the result of heat treatment like tempering; instead, it is a gradual chemical reaction with the environment.

What Are the Uses of Weathering Steel?

Weathering steel finds diverse applications due to its unique properties. It is commonly used in:

  1. Architectural structures like bridges and building facades.
  2. Outdoor sculptures and artworks due to its distinctive rust-colored appearance and corrosion resistance.
  3. Shipping containers.
  4. Railway wagons.
  5. Transmission Poles.

How Long Will Weathering Steel Last?

Corten steel has the potential to last for several decades to over a century. This is thanks to the protective rust layer that imparts corrosion resistance.

How Much Does a Weathering Steel Cost?

The cost of weathering steel can vary depending on several factors including: the specific type of weathering steel, the thickness and dimensions of the material, the supplier or manufacturer, the location where it's purchased, and any additional features or treatments applied to the steel. Generally, weathering steel may be slightly more expensive than traditional carbon steel due to its enhanced corrosion-resistant properties. However, the long-term savings in maintenance and replacement costs can make weathering steel a cost-effective choice for certain applications over its life span. It's advisable to obtain price quotes from suppliers or manufacturers based on your specific project requirements for a more accurate cost estimate.

What Is the Common Form of Weathering Steel?

The most common form of weathering steel is sheet or plate steel, often used in architectural and structural applications. This form allows for the creation of various structures, facades, sculptures, and other designs that benefit from both the unique aesthetic of the rusted surface and the material's enhanced corrosion resistance. 

What Are the Advantages of Weathering Steel?

Weathering steel offers several distinct advantages that make it a valuable material for a wide range of applications such as:

  1. Forms a protective patina on its surface which acts as a natural barrier against further corrosion. 
  2. The self-protecting nature of the rust-like patina significantly reduces the need for ongoing maintenance, such as painting and coating. This leads to cost savings and lower environmental impact.
  3. Weathering steel's resistance to corrosion and its durable nature contribute to a longer life span.
  4. The unique appearance of weathering steel adds an artistic and architectural appeal to structures.
  5. Maintains its strength and structural integrity even in challenging environments.

What Are the Disadvantages of Weathering Steel?

Weathering steel does have some drawbacks to consider such as:

  1. Has a higher initial cost than that of traditional carbon steel. 
  2. The initial rust runoff can stain surrounding areas. The early stages of rusting can also result in a less aesthetically pleasing appearance. 
  3. Requires careful consideration during the design phase to prevent water accumulation in crevices. 
  4. Certain climate conditions, such as high-chlorine environments, are not compatible with weathering steel and could lead to issues with corrosive resistance and durability. 

Is Weathering Steel Better Than Electrical Steel?

It depends. Weathering steel and electrical steel serve very different purposes and have distinct characteristics. They are designed for different applications and possess unique properties that are optimized for their respective uses. Weathering steel is engineered to develop a protective rust layer when exposed to the elements. This rust acts as a shield against further corrosion and gives the steel its characteristic appearance. Weathering steel finds its applications in outdoor structures, architectural projects, and bridges, providing durability and resilience in harsh weather conditions. On the other hand, electrical steel, also known as silicon steel, is tailored for the electrical industry. It is used to create the cores of transformers, electric motors, and electromagnetic devices. Its key attributes include low electrical resistance and high magnetic permeability, ensuring efficient transmission of magnetic fields and minimizing energy losses. 

Summary

This article presented copper, explained it, and discussed its various applications. To learn more about copper, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities, including casting and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

  1. COR-TEN® is a trademark of United States Steel Corporation (USS)

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Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.