What is Corten Steel?
What Is the Difference Between Mild Steel and Corten Steel?
This guide explains the key differences between Mild Steel and Corten Steel, helping you choose the right material for your project.
At Edensteel, we only work with high-quality, prime-grade materials. With over 25 years of combined experience in the steel industry, we believe that understanding your material is just as important as the finished product itself. Below, we break down the differences between the two carbon steels most commonly used in our fabrication work: Mild Steel and Corten Steel.
What Is Steel and How Is It Made?
Let’s start with the basics.
Metals fall into two main categories: Pure Metals and Alloys.
A pure metal contains at least 99% of a single element in its natural form. Examples include iron, aluminium, copper, gold, and lead. When two or more elements are combined, they form an alloy—and steel is one such alloy.
Steel is made by combining iron with carbon and other elements. Broadly speaking, most steel falls into three categories:
1. Carbon Steel
2. Tool Steel
3. Stainless Steel
For the purposes of this guide, we are focusing on Carbon Steel, as both Mild Steel and Corten Steel fall into this category.
What Is Corten Steel?
Corten Steel, also known as Weathering Steel, is a high-strength, low-alloy carbon steel that was originally patented in 1933 by the United States Steel Corporation.
Its typical chemical composition includes:
- 0.2–0.5% Copper
- 0.5–1.5% Chromium
- 0.1–0.2% Phosphorus
The name COR-TEN comes from:
- COR – Corrosion resistance
- TEN – Tensile strength
This combination gives Corten its exceptional durability and weather resistance.
How Does Corten Steel Resist Corrosion?
Corten’s unique chemical makeup allows it to form a protective rust-like patina on its surface. This patina acts as a barrier, significantly slowing further corrosion.
An important benefit is that if the surface is scratched, the exposed steel will naturally form a new protective layer—effectively making it self-healing.
How Long Does Corten Steel Take to Weather?
Corten Steel develops its patina through repeated cycles of wet and dry conditions. Typical British weather is ideal for this process.
All Edensteel Corten products are supplied in their raw steel state, meaning:
If installed outdoors in the open, patination begins within days, often after the first rainfall.
A full patina develops over approximately 6–9 months, depending on location.
The colour and finish continue to mature over up to 36 months.
Factors that affect weathering include:
- Heat
- Humidity
- Rainfall
- Proximity to open water
- Coastal exposure and salt air
If your product is installed indoors or undercover, the patina may not develop naturally. In these cases, Edensteel can pre-treat the steel with a patination solution before delivery.
Why Does Some Corten Steel Develop a Predominantly Black Patina?
While Corten Steel is best known for its rich orange and brown tones, some installations develop a much darker, almost black patina over time. This is completely natural and can occur for several reasons:
1. Natural Aging
Over many years, the patina can continue to darken, often moving from orange and brown tones towards deep brown and black shades.
2. Environmental Conditions
Lower temperatures, high humidity, and airborne pollutants (such as in urban or industrial environments) can all contribute to a darker overall finish.
3. Lack of Proper Wet/Dry Cycles
Corten relies on repeated wetting and drying to form its stable protective layer. If the steel remains constantly wet or damp—for example:
- Buried in saturated soil
- Subject to pooling water
- Installed in permanently shaded, poorly ventilated areas
The patina may not stabilise correctly, leading to a darker, less stable rust finish.
4. Mill Scale on New Steel
New hot-rolled Corten Steel is supplied with a dark blue-black surface layer called mill scale, created during manufacturing.
If this mill scale is not naturally broken down or removed before weathering:
- It can flake off unevenly
- It may interfere with patina development
- It can result in patchy black areas on the surface
5. Sealing at a Specific Stage
If a sealant or varnish is applied to the steel at a particular stage of weathering, this will lock in the existing colour—often deepening it into a darker or near-black tone.
At Edensteel, we are always happy to advise on expected colour outcomes based on installation environment and finishing options.
Coastal and High-Moisture Environments
We generally do not recommend Corten Steel within 2km of the coastline or in permanently wet environments. Salt in coastal air can prevent the steel from fully drying, which is essential for correct patina formation.
That said, Corten is widely used in:
- Shipping containers
- Coastal bridges
- Marine infrastructure
It will still perform well, but its protective qualities may develop differently.
Why Is Corten Steel So Popular in Landscaping and Design?
Quite simply—it looks exceptional.
The rich orange, red, brown, and darker evolving tones of the patina complement:
- Greenery
- Natural stone
- Timber
- Contemporary garden designs
Corten provides an all-year-round visual feature, ageing beautifully with its surroundings.
Will Corten Steel Stain Surrounding Surfaces?
Both Corten Steel and Mild Steel produce “run-off” during the early stages of weathering. This is rainwater that carries fine rust particles and can stain paving, concrete, and light stone surfaces.
To manage this:
- Install products on gravel or chippings that can be turned or replaced
- Use a sacrificial slab beneath the product
- Allow runoff to settle before permanent surface installation
What Is Mild Steel?
Mild Steel is the most commonly used steel across industries including:
- Construction
- Manufacturing
- Automotive
- It contains approximately 0.05–0.25% carbon, making it:
- Highly workable
- Readily available
- More cost-effective than Corten Steel
At Edensteel, we use structural-grade Mild Steel, giving our products comparable strength and performance to Corten equivalents at a lower cost.
How Long Does Mild Steel Take to Weather?
Mild Steel will naturally begin to oxidise when exposed to the elements:
Initial weathering typically begins within weeks
A full patina generally develops over 12–18 months
Unlike Corten, Mild Steel continues to corrode over time unless sealed or protected
Is There a Difference in Appearance Between Mild Steel and Corten Steel?
In the early stages, yes—but long-term, the finishes are very similar.
Corten Steel often starts with lighter yellow-orange tones
Mild Steel develops deeper oranges, reds, and browns earlier
Once both reach a mature patina, they are often visually indistinguishable. The key difference is that Corten stabilises, while Mild Steel continues to weather unless sealed.
Many customers choose to apply a clear rust sealant to Mild Steel once their desired finish is achieved.
How Sustainable Are Mild Steel and Corten Steel?
Steel production is energy-intensive, but it offers a major sustainability advantage:
✅ Steel is 100% recyclable and can be recycled indefinitely without any loss of quality.
At Edensteel, we actively minimise our environmental footprint:
- Certified renewable energy
- Full recycling of waste steel
- Electric material-handling equipment
- UK-based fabrication in the West Midlands
Summary
- Mild Steel and Corten Steel are both carbon steels
- Corten forms a protective corrosion-resistant patina
- Mild Steel does not stabilise, but has a similar appearance
- Corten weathers faster and stabilises
- Mild Steel is more cost-effective
- Both materials can cause run-off staining
- Patina can naturally darken to deep brown or black depending on environment and finishing
- Both steels are 100% recyclable
All Edensteel products are fabricated to the same high standard, regardless of material choice
Need Advice?
If you’re still unsure which material is right for your project, our team is happy to help.
📧 Email: sales@edensteel.co.uk
📞 Phone: 0121 517 2229