Everything You Need to Know About Welding Corten Steel

Corten steel, also known as weathering steel, is an extraordinary material appreciated for its rustic appearance and exceptional durability. Its warm, natural tones and ability to form a protective patina make it ideal for architectural structures, sculptures, and outdoor furniture. However, when it comes to welding, Corten steel requires particular care. Its unique composition affects not only its oxidation process but also the techniques and precautions needed to preserve its corrosion resistance after welding.

Many people assume that welding Corten steel is the same as welding conventional carbon steel, but this is a mistake. Improper welding techniques or unsuitable filler materials can compromise the metal’s self-protective patina. Similarly, post-welding treatments must be chosen carefully: acid-based rust accelerators can seriously damage the steel, while professional acid-free solutions like those from Corten+ maintain the integrity of the material.

This complete guide explores everything about welding Corten steel — from its composition and challenges to recommended techniques and post-weld treatments. It also explains why using the Corten+ system ensures long-lasting protection and an authentic aesthetic finish without any risk to the steel.

Understanding the Composition and Behavior of Corten Steel

Before exploring welding techniques, it is essential to understand what makes Corten steel so unique. Unlike standard carbon steel, Corten steel contains alloying elements that enable it to form a stable layer of protective oxidation called patina.

The main components of Corten steel include:

Iron, which forms the base of the alloy.
Copper, which increases corrosion resistance and gives the surface its characteristic orange-brown tone.
Chromium, which helps create a dense and adherent oxide layer.
Nickel, which enhances strength and contributes to the uniformity of the patina.
Phosphorus, which improves the stability of the oxide layer in atmospheric conditions.

When exposed to alternating wet and dry environments, these elements react with oxygen and humidity to form a compact rust layer that prevents further corrosion. This property gives Corten steel its durability and self-protection.

However, welding alters the composition and structure of the metal locally. The heat of the welding process changes the crystalline structure of the steel and can affect the behavior of the protective elements. Therefore, understanding how to weld Corten steel properly is crucial to maintaining its natural resistance.

The Challenges of Welding Corten Steel

Welding Corten steel presents certain challenges that differentiate it from ordinary steels. These challenges are mainly linked to its composition and the properties of its protective layer.

When welded, the heat-affected zone (HAZ) experiences a modification in its microstructure. The copper, nickel, and chromium elements that normally form the protective patina may redistribute unevenly. This area, once cooled, may not rust in the same way as the rest of the steel. If not properly managed, this can create visible differences in color and, more importantly, areas less resistant to corrosion.

Another challenge is the risk of brittleness in the welding zone. Because Corten steel contains specific alloying elements, using inappropriate filler materials or incorrect techniques can lead to cracking.

Finally, post-weld oxidation is a sensitive step. The welded area must be cleaned and treated to ensure that the patina develops uniformly over the entire surface. Using acid-based accelerators at this stage is dangerous, as they damage the metal and cause long-term corrosion. Instead, an acid-free system such as Corten+ ensures a safe and even finish.

Choosing the Right Welding Technique

Several welding methods can be used for Corten steel. The choice depends on the thickness of the material, the type of joint, and the equipment available. Each process has advantages and specific recommendations.

MIG Welding (Metal Inert Gas)

MIG welding is one of the most common methods for Corten steel, especially for medium-thickness parts. It allows precise control of the weld and produces clean, consistent seams.

When using MIG welding, the filler wire must be carefully selected. It is essential to use wire specifically designed for weathering steel, typically identified as ER80S-G or ER70C-6M. These wires contain alloying elements compatible with Corten steel and ensure that the weld develops a protective patina similar to the rest of the structure.

MIG welding requires the use of inert gases such as argon or a mixture of argon and CO₂ to protect the weld from oxidation during the process.

TIG Welding (Tungsten Inert Gas)

TIG welding is preferred for thin sheets and decorative applications because it provides exceptional precision and a visually clean seam. As with MIG welding, the filler material must match the composition of Corten steel. A compatible rod containing similar alloying elements (copper, nickel, and chromium) should be used.

TIG welding also uses inert gas protection, usually pure argon. Although slower than MIG, it offers superior control and aesthetic quality, making it ideal for visible joints or artistic pieces.

Stick Welding (SMAW or Manual Arc Welding)

Stick welding is a versatile and widely used technique, especially for outdoor or structural applications. It is suitable for thicker sections of Corten steel.

To preserve the weathering properties, use special electrodes designed for Corten steel, such as E8018-W or E7018-W. These electrodes produce a weld with similar composition to the base metal, ensuring consistent corrosion resistance.

Stick welding requires experience to control the arc length and avoid porosity. Nevertheless, it remains one of the most practical methods for on-site work.

Flux-Cored Arc Welding

Flux-cored welding combines the speed of MIG with the flexibility of stick welding. It is well-suited for thick Corten steel and outdoor applications where wind or humidity can affect gas shielding.

Again, the filler wire must be weathering steel compatible, typically identified as E71T1-W. This ensures that the weld area develops the same protective patina as the base steel.

Preparation Before Welding

Preparation is crucial to achieving a high-quality weld on Corten steel. Any impurity, grease, or mill scale will interfere with the welding process and affect both penetration and corrosion resistance.

Before welding, the surface should be cleaned using mechanical or chemical methods. For hot-rolled Corten steel, there is usually a dark mill scale that must be removed. Sandblasting is the most effective method, as it eliminates the scale completely and leaves a slightly rough texture that promotes adhesion.

If sandblasting is not available, the Corten+ Descaler provides an excellent alternative. It chemically removes the mill scale without damaging the metal. Once applied, it must be rinsed with water to ensure the surface is perfectly clean.

Cold-rolled Corten steel, on the other hand, is often coated with industrial grease to prevent premature oxidation. This grease is extremely resistant and cannot be removed with ordinary degreasers. The Corten+ Degreaser is designed specifically for this purpose. It dissolves even the most persistent oils, preparing the surface for welding or oxidation.

Cleanliness is the foundation of strong, corrosion-resistant welds. Only after a thorough cleaning can welding proceed without risk of contamination or uneven oxidation.

Post-Welding Treatment and Oxidation

Once the welding process is complete, the steel needs to be cleaned again to remove any slag, oxidation residues, or contaminants from the weld. This step is vital to ensure that the protective patina forms uniformly across the welded joints.

The most common mistake at this stage is using acid-based solutions to accelerate the rusting process. While acids may give quick results, they attack the alloying elements in Corten steel — copper, chromium, and nickel — which are essential for forming a protective patina. The acid continues to corrode the metal beneath the surface, leading to internal degradation.

An acid-treated weld area will never develop the same corrosion resistance as untreated Corten steel. Within a few years, perforations, cracks, and discoloration appear.

To avoid these problems, the Corten+ system offers an acid-free oxidation process that reproduces the natural weathering of steel without damaging it.

Using the Corten+ Rust Accelerator

The Corten+ Rust Accelerator is applied to clean, dry steel after welding. It activates the natural oxidation process through a chemical reaction that mimics natural weathering conditions, but in a controlled way.

Within hours, the steel begins to develop its characteristic orange tone. In 24 to 48 hours, the patina becomes uniform and stable, identical in color and texture to naturally aged Corten steel.

Because it contains no acid, the Corten+ Rust Accelerator does not harm the weld or surrounding metal. The oxidation stops naturally once the protective layer has formed, preserving the mechanical integrity of the steel.

Protecting the Weld with the Corten+ Sealer

After oxidation, it is essential to protect the weld and the rest of the steel with the Corten+ Sealer. This transparent coating seals the surface, preventing water from penetrating and carrying rust particles to other areas.

It also prevents run-off stains on surrounding materials such as concrete, stone, or wood. Additionally, it locks the color at the desired stage, preventing the patina from darkening excessively over time.

The Corten+ Sealer ensures that the welded areas remain as durable and visually harmonious as the rest of the steel, even in harsh weather conditions.

The Importance of Using the Right Filler Material

Using the correct filler material during welding is essential for both strength and corrosion resistance. A mismatch between the filler and the base metal creates a weak point where corrosion can start.

For Corten steel, the filler should have a composition that closely matches the base metal, particularly in terms of copper, nickel, and chromium content. This allows the weld to develop a similar patina when exposed to weather.

If a standard carbon steel filler is used, the weld will not form the same protective oxide layer. It will corrode faster and appear as a different color compared to the surrounding steel. This not only affects the visual uniformity but also shortens the lifespan of the structure.

Always check the specifications of the filler material and choose one designed for weathering steel. Manufacturers usually mark these products with the suffix “W” to indicate their compatibility with Corten steel.

Controlling the Appearance of Welds

Beyond strength and resistance, the aesthetic quality of the weld is important for architectural and artistic projects. The goal is to ensure that the welded joints blend seamlessly into the overall surface once oxidation occurs.

After welding, grinding and cleaning should be done carefully to smooth the seams without thinning the steel excessively. Any sharp edges or weld spatter should be removed.

Once the Corten+ Rust Accelerator is applied, the welded area begins to oxidize in the same way as the rest of the steel. Because the Corten+ formula activates the natural oxidation process rather than corroding the surface, the weld and base metal develop a uniform appearance.

This uniformity is difficult to achieve with acid-based treatments, which often leave irregular color patches and accelerate localized corrosion. The acid also reacts differently with the welded metal, creating visible discoloration. With Corten+, the patina develops evenly, resulting in a professional and natural finish.

Safety and Environmental Considerations

Corten steel is often used in outdoor or public installations, which makes safety and environmental responsibility essential. Traditional acid-based treatments release harmful vapors and residues that can endanger both the user and the surrounding environment.

The Corten+ Rust Accelerator and Sealer are completely acid-free and non-toxic. They can be used safely both indoors and outdoors without protective gear or special ventilation. They also leave no harmful residues that could contaminate soil or water.

For projects in sensitive environments, such as gardens, parks, or near natural water sources, using an acid-free system is not just a matter of quality but also of environmental ethics.

Maintenance After Welding

Once welding and oxidation are complete, the maintenance of Corten steel is simple but necessary to ensure its longevity. The main goal is to preserve the protective patina and prevent the accumulation of moisture or contaminants.

Inspect the welded areas regularly to ensure that the sealer remains intact. If small scratches or damage occur, they can be repaired easily by cleaning the area and reapplying the Corten+ Sealer.

Avoid using abrasive cleaners or solvents that could damage the patina. Water and a soft cloth are usually sufficient for cleaning.

If the project is located in a highly humid or coastal environment, reapplying the sealer every few years will maintain optimal protection and color stability.

Why Choose Corten+ for Post-Welding Treatment

The Corten+ system provides a comprehensive, professional solution for treating Corten steel after welding. It ensures that the natural patina develops safely and uniformly, without the risks associated with acids.

Key advantages include:

100% acid-free and non-corrosive formulation.
Safe for users and the environment.
Uniform, natural oxidation matching the rest of the steel.
No need for neutralization or rinsing.
Enhanced durability through long-term protection with the sealer.

In contrast, acid-based products permanently alter the steel’s structure, cause irregular oxidation, and dramatically shorten its lifespan. A surface treated with acid may look rusted at first but will continue to corrode until it fails.

Corten+, by respecting the natural chemistry of the metal, ensures that both the welded areas and the original steel remain stable and durable for decades.

Conclusion

Welding Corten steel requires precision, the right materials, and an understanding of how this unique alloy behaves. When welded correctly, Corten steel retains its strength, corrosion resistance, and aesthetic appeal.

However, the weld area must be treated with care. Acid-based products should never be used to accelerate rusting after welding, as they destroy the protective elements that give Corten steel its resistance. This leads to premature corrosion, discoloration, and structural weakness.

The Corten+ system provides a safe, acid-free alternative that reproduces natural oxidation in a controlled, rapid, and environmentally friendly way. It allows welded areas to blend seamlessly with the rest of the steel while maintaining full corrosion resistance.

By combining proper welding techniques with the Corten+ Rust Accelerator and Sealer, you ensure that your Corten steel projects — whether architectural, structural, or artistic — maintain their integrity and beauty for a lifetime.

Corten+ products, including the Descaler, Degreaser, Rust Accelerator, and Sealer, are available on our official website, with fast worldwide delivery. They represent the safest and most effective solution for preparing, oxidizing, and protecting Corten steel before and after welding.

With Corten+, you can weld, rust, and protect your Corten steel with complete confidence — preserving both its aesthetic charm and its legendary durability.