Understanding the Natural Weathering of Corten Steel

One of the distinctive features of Corten steel is that its appearance is designed to change when exposed to the elements.

Rather than being protected by a conventional painted or coated finish, Corten develops its own rust-coloured protective patina. This dense surface layer adheres to the steel and helps significantly slow down further corrosion underneath.

Because this finish develops naturally, Corten steel won't always have a perfectly even colour — particularly during the early stages of weathering.

You may notice:

• Dark or damp-looking patches

• Variations between orange, brown and darker rust tones

• Rain marks and streaking

• Sections that darken temporarily after watering

• Different areas of the steel weathering at different speeds

These variations are a normal part of the character of genuine weathering steel.

Why Does Corten Steel Develop Dark Patches?

Dark patches on Corten steel are usually caused by differences in surface moisture.

When part of the steel becomes wet, the patina can appear much darker than the surrounding dry areas. Once the surface dries, the colour will usually lighten again.

This can make a planter look temporarily patchy after rainfall or watering, particularly while its patina is still developing.

Rather than aiming for a perfectly uniform surface, Corten is intended to develop naturally in response to its surroundings.

The Importance of Wet and Dry Weather

Corten steel performs best when it is allowed to alternate between getting wet and drying out.

Rain and moisture encourage oxidation, creating the characteristic rust-coloured surface. Dry periods then allow this layer to stabilise and form the protective patina for which Corten steel is known.

Normal British weather is therefore well suited to the weathering process.

Rainfall itself isn't something you need to protect your Corten planter from. What is more important is allowing the steel to dry periodically rather than leaving it continuously exposed to standing water.

Good Drainage Helps Your Corten Planter Weather Properly

Corten steel is extremely durable, but it is not intended to remain permanently waterlogged.

Conditions that can interfere with the normal weathering process include:

• Standing water around the planter

• Poorly drained ground

• Water being continually trapped against the steel

• Permanently damp soil or debris against exposed surfaces

• Irrigation repeatedly soaking the same area

• Prolonged exposure to coastal salt

If moisture remains against the steel continuously, the protective patina may not stabilise as effectively and corrosion can continue rather than naturally slowing.

Our Corten steel planters are therefore supplied with drainage holes to help excess water escape.

When positioning your planter, we also recommend choosing somewhere that doesn't regularly develop standing puddles during prolonged wet weather.

For additional separation between the compost and the inside of the planter, fabric planter liners are also available.

Why Doesn't All of My Corten Planter Look the Same Colour?

No two pieces of naturally weathered Corten steel develop in exactly the same way.

Rainfall, sunlight, airflow, temperature and moisture all influence the surface. Even different sides of the same planter can weather at different rates.

For example, a side exposed to frequent rainfall may initially become darker than a sheltered side. Areas receiving more sunlight may dry more quickly and consequently develop a slightly different tone.

As weathering continues, the finish generally matures into a richer rust-brown appearance. Some natural variation will remain, however, and this is an intentional part of the material rather than a defect.

A Finish That Continues to Develop

Unlike a painted planter, Corten steel doesn't have a static finish.

Its changing tones, natural markings and subtle differences in colour are part of its appeal. Dark areas following rainfall, water marks and variations in the patina should all be expected as the surface responds to the weather.

With good drainage and normal periods of wet and dry weather, Corten steel can continue developing its distinctive protective finish while providing excellent durability for many years.