Unlike ordinary mild steel, Corten steel is designed so that its initial surface rust develops into a dense, adherent protective patina.

This rust-coloured patina acts as a protective layer, significantly slowing further corrosion of the steel underneath.

As the planter is exposed to the weather, its appearance will naturally change. During this process you may see:

• Darker wet patches

• Uneven rust colouring

• Water marks or streaks

• Areas that temporarily look darker after rain

• Differences in colour between wetter and drier parts of the planter

These are normal characteristics of weathering steel.

Are Wet Patches Corrosion?

In most cases, visible wet patches are simply discolouration caused by moisture rather than harmful corrosion.

Corten steel naturally develops a protective rust-coloured patina that helps resist further corrosion. The colour and appearance of this patina can vary depending on how recently different areas of the planter have been exposed to water.

A damp section will often look considerably darker than the surrounding dry steel and will lighten again once it dries.

Corten Steel Needs Wet and Dry Cycles

An important part of the Corten weathering process is that the steel should experience alternating wet and dry cycles.

Moisture encourages the initial oxidation of the surface, while drying allows the protective patina to stabilise.

This means that occasional rain, watering and normal outdoor exposure are exactly what Corten steel is designed to cope with.

However, Corten steel should not ideally remain permanently wet.

What Happens if Corten Steel Stays Wet?

Although Corten steel is highly durable, its protective properties rely on the surface being able to dry periodically.

Constant moisture can prevent the protective patina from developing or performing as intended.

Particular conditions to avoid include:

• Permanently waterlogged areas

• Poor drainage

• Water continuously trapped against the steel

• Soil or debris remaining permanently wet against exposed surfaces

• Constant irrigation onto the same area

• Prolonged coastal salt exposure

Under these conditions, corrosion can accelerate rather than stabilise.

This is why we always supply our Corten planters with adequate drainage holes, and we always advise that they are not situated in areas where puddles are likely to develop in extended spells of wet weather. We also offer fabric planter liners to add another layer of protection for your planter.

Why Does My Planter Look Patchy?

Corten steel is intentionally a living finish rather than a uniform painted surface.

Different areas weather at different rates depending on rainfall, sunlight, airflow and moisture. One side of a planter may therefore initially appear darker, lighter or more orange than another.

Over time, weathering generally produces a deeper and more established rust-brown finish, although natural variation will always remain part of the appearance of genuine Corten steel.

Wet patches and changes in tone should therefore be expected — they are part of what gives Corten steel its distinctive character.

In Summary

With appropriate drainage and normal outdoor exposure, your Corten steel planter will continue to develop its characteristic weathered finish while remaining extremely durable for many years.