No, copper does not rust. Rust is iron oxide, so only iron and metals that contain iron, like steel, can rust. Copper does corrode, though. Over time it darkens, then slowly turns a blue-green color called patina, and that patina actually protects the copper underneath from further corrosion.
If you’re an architect, builder, or contractor looking at architectural copper for a project, that’s the short answer to the question we hear most. Here’s what’s really happening when copper changes color, and what speeds it up.
First, What Exactly Is Rust?
Before we can even answer the question of whether or not architectural copper rusts, it’s important to first understand what rust is. Rusting is what occurs when iron or other metal alloys are exposed to oxygen, whether it be from air or by exposure to water. Rust produces an unsightly yellow, red, or brown when iron oxidizes.
With that said, not all metal oxidation creates rust, as rust is unique to iron and iron alloys only. Other metals undergo the same oxidation process, but it leads to corrosion, rather than rust. Thus, copper does not rust!
So Then What Is Corrosion?
Rust is simply a form of corrosion that’s unique to iron and iron alloys. So then what does it look like on architectural copper? As copper oxidizes, rather than receiving the reddish brown hues, architectural copper will become a stunning blue-green color known as patina following oxidation.
What Can Accelerate Copper Corrosion?
Copper and copper alloys will naturally corrode over time when exposed to the atmosphere, however some key elements that may accelerate copper corrosion include:
- Heat
- Seawater & Rain
- Acidic Compounds
Copper will first tarnish into a darker color before settling on the patina color.
Is Corrosion Bad For Architectural Copper?
Whereas rust will significantly impact iron and iron alloys’ utility, copper oxidizing actually has some unique benefits. Not only does it enjoy the aesthetic benefits of the patina, but oxidized copper will actually protect it from further exposure to oxygen, stopping corrosion from occurring further.
Frequently Asked Questions
Does copper corrode?
Yes. Copper reacts with oxygen, moisture, and pollutants in the air. It first tarnishes to a darker brown, then develops the blue-green patina over time. Unlike rust on steel, this layer doesn’t eat through the metal. It seals the surface and slows further corrosion.
Why does copper turn green?
The green is patina, a thin layer that forms as copper reacts with moisture, carbon dioxide, and sulfur compounds in the air. It’s the same process that turned the Statue of Liberty green.
Can copper rust in water or outdoors?
No. Water and weather speed up corrosion and patina, but copper still can’t rust, because rust only forms on iron. Heat, rain, seawater, and acidic compounds all make copper change color faster.
Do copper alloys like brass and bronze rust?
Generally no. Brass and bronze are made mostly of copper and contain little or no iron, so they tarnish and form patina rather than rust.
Is patina bad for architectural copper?
No. Many architects choose copper for the patina itself, and the patina layer protects the metal underneath, which is a big part of why copper roofing and cladding last so long.
Interested in learning more about architectural copper for your next project? No worries! Contact Wieland-Wrisco today to discover how we can assist in supplying your next project.