Meta Description: Does bronze develop a patina like pure copper? Read on to find out if bronze also turns green in the presence of oxygen and contaminants. 

Does Bronze Turn Green Like Copper

Many metals when exposed to air or moisture form protective oxide layers that have varying colors. Copper is not an exception and the green color it forms after using for years has caught the attention of manufacturers and metal fabricators. However, it is normal to wonder if bronze turns green too since it contains copper as the major element.

While both bronze and copper develop a patina, the composition and rate of formation differ. This is because bronze has other elements that slow down the reaction and make the copper oxide layer stronger. Read on to find out all about copper and bronze metals, why they turn green, copper vs bronze color, and how to prevent patina formation. 

What is Bronze

Bronze is an alloy that contains mainly copper as the base metal. It also has tin, aluminum, silicon, phosphorus, and nickel in varying amounts depending on the specific type of bronze. The use of bronze dates back to ancient civilization and its unique properties make it relevant in modern manufacturing. To manufacture bronze, you heat copper, tin, and other metals together at a specific temperature before cooling off. This gives a reddish-brown metal alloy with superior mechanical properties. 

There are different types of bronze available and the most common ones are aluminum bronze, nickel-aluminum bronze, phosphor bronze, and silicon bronze. They all have unique compositions and are used for different applications. Bronze alloys have a density of around 6-9 g/cc with a tensile strength ranging from 400-1000 MPa. They have good corrosion resistance properties and conduct heat and electricity moderately. 

Pros

Aesthetic Appeal: Bronze components are aesthetically pleasing. They have a warm and attractive color. Additionally, bronze produces a good surface finish which makes it suitable for decorative items and architectural structures.

Easy to Work on: You will find bronze easy to work on. It has excellent machinability properties allowing it to be shaped into complex parts. Furthermore, it is not labor-intensive to cast and form bronze alloys. 

Low Friction Coefficient: Bronze has a low friction coefficient. They minimize mechanical friction and decrease heat generation. This property makes bronze useful when manufacturing bushings and bearings. They reduce wear and tear which prolongs the lifespan of the parts. 

Recyclable: Bronze is an environmentally friendly metal. It is easy to recycle and reuse bronze without losing its properties. Recycling of bronze reduces waste and pollution. Furthermore, it requires less energy and can be cheaper than primary production.

Antimicrobial Properties: Bronze alloys have inherent antimicrobial and antibacterial properties. They can kill harmful microbes that make surface contact. Hence, it is a popular choice when manufacturing parts used in hygiene-sensitive applications. 

Cons 

Cost: The price of bronze alloys is on the high side. When compared to most metals such as aluminum or steel, bronze is more expensive. This is because the main raw materials; copper and tin are not cheap. 

Weight: Bronze is dense and relatively heavy. This is a limitation in applications where lightweight material is needed such as in automotive or aerospace part manufacturing. 

Limited Use in High-Temperature Applications: Bronze has a low melting point which limits its use in high-temperature environments. It can expand and degrade when there is a significant increase in temperature. 

What is Copper

Copper is a reddish-orange metal that can be found in its pure form. You can use copper metal alone with the most common application being in electrical connection. It is soft, malleable, and conducts heat and electricity perfectly. This makes copper ideal for the production of radiators, heat exchangers, and heat sinks. Additionally, copper wiring minimizes energy loss and ensures consistent connections.

Furthermore, copper is mostly alloyed with other elements to improve its mechanical properties. The most common copper alloys are brass, bronze, and cupronickel. Brass contains zinc while bronze is made up of tin as the major additional element. However, cupronickel is a mixture of copper and nickel. They all have unique properties because of their different chemical composition. 

Pros

Corrosion Resistant: Copper has good corrosion resistance properties. When exposed to corrosive attacks, it forms a protective film that prevents further oxidation and damage. This makes copper suitable for use in outdoor applications. 

Durable: The durability of copper metal is second to none. Copper parts are strong and you can use them for an extended period. Copper can handle physical stress while retaining its mechanical properties. 

Ductility: Copper is a highly ductile metal. This means that you can easily stretch it into thin wires without breaking it. This property makes copper metal useful in certain industries including plumbing and electrical.

Low Thermal Expansion: Copper has a relatively low coefficient of thermal expansion. It does not expand and contract significantly with temperature changes. When used in demanding conditions, copper does not warp or degrade. 

Non-magnetic: Copper is nonmagnetic and does not interact at all in the presence of the magnetic field. This feature is important in electrical applications where there should be a total absence of magnetic interference for the parts to function properly.

Cons

Softness: The softness of copper metal can be a limitation in many instances. Copper is prone to scratches and dents especially when it is in contact with abrasives. Furthermore, you may find it challenging to machine copper as it tends to stick to the cutting tools.

High Density: Copper is a dense and relatively heavy metal. Hence, it is difficult to handle and not advisable to use in lightweight applications. Additionally, the density of copper metal increases the cost of transportation and logistics which is a shortcoming. 

Does Bronze Turn Green Like Copper

Yes, bronze can turn green like copper in similar conditions. The green color, called a ‘patina’, occurs when bronze or any alloy containing copper reacts with oxygen, moisture, or pollutants. The extra green layer serves as a means of protection from further oxidation. For a better understanding, when exposed to atmospheric conditions, the copper content in bronze reacts to form copper oxide. This oxide layer is usually greenish or bluish-green which takes years to form. 

Note that even though bronze turns green like copper, there are still some differences. The composition of the green patina is somewhat different because of the presence of tin. Tin is more resistant to corrosive attacks. Hence, it takes longer for bronze to develop a patina, especially in harsh conditions. Additionally, the patina on bronze is harder and more durable when compared to pure copper. 

Furthermore, sometimes this copper oxide layer is desirable. The green patina on bronze sticks well to the base metal and forms a barrier for corrosive attacks. The thicker the copper oxide layer, the more protection it offers. It is durable and does not flake off. However, you may not want the green patina on your part in certain instances. It is not aesthetically pleasing when bronze jewelry turns green after reacting with chemicals on the skin. To prevent patina, consider waxing the part or using a protective finish and sealants to keep away oxygen and moisture. 

Conclusion

Bronze contains up to 88% of copper. Hence, it turns green just like pure copper in the presence of oxygen or humid conditions. The formation of the patina occurs in stages and takes several years. It continues to grow in thickness over decades and centuries. In custom metal fabrication, patina can be desirable when using bronze for statues, coins, or architectural work. This copper oxide layer provides aesthetics and prevents further oxidation.