Bronze Anodizing Explained: How's it Done

Anodizing is a popular manufacturing technique used to apply a coat of finishing layer on a metal surface.

Bronze anodizing is an electrochemical process that forms a smooth and protective oxide layer over a bronze part, making it more resistant to corrosion and abrasion. This layer is generally harder than bronze and offers a great visual appeal.

This article discusses the process of anodizing bronze, its purpose, applications, and alternatives in detail.

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Anodizing Bronze: Explained

The constituent elements of bronze are tin and copper, which do not oxidize to form a protective layer upon anodization. They only oxidize naturally.

Hence bronze parts are added with about 5 - 11% aluminum in its overall composition to help with the anodization process.

This new material is called copper-aluminum bronze. It anodizes slowly, making bronze anodizing more expensive and challenging.

Aluminum readily oxidizes and forms an oxide layer during this process, thus providing an external layer over the bronze surface.

Besides the rapid oxidation of aluminum, aluminum parts are more common in industries, thus rendering aluminum anodization more popular.

A bronze anodized surface has a light gold orangish color with a scattered reddish texture.

Bronze anodizing of parts involves various factors that govern anodizing cost, and therefore must be carefully considered before outsourcing the job.

Anodizing Bronze - The Process

In large-scale industries, overhead cranes lift, move and dip the bronze components or parts in the electrolyte tanks.

Hobbyists can use tongs or safety gloves to dip and anodize bronze workpieces.

Cleaning/Washing

Cleaning the copper-aluminum bronze component in a detergent bath is the first step in bronze anodizing.

It removes grease, dirt, and dust from the bronze surface, ensuring uniform exposure to the electrolyte solution.

Etching

After cleaning, the components are bathed in separate tanks containing caustic, and acid etches solutions, both of which remove a thin layer of bronze, especially the aluminum constituent.

This step, in turn, removes any non-uniformities and defects present on the bronze surface.

You can also perform bright dipping to achieve a brighter output color, known as the bright dip anodizing process.

Once etching is done, the components are dipped in a desmutting solution to remove any remains from the etching bath.

Anodizing

In the anodizing stage, the bronze part is submerged in the electrolyte, which is usually an acid solution, like sulfuric acid.

The current per unit surface area used for each job varies according to the bronze composition and surface area. However, it is usually between 50 and 500 A/m2 (ampere per square meter).

Anodization reaction dissipates significant heat, so you must keep the electrolyte refrigerated.

This process is quite slow and can take several hours to complete.

Once the process is completed, the components are rinsed in deionized water to remove any residue from the electrolysis process.

Coloring 

The resulting oxide layer is porous so that you can fill it with anodizing dyes. It allows you to provide a more aesthetically pleasing surface with different colors.

Generally, bronze anodizing provides comparatively fewer color options than anodized aluminum colors.

Sealing

Lastly, the components are dipped in a nickel acetate solution to close off all the surface pores and seal any remaining cavities.

The resulting bronze surface is then non-porous, smooth, and uniform.

Why Should You Anodize Bronze? - The Purpose

Bronze having tin and copper as its only composition is vulnerable to wear and tear and has low ductility.

A copper-aluminum anodized bronze possesses many favorable properties that make it useful for various applications.

In case of bronze, the anodizing thickness is about 0.2 - 8 mm thick, with a smooth and shiny texture.

Unlike anodizing titanium, which is solely performed to improve the aesthetics of the part, bronze anodizing is performed to improve the abrasion resistance and durability of the part.

Properties

Property	Explanation
Malleable	Makes it easily machinable
Ductile	Makes it durable
Higher melting point	Can withstand high temperatures
Minimal sparking on impact	Can be easily stored with flammable materials
High electrical conductivity	Can be used in electrical applications
Aesthetic surface	Can be used for decorative purposes

Anodized bronze surface possesses many beneficial properties that render it suitable for numerous applications.

The anodic layer makes the bronze part more durable, enabling it to withstand significant heat, wear and tear forces, and pressure.

Moreover, due to this layer, it does not generate any spark when surfaces slide or hit the bronze surface. This makes it suitable to use in fire-hazardous situations.

The external oxide layer formed while anodizing bronze is smooth, shiny, and uniform, which are vital factors to look for in decorative applications.

Applications

The anodic layer of copper aluminum bronze is visually aesthetic and has a smooth texture, making it suitable for decorative purposes. 

Anodized bronze is also highly conductive and durable, making it useful in electronic and moving machinery components.

It has applications in industries like aircraft, automobiles, decor, electronics, furniture finishing, etc.

Alternatives to Bronze Anodizing

Alternative processes to bronze anodizing are preferred in some cases, especially by hobbyists, as these alternatives are often cheaper, simpler, safer, and less time-consuming.

Brush Finishing

In brush finishing, a roller brush is pressed against the bronze surface and moved along it to remove any impurities, making the surface shiny and uniform.

This process is purely mechanical and does not result in any addition of material to the external surface.

Polish Finishing

A sandpaper or cloth with tiny abrasives is rubbed against the bronze surface after adding some polish solution, which usually contains some detergent bath and oil mixture.

This process fills up any pores and cavities on the bronze surface while making it uniform and shiny.

Painting

The painting process is the cheapest, fastest, and most straightforward option, making it a good choice for hobbyists.

In this process, the surface is thoroughly painted with a color of your choice and then left to dry.

An additional layer of paint will provide extra protection against surface damage, unlike the previous two methods. Unfortunately, it quickly wears off over time.

Powder Coating

This process involves electrostatic charging, in which the bronze surface is given a specific charge, and powder particles with an opposite charge are sprayed upon it via a nozzle.

The resulting surface is uniform and firmly bonded to the original bronze surface. The powder particles are usually polymer resins.

This process is more expensive than bronze anodizing but quick and efficient, as this layer will be more durable and last longer.

When comparing powder coating with anodizing, powder coating is straightforward and does not require separate steps for adding color and sealing the surface.

Bronze Anodizing Dye

This process is similar to the dye-based gold anodizing process and involves dipping an aluminum component into a bronze-colored dye. 

The aluminum component is first anodized in an acid electrolyte at ambient temperature for about 2 hours.

Afterward, the part is dipped in the bronze-colored dye for about 15 - 20 minutes at a temperature of 60 °C (333 K).

Lastly, the component is dipped in a sealer bath, like a nickel acetate solution, to close off any pores.

This process is relatively shorter and cheaper in terms of running costs.

The resulting surface is a bronze layer thoroughly bonded to the original surface.

Anodized Bronze Paint

Bronze paint is light gold orangeish in color, as it contains bronze as a constituent.

Components like an anodized aluminum workpiece can be painted using bronze paint.

This process will be faster, cheapest of all, and relatively simple. It results in a thick and durable protective layer over the part.

Final Thoughts

Bronze anodizing is a beneficial electrochemical finishing process but has drawbacks like high cost and long processing time.

Alternative methods like dyeing or painting are preferred when just an aesthetic appeal is needed.

For hobbyist use, cost, time, and simplicity are key factors to consider, so based on that, painting followed by dyeing and then anodizing should be the way to go.

Frequently Asked Questions

Can every metal undergo anodizing?

No, all metals can not undergo anodizing. Ferrous metals like iron cannot undergo this process as the iron oxide layer will easily slide off or remove. Some metals like copper and tin oxidize very slowly during the anodization process.

What are the safety precautions that need to be followed during the bronze anodizing process?

The safety precautions that need to be followed during the bronze anodizing process include wearing a face shield, gloves, and safety boots and maintaining a safe distance from the acidic electrolysis tank.

Can an anodized bronze surface be further painted?

Yes, you can further paint an anodized bronze surface via electrostatic spray painting or other general painting techniques. The evenness of the paint will depend on its quality.