Metal burnishing

Metal burnishing is a chemical or electrochemical process used to impart a dark finish, usually black or brown, to metal surfaces. This treatment not only improves the aesthetic appearance of the metal, but also provides some protection against corrosion. It is commonly used on steel, copper, brass and other metal alloys.

The burnishing of metals occurs through a chemical reaction between the metal surface and specific chemical compounds. This process forms an oxide layer on the surface of the metal that is generally darker than the native metal. Depending on the metal and the method used, this protective layer can vary in thickness and uniformity.

Applications

Metal burnishing finds application in a wide range of industrial and artisan sectors:

Firearms: One of the best-known applications of burnishing is in the treatment of firearm barrels. The dark coating not only provides protection against corrosion but also reduces light reflection, making the weapon less visible.

Mechanical Components: Parts of machinery and tools are often burnished to improve their resistance to corrosion and to reduce wear.

Decorative Objects: Burnishing is used to give an antique or refined look to decorative metal objects, such as jewelry, furniture and accessories.

Musical Instruments: Some musical instruments, such as brass wind instruments, can be burnished to improve aesthetics and protect the underlying metal.

Benefits of Burnishing

Corrosion Protection: The oxide layer created by burnishing acts as a barrier, protecting the underlying metal from oxidation and corrosion.

Friction Reduction: In some applications, burnishing can reduce friction between metal parts, improving their durability and functionality.

Improved Aesthetics: Dark finishing can improve the appearance of metal components, giving them a more professional and refined look.

Easy Maintenance: Burnished surfaces tend to show fewer fingerprints and signs of wear, making maintenance and cleaning easier.

Burnishing methods

Cold Burnishing:

Cold burnishing uses chemical solutions that react with the metal surface at room temperature to create a dark oxide layer. This method is often used for its simplicity and the possibility of applying it without the need for heating. The main chemical compounds used in cold burnishing include:

Selenic Acid: One of the most common chemicals for cold burnishing of steel. It reacts with iron to form a dark oxide layer.

Phosphoric Acid: Used in some cold burnishing solutions, particularly for steel. It can create a durable black coating.

Nitrates and Nitrites: Compounds such as copper or zinc nitrate can be used to create a dark finish on metals such as steel and copper.

Chlorides and Bromides: These salts, such as ferric chloride, can be used for cold burnishing, providing a protective and decorative coating. More information here:

https://www.surface-treatment.it/l/dark-etching/

Copper-Based Solutions: For burnishing brass and copper, solutions containing copper compounds that react with the metal to form a dark patina can be used.

Procedure

Surface Preparation: The metal must be cleaned thoroughly to remove oils, greases and surface oxides. This can be done by solvent washing, acid pickling or mechanical abrasion.

Solution Application: The cold burnishing solution is applied to the metal surface by dipping, brushing or spraying. It is important to follow specific product instructions to ensure even coverage and the desired color.

Chemical Reaction: The solution reacts with the metal surface, forming an oxide layer. The reaction time varies depending on the solution and the metal, but generally lasts a few minutes.

Rinsing and Drying: After the reaction is complete, the metal is rinsed with clean water to remove any chemical residues. Afterwards, it is dried thoroughly to prevent water spots from forming.

Application of a Protectant: To improve corrosion resistance and durability of the finish, a protective layer of oil, wax or clear lacquer is often applied.

Advantages and Limitations

Advantages:

Simplicity: Does not require special heating equipment.

Convenience: Ideal for small batches or field repairs.

Flexibility: Can be used on a variety of metals.

Limitations:

Lower Durability: The resulting oxide layer may be less durable than hot burnishing.

Uniformity: It can be more difficult to achieve a uniform finish, especially on complex surfaces. 

Hot Burnishing:

Hot burnishing is a process used primarily for steel, which involves immersing the metal in heated chemical solutions. These solutions react with the metal surface to form a dark, protective oxide layer. Below are the main chemical compounds used:

Sodium Hydroxide (NaOH): A key component of hot burnishing solutions. It serves as a strong base that helps dissolve nitrates and keep the solution alkaline.

Sodium Nitrate (NaNO3): Acts as an oxidant, contributing to the formation of the oxide layer on the surface of the metal.

Sodium Nitrite (NaNO2): Works in conjunction with sodium nitrate to control the rate of oxidation and ensure a uniform finish.

Procedure

Surface Preparation: Before proceeding with burnishing, the metal must be thoroughly cleaned to remove oil, grease, rust and surface oxides. This is usually done by acid pickling or solvent degreasing.

Solution Preparation: The burnishing solution is typically composed of a mixture of sodium hydroxide, sodium nitrate and sodium nitrite dissolved in water. The solution is heated to a temperature between 135°C and 155°C.

Metal Immersion: The metal is immersed in the heated solution for a period ranging from 15 to 30 minutes. During immersion, a layer of iron oxide (Fe3O4), also known as magnetite, forms on the surface of the metal.

Rinsing: After immersion, the metal is thoroughly rinsed with clean water to remove residual chemical solution.

Neutralization: Often the metal is then immersed in a neutralization solution, such as deionized water with a corrosion inhibitor, to prevent further chemical reaction.

Drying: The metal is dried completely to prevent rusting.

Application of a Protective: Finally, a layer of oil or wax is applied to improve corrosion resistance and give a shiny, protective finish.

Advantages and Limitations

Advantages:

Durability: Hot burnishing produces a stronger and more durable oxide layer than cold burnishing.

Uniformity: Tends to provide a more uniform, high-quality finish.

Improved Protection: Offers superior corrosion protection compared to other methods.

Limitations:

Equipment Required: Requires special equipment to heat and maintain the solution at elevated temperatures.

Safety: The use of caustic chemical solutions at high temperatures requires significant safety precautions to prevent burns and inhalation of harmful vapors.

Cost: This is more expensive than cold burnishing due to the equipment, temperatures and safety precautions required.