Brass Patina: Formation Causes, Application Value, Removal and Protection Methods
Brass patina, this term may sound unfamiliar, but if you mention the mottled green rust on ancient copperware, I believe many people can immediately draw out its unique image in their minds. As a green oxide layer naturally formed on the surface of brass under specific conditions, it has both disadvantages and unique values. This article will reveal the formation causes of brass patina, explore its application value, and provide practical removal and protection methods.
1.Which is more likely to produce patina, brass or pure copper?
The following is a comparison between brass and pure copper in terms of producing patina:
(1) Pure copper:
Pure copper reacts with oxygen, carbon dioxide and water in a humid environment to produce basic copper carbonate (patina), whose chemical formula is Cu2(OH)2CO3.
Pure copper is relatively inactive, so its surface is not easy to rust in a dry environment. However, in a humid environment, pure copper will gradually produce patina.
(2) Brass:
Brass is an alloy of copper and zinc. Zinc is more active than copper, so zinc will react with oxygen in the air first to form a protective film, thereby slowing down the oxidation process of copper.
Due to the presence of zinc, brass is usually more resistant to corrosion than pure copper. Therefore, under the same environment, brass is less likely to produce patina than pure copper.
In summary, brass is less likely to produce patina than pure copper due to the characteristics of its alloy composition. Pure copper is more likely to react with oxygen, carbon dioxide and water to produce patina in a humid environment. Nevertheless, brass will still produce patina in certain specific environments. Please see the following introduction to brass patina.
2.Analysis of the causes and influencing factors of brass patina
(1) Causes of brass patina:
The formation of brass patina is a complex chemical process, mainly caused by the reaction of copper in brass with oxygen, carbon dioxide and water vapor in the air to produce basic copper carbonate (Cu₂(OH)₂CO₃), commonly known as patina.
This process is usually accelerated in a humid environment, especially in air containing hydrogen sulfide or sulfur dioxide, the formation rate of patina will be faster. The color of the patina can range from light green to dark green, and sometimes even bluish green, depending on its composition and the conditions under which it forms.
(2) Factors affecting the formation of brass patina
1) Humidity:
Brass is more likely to form patina in a humid environment because water is an indispensable substance in chemical reactions.
2) Temperature:
Higher temperatures accelerate the rate of chemical reactions, so brass patina forms faster in warm climates.
3) Air quality:
The concentration of carbon dioxide in the air affects the rate of patina formation, and high concentrations of carbon dioxide promote the formation of patina.
4) Alloy composition:
Although brass itself is more corrosion-resistant than pure copper due to the presence of zinc, if the zinc content in brass is insufficient or the zinc is consumed, the copper will begin to oxidize and form patina.
5) Surface treatment:
The finish of the brass surface and whether there is a protective layer will also affect the formation of patina. If the surface is scratched or the protective layer is damaged, patina is more likely to form in these places.
6) Contact material:
If brass comes into contact with acidic substances, the corrosion process will be accelerated and patina will be generated faster.
3.What is brass dezincification corrosion?
Brass dezincification corrosion is a typical selective corrosion phenomenon, which mainly occurs in copper-zinc alloys. The following are some core contents of brass dezincification corrosion:
(1) Definition and form:
Brass dezincification corrosion refers to the preferential dissolution of zinc in the alloy, leaving a loose and porous copper layer. Its manifestations mainly include uniform layered dezincification and uneven banded or plug-shaped dezincification.
(2) Occurrence conditions:
Brass dezincification corrosion is prone to occur in brass with a high zinc content, especially when the zinc content is greater than 20%. Corrosive media include neutral salt aqueous solutions, seawater, river water, etc.
(3) Corrosion mechanism:
The main corrosion mechanisms include preferential dissolution mechanism and dissolution-redeposition mechanism. The preferential dissolution mechanism assumes that zinc dissolves preferentially from brass, leaving behind a copper layer; while the dissolution-redeposition mechanism assumes that zinc and copper dissolve simultaneously, but copper is redeposited on the alloy surface.
(4) Influencing factors:
The factors affecting brass dezincification corrosion include alloy structure, pH value of the medium, temperature, etc. Increased temperature will accelerate the dezincification rate, resulting in the formation of brass patina.
(5) Preventive measures:
The methods to prevent brass dezincification corrosion mainly include adding a small amount of alloying elements such as arsenic and boron to the alloy, as well as using corrosion inhibitors, cathodic protection and other measures.
4.Application value of brass patina
(1) Artistic decoration:
The unique green color of brass patina makes it widely used in the field of artistic decoration. It can be used in sculptures, architectural decorations, etc., to add a simple and natural color to the work. For example, the patina formed on the surface of some ancient copper roofs and statues has a good decorative effect.
(2) Furniture design:
In furniture design, brass patina is also widely used on table legs, drawer handles and other parts. After a long period of oxidation and corrosion, these parts will form a unique green rust on the surface, which is not only beautiful and durable, but also adds a retro atmosphere to the furniture.
(3) Chemical reagents:
In chemical experiments, brass patina can be used as an analytical reagent. For example, it can be used to detect the components in certain gases or solutions because its color change can indicate the occurrence of chemical reactions.
(4) Fungicides:
Brass patina has a certain bactericidal effect. In the agricultural field, it has been used as a fungicide to prevent and control some crop diseases, such as grape downy mildew.
(5) Pigments:
Due to its bright green color, brass patina can also be used as a pigment. In some paintings and coatings, brass patina can be used to create special color effects.
5.Removal, protection and utilization of brass patina
(1) Patina removers:
There are special removers for patina on the market. These removers usually contain chemical components that can react with patina. When using, just apply the remover to the patina surface, treat it according to the instructions, and then rinse it with clean water.
(2) Chemical cleaning method:
Chemical cleaning method is a common method for removing brass patina. This method usually uses an acidic solution (such as dilute hydrochloric acid, dilute sulfuric acid, etc.) or an alkaline solution (such as sodium hydroxide, sodium carbonate, etc.) to clean the brass surface. These solutions can react chemically with the patina to dissolve and remove it.
However, chemical cleaning method will cause certain damage to the brass surface, so it needs to be used with caution. In addition, the brass surface after chemical cleaning needs to be properly protected to prevent it from rusting again.
(3) Physical cleaning method:
Physical cleaning method mainly includes mechanical grinding and ultrasonic cleaning. Mechanical grinding is to use sandpaper, wire brush and other tools to grind the brass surface to remove the patina.
Ultrasonic cleaning uses the vibration of ultrasound to peel off the patina on the brass surface. The advantages of physical cleaning method are simple operation, no need to use chemical reagents, and suitable for rust removal of complex shapes and small parts.
(4) Electrochemical protection method:
Electrochemical protection method is a method of preventing metal corrosion by controlling the potential. For brass products, it can be connected to a more active metal (such as zinc) to make it a cathode, thereby preventing it from being oxidized and rusted. This method is often used to protect brass products in marine environments.
(5) Coating protection method:
Coating protection method is to apply a protective film on the surface of brass to prevent it from reacting with oxygen, water vapor, etc. in the air. Commonly used coating materials include varnish, paint, wax, etc.
These coating materials can form a dense protective film on the surface of brass, effectively preventing the penetration of oxygen and water molecules. However, the coating protection method requires regular maintenance and renewal of the coating to ensure its protective effect.
(6) Using rust for artistic creation:
In addition to the above protection methods, we can also use brass patina for artistic creation. By controlling the oxidation conditions and environment, we can artificially form patina of different colors and textures on the surface of brass, thereby creating works of art with unique beauty.
For example, some artists use brass plates for carving and oxidation to create exquisite decorative paintings and screens.
6.Summary
The formation of brass patina is the natural result of the interaction between copper products and the natural environment. Through the introduction of this article, you not only understand the formation reasons and application value of patina, but also master effective removal and maintenance methods. This knowledge will help you better protect brass products, extend their service life, and make full use of the unique charm of brass patina.