Black Anodizing: A Comprehensive Guide
Black anodizing is a surface treatment that involves anodizing an aluminum part and then dyeing it black using a suitable dye. This process provides the benefits of standard anodizing, including improved wear and corrosion resistance, as well as additional benefits, such as light absorbency and heat radiance. In this article, we will delve deeper into the details of black anodizing, its benefits and limitations, and where it is commonly used.
What is Black Anodizing?
Black anodizing is the process of blackening an aluminum part by first performing standard anodizing to create a semi-porous aluminum oxide layer, followed by dyeing the oxide layer black using one of three dyeing techniques: organic dyes, inorganic dye, or electrolytic dyeing using metal salts. Anodizing creates a uniform, protective oxide layer on the surface of the aluminum, making it more durable and resistant to wear and corrosion. The dyeing process then adds a black color to the oxide layer, which can provide additional benefits, such as improved light absorbency and heat radiance.
What is the Purpose of Black Anodizing?
Black anodizing is performed to achieve an aesthetic or functional black color on an aluminum part while also improving the corrosion and wear properties of the underlying aluminum component. It is commonly used in applications where the black coating is beneficial, such as satellite components that rely on the black anodic coating to more efficiently radiate heat back into space, optical equipment that relies on the ability of black coatings to absorb any stray reflective light, and architectural trimmings in which the deep black colors are both aesthetically pleasing while also having excellent color fastness, and the surface finish provides improved abrasion resistance.
What Metals Can Be Anodized Black?
The metals that can be black anodized are aluminum, magnesium, and titanium. Aluminum is the most commonly used material for black anodizing, and only aluminum series 5, 6, and 7 can be anodized. It is important to note that not all grades of aluminum can be black anodized.
How Does Black Anodizing Work?
Black anodizing works by first anodizing the part using the standard anodizing process with either a type II or type III anodizing process. A minimum anodized thickness of 25 microns or more is recommended for optimal color fastness. Once anodized, the part will have a semi-porous surface structure. These pores are utilized to hold the black dye. There are three types of dyeing techniques:
Organic Pigments: These dyes are dissolved in warm water and then placed into the dye bath. The part is placed into the dye bath and the dye fills in the semi-porous surface of the anodic layer. Typically, black dyeing requires additional dyeing time compared to lighter colors. Organic-dyed parts are not recommended for outdoor usage.
Inorganic Pigments: These pigments are not soluble in water. Inorganic pigments produce coatings with excellent light fastness. For black anodizing, parts can be produced with inorganic salts of cobalt sulfide.
Electrolytic Dyeing: After normal anodizing, parts are placed in another electrolysis bath with a solution containing heavy metal salts. The anodic layer acts as the cathode, and a stainless steel electrode is placed in the bath. The metal salts are then deposited into the bottom of the pores. Nickel, cobalt, and tin salts are used to produce different colors, a proprietary process that produces parts with exceptional color fastness.
After dyeing, the pores on the anodic layer must be sealed to prevent color leaching. Sealing is typically done via hydration of the
aluminum oxide layer using either hot water or steam. The sealing process not only improves the colorfastness of the anodic layer, but also enhances its resistance to corrosion and abrasion. Sealing can also increase the thickness of the anodic layer, which can further improve the protective properties of the coating. Depending on the specific application and performance requirements, different sealing methods and conditions may be used. For example, some sealing processes may involve the use of chemicals or electrochemical treatments. Overall, the sealing step is a crucial part of the anodizing process, as it helps to ensure that the final product meets the desired performance and durability standards.
How to Achieve High-Quality Black Anodizing
Black anodizing is a specialized process that requires attention to detail and expertise to achieve high-quality results. Here are some tips to ensure that your black anodized parts meet your specifications:
Use High-Quality Aluminum
Not all grades of aluminum can be black anodized, and even among the grades that can be anodized, there can be variations in the quality of the aluminum. It is important to use high-quality aluminum with a uniform surface finish to ensure that the anodized coating is uniform and free from defects.
Properly Prepare the Surface
The quality of the anodized coating is directly related to the quality of the surface preparation. The surface of the aluminum must be clean and free from any oils, greases, or other contaminants that can interfere with the anodizing process. Proper cleaning and etching of the surface are critical to ensure that the anodic coating adheres properly to the aluminum substrate.
Use the Correct Anodizing Process
There are two types of anodizing processes commonly used for black anodizing: Type II and Type III. Type II anodizing is a conventional anodizing process that produces a thinner anodic coating, while Type III anodizing produces a thicker, more durable coating. For black anodizing, Type III anodizing is preferred because it provides better color fastness and wear resistance.
Optimize the Dyeing Process
The dyeing process is critical to achieving high-quality black anodizing. The dyeing solution must be properly formulated to ensure that it penetrates the semi-porous anodic coating and produces a uniform, deep black color. The dyeing process must also be carefully controlled to ensure that the parts are dyed for the correct amount of time and at the correct temperature to achieve optimal color and color fastness.
Seal the Anodic Coating Properly
After dyeing, the anodic coating must be properly sealed to ensure that the color is permanent and the coating provides maximum wear and corrosion resistance. The sealing process can be done either by hydration or chemical sealing. Hydration sealing is the most common method and involves immersing the parts in hot water to close the pores in the anodic coating. Chemical sealing involves the use of nickel acetate or dichromate solutions to seal the pores in the coating.
Black anodizing is a specialized process that can provide many benefits to aluminum parts, including increased wear and corrosion resistance, improved heat emissivity, and improved color fastness. Achieving high-quality black anodizing requires attention to detail and expertise in the anodizing process, dyeing process, and sealing process. By following these tips, you can achieve the desired results and produce high-quality black anodized parts that meet your specifications.