Black anodizing is a surface finish commonly used on aluminum parts. This finish gives the metal surface an aesthetic appearance and improves resistance to wear and corrosion. These beneficial properties make black anodizing the preferred finish for companies in the automotive, aerospace, etc. sectors.
What is black anodizing?
Black anodizing is the electrolytic process of darkening a metal part, usually aluminum. However, before the darkening process, the aluminum is first subjected to a standard anodization to create a semi-porous layer of aluminum oxide.
This aluminum oxide layer is then colored black using electrolytic coloring with metallic salts, inorganic dyes or organic dyes.
It is important to highlight that not only aluminum can go through this process. Other metals, such as magnesium and titanium, can also be anodized black.
How does black anodizing work?
An aluminum component is first subjected to the standard anodizing process before being colored black by a layer of semi-porous aluminum oxide. Here are the steps for anodizing black aluminum.
1. Anodizing
Aluminum is anodized in the acid electrolytic bath and serves as the anode. Aluminum oxide is created through electrolytic conversion of the outer aluminum layer during anodizing.
Specifically, aluminum oxide is formed when oxygen molecules interact with the surface of aluminum in the bath during the passage of an electrical current. This oxide deposit penetrates the pores and covers the surface of the aluminum.
2. Coloring
After anodizing, the aluminum part would have a semi-porous surface. The purpose of these pores is to store the black dye used during the black anodizing process. Three dyeing techniques are used:
2.1 Electrolytic dyeing
In this coloring technique, the anodized aluminum part is immersed in another electrolysis bath. This bath contains heavy metal salts and a stainless steel electrode. Metal salts may contain tin, nickel and cobalt.
In this new bath, anodized aluminum serves as the cathode. When electrolysis begins, salt deposits occur in the pores of the anodized aluminum. Electrodyeing often creates pieces with unique color fastness.
2.2 Inorganic Coloring
These are non-water-soluble pigments, but have exceptional color fastness. An example of an inorganic salt used in this process is cobalt sulfide. Black anodizing with inorganic dyes creates pieces with colors that don't fade easily.
2.3 Organic dyeing
In contrast to inorganic dyes, organic dyes are soluble in water. To use organic dyes, dissolve them in warm water before adding them to the dye bath. Then, dip the anodized aluminum part into the paint bath to allow the organic paint to penetrate the semi-porous layer of the part. The darker the color, the longer organic dyeing takes.
3. Sealing
This is the final step in the black anodizing process and involves closing the pores in the aluminum oxide part to trap the black dye inside. Sealing is done in nickel acetate or acid bath and helps harden the coating. Curing prevents colors from bleeding or fading.
Materials and tools required for the black anodizing process
Certain materials are required for successful black anodizing. While the process from pre-to-post black anodizing is simple, the materials listed below take facets of the process into consideration. These materials include:
- Sulfuric Acid Electrolyte
- cathode
- Dye bath
- dye color
- DC power source to provide direct current
- Acid-resistant container for storing the electrolyte
- Degreaser to clean the tank after the process
- Conductor wire for hanging the aluminum part
Advantages of black anodized aluminum
1. Aesthetic appearance
Black anodized aluminum has a beautiful matte black finish for decorative and cosmetic purposes. It also offers a sophisticated powder-coated look.
2. Corrosion resistance
The anodized coating protects the aluminum from corrosion caused by oxidation and environmental influences. In demanding outdoor environments, it protects against damage caused by chemical compounds and moisture. With black anodizing, corrosion does not damage the base metal. Instead, it only affects the applied coating.
3. Value for money
An economical way to color aluminum is black anodizing. It offers better performance and aesthetics than powder coating and provides a better finish at a lower cost for aluminum anodizing. This process also uses cheaper dyes and tools.
4. Thermally stable
Properly sealed, black anodized aluminum is heat resistant and will not corrode or discolor. Furthermore, the stable coating does not burn and does not lose its adhesion even at permanently high temperatures, which would damage paint or powder coatings.
5. High color fastness
Black anodized components exhibit high color fastness when colored with inorganic or metallic salts. Therefore, they normally retain their color when exposed to UV (ultraviolet) radiation. In other words, black anodized metal parts do not fade easily, especially when using inorganic dyes.
Restrictions on black anodized aluminum
After exploring the benefits of black anodized aluminum parts, here are some limitations that can arise with this finish.
1. Difficult process
Successful anodizing requires thorough cleaning and pre-treatment of the aluminum, as well as careful anodizing and coloring treatment. Furthermore, inadequate processing and protection lead to poor results with uneven coating. Experienced operators and well-maintained machines are essential to a successful black anodizing process.
2. Fading
When used outdoors, gradual fading over time is inevitable. Black anodized items will also fade if organic dyes are used as the dye. The reason for this is that organic dyes do not tolerate UV light.
Additionally, poorly sealed or thin anodized layers may fade or lose color when exposed to strong light. High quality dyes and a strong seal are required for maximum light fastness.
3. Cracks on the surface of the material
With black anodized parts, cracking of the anodized coating may occur when exposed to temperature changes. The different rates of thermal expansion of the substrate and coating lead to cracking.
4. Material limitation
Black anodizing is not possible with all metals. It is only possible with aluminum, magnesium and titanium. Furthermore, not all types of aluminum alloys or series can be subjected to this process. Only works on Series 5, 6 and 7 aluminum, with Series 6 being the most popular.
Differences between black oxide coating and black anodizing
Black anodizing versus black oxide coating is a popular debate among manufacturers, but which is better? Let's take a look at both methods.
Black oxide coating, also known as blackening, is ideal for coating ferrous materials, stainless steel, copper alloys, copper, zinc and silver solder. It gives metals a slight resistance to corrosion, minimizes light reflection and improves aesthetics. Unlike black anodizing, black oxide coats materials through a chemical transformation. Depending on the temperature used, there are three black oxide coating methods: hot, medium and cold.
Black anodizing is an electrochemical process that makes aluminum more beautiful and durable. This process makes the surface of metals weather-resistant for a long time. For best results, you must ensure that all surfaces to be anodized are clean, free of contaminants and dry.
Application areas for black anodized surfaces
Architectural Parts : Exterior components are suited to the durable and attractive black finish through anodizing. These exterior components include windows, gutters, garage doors, trim, gutters, signage, doors, railings, etc.
Automotive Components : Black anodizing creates an attractive black coating that is weather and chemical resistant. The black anodized components of the car include speaker grills, pedals, shifter, grille grilles, etc.
Medical Care : Anodizing creates a black surface that is sterile, corrosion and impact resistant, making it ideal for making wheelchairs, canes, medical equipment, oxygen cylinders, etc.
Electronic housing : These include, among other things, smartphones, tablets, laptops and stereo systems. The cases benefit greatly from the lightweight, reusable and heat dissipation properties of anodized aluminum.
Industrial equipment and machinery : Anodized aluminum is suitable for industrial tools and machinery due to its excellent mechanical properties and chemical/abrasion resistance. It is used for finishing industrial machine components such as replacement valves, rollers, pulleys, seals and covers.
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Common questions
Does the black anodized surface fade, rust or scratch?
Anodized aluminum does not corrode, fade, scratch or rust easily. The term “rust” generally describes the development of a damaging, flaking oxide layer on ferrous metals. Anodizing aluminum creates an oxide layer that adheres to the surface and prevents the base metal material from oxidizing further.
How long does it take to anodize aluminum black?
The desired thickness of the anode layer, the staining method, and the sealing method after staining all affect the time it takes to anodize a part black. 2.5 microns are typically anodized in five minutes.
Therefore, it takes about 1 hour to anodize a black anodizing layer of at least 25 microns. Up to 20 minutes may pass during the staining process. Washing and rinsing processes are also taken into account in these times.
Can steel be anodized black?
Black anodizing of steel is not possible. There are three metals that can be anodized black: aluminum, titanium and magnesium.
