Black Anodized Aluminum Basics: Understanding Black Anodizing Process

In mechanical design, the black anodizing process is often performed on the surface of the aluminum part. Generally, black anodizing is a process that first anodizes an aluminum part and then dyes it black with an appropriate dye. Conducting black anodizing on an aluminum part can obtain an aesthetic or functional black color while enhancing its corrosion and wear resistance. This process is frequently used to coat parts for industries like aerospace and automotive, but it can also be applied to everyday objects such as appliances and furniture.

This article will dive into black anodizing, how black anodized aluminum is made, its benefits and limitations, and its typical applications. Let us get started!

#1 An Overview of Anodizing

Before we want to understand what is the black anodizing process, we should comprehend what is anodizing at first. Here is an overview of the anodizing process:

Anodizing is an electrochemical process that transforms a metal’s surface into a durable, high-performance aluminum oxide layer. This layer will not peel or chip since it is incorporated into the metal rather than applied on the surface. The protective layer makes it extremely hard and durable, improving its corrosion resistance. 

In simplest terms, anodizing is the tightly controlled enhancement of a naturally occurring phenomenon – oxidation. At extremely low temperatures, the aluminum is submerged in an acid electrolyte solution through which connected electrodes carry an electric current. This results in a hard coat surface with outstanding performance. Nonetheless, the metal remains porous, allowing it to be colored, sealed, or subjected to further processing if desired.

Anodizing may be done on various metals, including magnesium and titanium, yet it is best suited to aluminum. There are three typical types of anodizing process: Type I – chromic acid anodize, Type II – sulfuric acid anodize, and Type III – hardcoat anodize. 

Type I – Chromic Acid Anodize

Even though chromic acid produces the thinnest anodizing layer of the three types, the metal is equally protected against corrosion when properly sealed. Type I absorbs less color when dyed, hence not recommended for aesthetic applications. It is possible to dye it black, but the temperature of the chromic acid must be raised.

Type II – Sulfuric Acid Anodize

Type II is the most common method for anodizing in which sulfuric acid solution is used as the electrolyte instead of chromic acid. This type of anodizing creates thicker layers than Type I, leading to greater resistance to abrasion and other forms of wear. Furthermore, Type II has more color alternatives than Type I due to creating deeper pores in the oxide layer, allowing for greater dye absorption into the metal part.

Type III – Hardcoat Anodize

Hardcoat anodize produces a thicker and denser layer than sulfuric acid anodize. When a metal is subjected to corrosive or strenuous situations, hardcoat anodize is preferred because of its durable layer. Additionally, it can be utilized when advanced electrical insulation is required.

#2 What is Black Anodizing Process?

Black anodizing refers to an electrolytic coloring process that converts the surface of aluminum into a durable black oxide finish. By anodizing and dyeing aluminum, then sealing the color within the pores of the coating, black anodized aluminum achieves a premium appearance with strong performance properties. Anodized aluminum has enhanced wear and corrosion resistance, while black anodized aluminum has the additional benefit of being light-absorbing and heat-radiating effectively. 

Anodized metals are better suited for high-heat and high-light environments, with black anodized materials ideally suited for spacecraft, optical, and healthcare devices. In addition to functioning as semiconductors or when exposed to electricity, black anodizing boosts resistance to high heat and powerful cleaning products in each situation. 

#3 How to Black Anodize Aluminum?

Black anodizing works by performing the normal anodizing process on an aluminum part and then dyeing it black through a semi-porous aluminum oxide layer. The following key steps show how to black anodize aluminum:

Step 1: Anodizing – Anodizing uses an electrolytic process to convert the outer layer of aluminum into aluminum oxide. The aluminum part serves as an anode in an acid electrolyte bath. When an electric current is passed through the bath, oxygen molecules combine with the aluminum surface to form aluminum oxide. This oxide buildup penetrates the pores and coats the surface of the aluminum.

Step 2: Dyeing – The aluminum part will have a semi-porous surface structure once it has been anodized. The pores are used to store the black dye. There are three dyeing techniques: organic, inorganic, or electrolytic dyeing using metal salts. 

Organic Dyes: This technique involves dissolving the dyes in warm water before being added to the dye bath. The aluminum part is immersed in a dye bath, and the dye penetrates the semi-porous surface of the anodic layer. Black dyeing often takes more dying time than lighter hues. It is recommended not to use organic-dyed parts outdoors.

Inorganic Dyes: Generally speaking, inorganic pigments are not water-soluble. However, coatings using inorganic pigments provide exceptional light fastness. Light fastness means that the colors tend not to fade as quickly. Parts for black anodizing can be made with inorganic salts of cobalt sulfide.

Electrolytic Dyeing: The aluminum part is immersed in another electrolysis bath containing heavy metal salts after standard anodizing. The anodic layer serves as the cathode, and an electrode made of stainless steel is set in the bath. The metal salts are subsequently deposited at the pores’ bottoms. Using nickel, cobalt, and tin salts to produce various colors is a unique method that yields parts with exceptional color fastness.

Step 3: Sealing – The dyed part is sealed in an acid or nickel acetate bath. Sealing closes the pores in the aluminum oxide and traps the black dye within. It helps harden the coating and prevents color from bleeding out and fading. Proper sealing is essential for maximizing color durability.

#4 What Metal Can Be Black Anodized?

The most common metals for black anodizing are aluminum, magnesium, zinc, titanium, niobium, and tantalum. Of these, aluminum works the best for anodized coatings dyed fully black with the best color uniformity, light fastness, and hardness. But for high temperature or unusually harsh uses, titanium, niobium, or tantalum provide exceptional durability if processes can be optimized for dyeing and sealing these more difficult substrates.

Aluminum 

Aluminum is the most common metal for black anodizing. Aluminum alloys readily form a thick, porous oxide coating during anodizing that absorbs the black dye. Black anodized aluminum provides an attractive, durable finish at a lower cost than other metals.

Magnesium 

Magnesium can be anodized black, but the resulting coating is less durable and scratch-resistant than aluminum. The porous magnesium oxide requires extra sealing for water resistance and colorfastness.

Zinc 

Zinc can be black anodizing but with limited coating durability. The zinc oxide initially prevents corrosion but breaks down faster with exposure. Additional dyeing and sealing processes are needed for quality black results.

Titanium

Titanium is difficult to dye black owing to its dense, non-porous oxide fully. However, titanium produces an extremely hard, wear-resistant anodized coating. Stronger acids and longer anodizing times are necessary to open up the surface and enable black dyeing effectively. When successful, titanium provides unparalleled coating durability.

Niobium

Niobium can achieve an appealing black anodized finish using special electrolytes and dyes, especially those containing nickel sulfate, which helps absorption. Niobium can produce a matte black coating with superior durability for high heat and chemical resistance. 

Tantalum 

Like niobium, tantalum can be black anodized with modified processes. Tantalum typically produces a dark grayish-black tone due to the challenges of fully dyeing the metal black. It offers comparable durability to niobium for high-temperature tolerance.

#5 Benefits of Black Anodized Aluminum

Black anodized aluminum offers an attractive and capable finish for interior and exterior products and parts. There are several benefits to black anodized aluminum:

Decorative Appearance

Black anodized aluminum provides an attractive matte black finish for decorative and cosmetic uses. It offers an upscale, powder-coated look at a lower cost.

Durability

The anodized aluminum oxide coating is integrated into the underlying aluminum for maximum abrasion and scratch resistance. It resists wearing away, marring, and color fading for many years when properly sealed.

Corrosion Resistance

The anodized coating protects the aluminum from oxidation and environmental corrosion. It prevents chemicals, humidity, and salt spray damage in harsh outdoor conditions. Only the coating is affected by corrosion, rather than the base metal.

Thermal Stability

Black anodized aluminum can withstand high heat without discoloring or breaking down. The stable coating will not burn off or lose adhesion at sustained elevated temperatures that would damage paints or powder coating. 

Cost-effectiveness

Black anodizing is an affordable aluminum coloring method. It provides an integral colored finish lower cost than powder coating, yielding good performance and appearance. Less expensive dyes and tooling are required. 

Eco-friendly

The anodizing process is an environmentally responsible coating method. It does not produce VOC emissions, hazardous waste, or toxic byproducts like some coating and painting processes. Anodizing uses recyclable acids and alkaline solutions.

Improved Adhesion

The anodized coating chemically bonds to the underlying aluminum for unparalleled adhesion. There is no risk of the coating chipping, peeling, or flaking off. Only severe abrasion can wear through the oxidized layer into the base metal. 

Malleability

Black anodized aluminum remains somewhat malleable and shapeable depending on the metal alloy and coating thickness. Less ductile alloys and thicker films limit formability more. Anodizing creates a hard, brittle oxide, but the base metal retains flexibility. 

Simple Maintenance

Wear and blemishes caused by handling, installation, fabrication, and surface cleansing disappear. To restore the black anodized aluminum to its original look, rinse or clean it with water and mild soap. If there are stubborn residues, apply mild abrasive cleaners.

Lubrication

Although anodizing aluminum provides higher corrosion protection, lubricating is also simple. When you anodize aluminum, you generate deeper pores that allow the lubricating film or oil to be retained. Generally, standard aluminum has a non-porous, smooth surface that cannot retain lubricating film or oil.

Heat Emissivity

To improve emissivity, aluminum components are frequently dyed black; examples include heat sinks and satellite components.

Excellent Color Fastness

Black anodized parts have high color fastness, particularly if dyed with inorganic or metal salts. Therefore, they will generally retain their color when exposed to UV (ultraviolet) light.

No Outgassing

If adequately sealed after dyeing, black anodized parts are not subject to outgassing when exposed to a vacuum. And the absence of outgassing aids in protecting sensitive equipment, particularly sensitive space-based equipment.

#6 Limitations of Black Anodized Aluminum

Although black anodized aluminum has several benefits, it also has some limitations. Here are some of the main limitations to consider with black anodized aluminum:

Limited Materials

Not all series of aluminum can be black anodized. In general, aluminum series 5, 6, and 7 can only be anodized, with series 6 being the most commonly anodized.

Surface Cracking

When exposed to thermal cycling, the anodic coating on black anodized parts will develop cracks. The cracks are caused mostly by the substrate’s and coating’s different thermal expansion rates. 

Color Fading

If organic dyes are used, black anodized parts will lose their color because organic dyes are not suited for UV light exposure. Improperly sealed or thin anodic coatings can fade or bleed color on exposure to bright light. High-quality dyes and robust sealing are required for maximum light-fastness. Some gradual fade over time with outdoor use is inevitable.

Coating Durability

While an anodized coating is durable, it will eventually wear away with progressive abrasion and scratching. The coating is hard but still prone to damage from rough or prolonged use. Re-anodizing or touch-ups may be needed for worn surfaces.

Loss of Malleability

Thicker anodic coatings reduce the formability and malleability of the aluminum somewhat due to the hard, brittle oxide. More ductile alloys also limit formability less, but in general, anodizing slightly hardens and stiffens the material. 

Challenging Process 

Successful anodizing requires proper cleaning and pre-treating of the aluminum, then carefully controlling the anodizing and dyeing. In addition, inadequate or inconsistent processing results in poor, uneven coatings with little protection. Skilled operators and well-maintained equipment are essential. 

Masking Required 

Any areas not intended to be anodized must be masked to avoid unwanted oxide and dye formation. Masking is time-consuming, especially for complex parts. Failure to mask properly means re-machining anodized surfaces. 

#7 Applications of Black Anodized Aluminum

Black anodized aluminum is specified for a wide range of architectural, transportation, electronic, industrial, and commercial applications where an attractive and durable black finish is required. The properties of anodized aluminum, including abrasion/corrosion resistance, heat tolerance, and longevity, make it well-suited to products and parts exposed to environmental or high-stress conditions. Furthermore, black anodizing provides a premium coating for aluminum components and enclosures across many market sectors. 

The following examples are some applications of black anodized aluminum:

• Architectural Components – Windows, doors, railings, garage doors, gutters, downspouts, signage, decorative trim, etc. The decorative black finish and durability of anodizing are well suited to exterior building products.
• Electronic Device Housings – Laptops, smartphones, tablets, stereo equipment, etc. Anodized aluminum’s lightweight, repairable, and heat-dissipating properties are ideal for enclosures. 
• Automotive Parts – Wheels, grills, trim, pedals, shifters, speaker grills, etc. Anodizing provides an attractive black coating that resists environmental damage from chemicals and weather. 
• Military and Defense Equipment – Flashlights, cases, firearm parts, tactical gear, etc. Anodized aluminum’s hardness, temperature tolerance, and corrosion resistance suit it well for many defense applications.
• Sporting Goods – Gym weights, baseball bats, lacrosse sticks, ski poles, firearm sling mounts, etc. Anodizing allows for a durable, matte black finish that prevents slipping and resists abrasion.
• Furniture – Chair and table bases, light fixtures, shelving, cabinet hardware, etc. The decorative black coating provides an upscale, powder-coated type appearance for furniture and fixtures. 
• Appliances – Oven handles, stove trim, refrigerator grills and panels, coffee maker parts, etc. Anodized aluminum offers a premium black finish that can withstand high heat without damage. 
• Industrial Machinery and Equipment – Valves, rollers, pulleys, gaskets, caps, replacement parts, etc. Anodized aluminum’s superior mechanical properties and chemical/abrasion resistance suit it ideally for industrial tooling and machinery. 
• Medical Equipment – Walkers, canes, wheelchairs, surgical tools, oxygen tanks, etc. Anodizing creates a sanitary, sterilizable black finish resistant to chemicals and daily use impacts.  
• Restaurant Supplies – Grills, griddles, ovens, food shields, extractor hoods, counter trim, etc. In commercial kitchen environments, anodized aluminum provides a durable black coating able to endure high heat, grease, and frequent cleaning.

#8 Black Oxide Coating vs Black Anodizing: What are Their Differences?

Black anodizing and black oxide coating are hot debates in the manufacturing industry. And they are both used to provide a black finish to a metal surface. Then which is better between them? Here goes into the differences between black oxide coating and black anodizing.

So first, what is black oxide? Black oxide, sometimes called blackening, is a conversion coating used on ferrous metals, stainless steel, copper and copper-based alloys, zinc, powdered metals, and silver solder. Unlike black anodizing, black oxide coats materials in a chemical conversion. Black oxide can improve aesthetics, provide modest corrosion resistance, have a low coefficient of friction, and reduce light reflection. It’s important to note that black oxide should be treated with oil or wax for maximum corrosion resistance. Blackening can be achieved in different ways, including hot bath, cold coating, and mid-Temperature Bath.

As mentioned before, black oxide is a conversion coating, so black oxide coats materials in a chemical conversion. How about black anodizing? Black anodizing is an electrolytic coloring process. That’s to say. Black anodizing converts the surface of aluminum into a durable black oxide finish through an electrochemical process. It helps make aluminum more beautiful, durable, and wear and corrosion-resistant. This process makes the surface of metals resistant to weather elements for a long time, such as high-heat and high-light environments. Overall, black anodized aluminum achieves a premium appearance with strong performance properties. Other metals like magnesium, zinc, niobium, tantalum, and titanium can also be black anodized.

Summary

Black anodizing is a specific process that colors aluminum black through an electrochemical reaction. It provides an attractive black matte finish that is durable, weather-resistant, and corrosion-resistant. The anodized oxide layer also improves aluminum’s hardness and wear resistance, making black anodizing suitable for a wide range of architectural, industrial, and consumer applications where aluminum components require an aesthetically-pleasing black coating.

LEADRP provides various manufacturing services for all your prototyping and production needs. Our engineering and manufacturing team can use professional knowledge to maximize the benefits of your anodizing process and apply anodized aluminum colors. Don’t hesitate to contact us today to discuss your project in more detail.

References

Everything You Need To Know About Black Anodizing – From Acktar

What is Black Anodizing? – From Coastal Machine & Supply