Anodizing aluminum forms an integral layer over the aluminum surface that protects it from corrosion and gives a pleasing texture.
You can further improve the aesthetics of anodized aluminum by adding colors to its surface layer.
Anodized aluminum colors include red, blue, black, green, gold, silver, purple, orange, pink, etc. Through the anodization process, these colors are permanently coated on an aluminum part surface and covered by a protective layer, improving their durability, stability, abrasion resistance, and surface finish.
This article discusses the color anodizing of aluminum by going through various color options, coloring techniques, and factors that determine the choice of color.
What's in this article?
- Anodizing color options available for aluminum
- Color anodizing aluminum
- Color matching on anodized aluminum
- Techniques to add color to anodized aluminum
- Color anodizing aluminum - The process
- Factors to consider when selecting the color for anodizing aluminum
- Final thoughts
- Frequently asked questions
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Color Options Available for Anodized Aluminum
What colors can you anodize aluminum?
You can apply a wide range of colors to the aluminum surface through the anodization process. Red, green, gold, bronze, blue, black, etc., are common aluminum anodized colors.
The underlying reason for the availability of such a wide choice of colors is the base metal (aluminum) itself.
Alloys of aluminum obtained from manufacturing processes, like the extrusion of billets, sheets, or coils, exhibit different hues of color.
The intensity of color depends on the thickness of the anodized layer, the concentration of the dye, immersion time, and the rating of the power source.
However, all these factors also affect the cost of anodizing, and it is important to regulate these factors to optimize the cost.
When the parts are anodized with sulfuric acid as an electrolyte, it produces brown and black, followed by the shades blue, red, bronze and gold.
Apart from these colors, there is clear anodizing, also known as silver anodizing.
It produces a clear, uniform film that exhibits a superior finish and enhances the durability and stability of the anodized product.
Anodized colors vary from light to dark depending on the type of alloy, the electrolyte's acid concentration, the coating's thickness, and the voltage flow.
About 80% of anodized industrial products adopt black, brown, and silver (clear) colors.
|Bronze and hues of red||Copper|
When the oxide layer formed on an aluminum stock is transparent, it is called clear-anodized aluminum. Because of this, it looks similar to regular aluminum with a silver finish.
Since they are transparent, a clear-anodized aluminum coating is hard to detect.
To know if an aluminum part is clear-anodized, measure the electrical resistance on the anodized surface using a voltmeter.
If the device displays a high resistance reading, the material is clear-anodized. Anodized parts are known to offer good resistance to electricity.
Due to this property, clear anodizing finds applications in electrical enclosures.
Further, anodized aluminum can also be multicolored. Feasible methods to carry out such coatings are still under study and have yet to become mainstream.
Following are the commercially available codes/method for custom color anodizing based on the thickness of the anodized parts.
|Category of color anodizing||Thickness of the layer|
|Protective and Decorative||0.0004"|
|Architectural Class II||0.0004" - 0.0007"|
|Architectural Class I||0.0007" or higher|
Protective and Decorative (less than 0.4 mils thick)
- A21 - Clear
- A22 - Integral Color
- A23 - Impregnated color (absorbed dye)
Architectural Class II (0.4-0.7 mils thick)
- A31 - Clear
- A32 - Integral Color
- A33 - Impregnated color
- A34 - Electrolytically deposited color (2-step)
Architectural Class I (0.7 mils and thicker anodic coating)
- A41 - Clear
- A42 - Integral Color
- A43 - Impregnated color
- A44 - Electrolytically deposited color (2-step)
Generally, standard colors like brown, gold, and black are chosen over custom-made colors due to the cost involved in maintaining specific shades in batch production.
The higher the volume of the anodized lot, the lower the cost involved in the process of coloring.
Color Anodizing Aluminum
Exposing aluminum to weathering forms a thin oxide film over its surface that prevents it from deteriorating due to corrosion.
Color anodizing aluminum is the process of artificially developing an oxide layer over the aluminum surface and adding color to enhance its functional and aesthetic properties.
This oxide layer exhibits excellent corrosion resistance and prevents the base metal from scraping.
Unlike other coatings, colored anodizing does not chip off or fade over time.
This is because, in techniques such as painting and plating, the layer is deposited over the material's surface.
In contrast, color anodizing involves seeping the dye/paint inside the porous oxide layer over the aluminum surface.
As a result, anodizing creates an integral layer over the base metal, which does not peel off under the action of force.
Since the color is a part of the anodized metal, it is abrasion- and fade-resistant, thus avoiding frequent recoat or maintenance.
Color Matching on Anodized Aluminum - Factors to Consider
While implementing the coloring of batch anodized parts, slight variations in the color scheme are possible.
To avoid any variation in colors, the following factors are given strident attention while anodizing.
Using dye of the same quality and concentration ensures consistency in color among different parts.
Color anodizing differs in quality and thickness of the coat based on the type of electrolyte and the input current chosen.
You can reduce the mismatch of colors among the parts to a greater extent by selecting the same input parameters of the process.
Dyeing or color anodizing the parts for the same duration plays an important role in maintaining the hue of the color.
The longer the process duration, the more uniform the coating thickness will be, showing proper color shade.
Therefore, it is important to regulate the duration precisely to get the same results every time.
Using the same sealing solution throughout the anodized lot will ensure preserving the colored anodized layer. Otherwise, the colors will vary.
Techniques to Add Color to Anodized Aluminum
Radiance to anodized products can be brought by adding colors to them.
Metallic salts or chemical compounds are deposited into the pores of the anodic layer to provide various colors.
However, adding color can only be done after anodizing the aluminum workpiece.
There are generally three types of techniques to add color to anodized aluminum.
In this method, the anodized part is immersed in a hot dye bath for about 15 minutes, followed by boiling the part for 30 minutes in de-ionized water.
Heating the dye increases the hue of the anodized part. However, it is not advised to heat the dye beyond 50℃.
The immersion coloring technique is mostly used for DIY applications.
In the electrolytic coloring technique, the anodized aluminum parts are dipped into colored electrolytic baths of metallic salts like tin, nickel, and cobalt.
Unlike plating operations, electrolytic coloring uses an AC power source rather than a DC power source. This results in a uniform coating with very few chances of defects.
Different tones of color are obtained based on the electric current's duration through the workpiece.
The longer the duration, the more the number of metal particles deposited in the pores of the anodized surface.
Generally, black and bronze color anodized aluminum parts are produced using the electrolytic coloring method.
Integral coloring is a combination of immersion coloring and electrolytic coloring methods.
It is a single-step process that yields a relatively thicker coat.
The integral coloring method is also known as the "duranodic process," as it enhances the anodized product's abrasion resistance.
Color Anodizing Aluminum - The Process
The electrolytic anodizing process is the most used technique. It is classified into three types based on the electrolyte and voltage used.
|Type I||Known as the light type|
Uses chromic acid as an electrolyte
|Type II||It is the most commonly used method|
Uses sulfuric acid as an electrolyte
|Type III||Known as the hard coat type.|
This method is adopted for heavy aluminum projects
Uses a high-voltage power source and sulfuric acid as an electrolyte
Color anodizing aluminum in a sulfuric acid solution (Type II) is the most common technique, and it consists of the following steps.
Step 1: Cleaning the Workpiece
The first step in the process involves cleaning the surface of the aluminum workpiece thoroughly to get rid of all the oils and grease deposits from the surface.
This enhances the deposition of oxide ions and helps produce a uniform oxide layer over the surface of the workpiece.
Apart from that, the moisture content on the surface can lead to white spots in the anodized layer, thereby reducing the quality of the workpiece.
Step 2: Etching the Workpiece
After cleaning the surface of the workpiece, it is immersed into an etching solution to eliminate natural oxides formed over the surface.
This process removes a fine layer of the workpiece, producing a uniform layer with no impurities or defects.
After the etching process, the part is rinsed again to remove any residues from the etching solution.
Step 3: Anodizing the Workpiece
After getting a smooth surface with no impurities, the workpiece is immersed in an electrolytic solution of sulphuric acid.
While preparing the electrolytic solution, add one part of sulphuric acid to three parts of water, not vice versa, because adding water to sulfuric acid creates an exothermic reaction producing hazardous gases.
The aluminum workpiece is connected to the anode to facilitate the deposition of oxide ions.
After connecting the anode, electricity is passed through the electrolytic solution, forming an oxide layer over the aluminum part.
The appearance of air bubbles near the anode hints the process is going well (formation of oxide layer).
If you do not observe any air bubbles, switch off the power supply and check if the connections are right.
Generally, the first layer of aluminum oxide is thin and uniform and is known as the barrier layer.
After the oxide layer's formation, the current's intensity is increased to create pores on the surface of the anodized layer, which facilitates the bonding of the colored layer.
Maintaining the current for a long duration (~ 45 minutes) changes the color of the anodized layer to brown and then to yellow.
After completing the anodizing process, the workpiece is thoroughly rinsed to remove any residue from the electrolyte.
Step 4: Adding Color
After completing the anodizing process, you can use a suitable method to add color to your workpiece.
The best-suited method for DIY projects is immersion coloring, whereas electrolytic coloring is suitable for commercial purposes.
However, if you want to maintain a natural color, you can skip the coloring phase, and subsequent rinsing and sealing processes can be performed.
Step 5: Sealing
Sealing is the most important step to be performed in the anodizing process. It improves the corrosion resistance of the anodic layer.
Immersing in a de-ionized water stream allows the aluminum oxide to hydrate, thereby increasing the volume and completing the pore’s closure.
The better the closure, the more integral the coating and the more desirable the material properties.
Factors to Consider When Selecting the Color for Anodizing Aluminum
Type of Application
Based on the application of the anodized parts, different colors are selected.
Anodized cooking utensils are coated black for better heat absorption and longer life.
Electronic gadgets need to have an attention-grabbing aesthetic appeal. So they come in a wide variety of colors and finishes.
Roofing with anodized aluminum needs good reflection properties and weather resistance. Hence light colors that exhibit good reflectivity are used for such applications.
Safety signs should adopt colors having a longer wavelength. Thus, they use red, yellow, and green colors.
The thickness of the Coating
The thickness of the color coat on the anodized surface affects the visual appearance of the part.
For example, electro-coloring can have a thickness of the order of 10 nm, which will affect the light reflection properties.
Furthermore, different shades, i.e., black, brown, grey, blue, etc., are possible with this method.
With the use of dyes, the thickness increase up to 50 µm. In addition, the use of metallic salts like cobalt and copper salts render blue and red hues.
Purpose of Anodizing
Anodizing performed solely to enhance the product's aesthetics can have different colors, irrespective of the thickness of the coating.
For decorative anodizing, the recommended layer of thickness is between 5µm to 25µm.
A thick oxide layer is required to protect the base metal from corrosive and abrasive agents.
For example, hard anodizing is the right choice for marine applications as it is continuously exposed to many corrosive agents.
In such cases, the thickness of the oxide layer must be between 25µm and 50µm.
Cost of the Process
The cost of coloring depends upon the type of color being selected and the number of workpieces.
Getting a unique color requires special dies that increase the cost of the process.
Batch production is preferable to lower the cost of coloring the anodized product when color uniformity is important.
A huge spectrum of colors is available for anodized aluminum parts. But, considering the application of the part and keeping the cost in control, it is best to choose standard colors like black, brown, and clear.
The thickness of the layer of color also plays an important role in determining the type of shades available for the workpiece.
Apart from that, the type of coloring technique implemented also affects the color outcome of anodized aluminum.
Therefore, it is important to select the right process for your application and maintain an appropriate oxide layer thickness.
Compared to other protective processes like painting and powder coating, the anodized coating process is long-lasting and reliable.
Frequently Asked Questions
Can you remove the color of an anodized part?
Yes, you can remove or change the color of an anodized part. You can use a grinding wheel to scrape out the thin layers of the coat or use a chemical stripper if the coat is relatively thick.
Why do small white spots appear on the finished anodized product?
The appearance of white spots on the finished anodized product shows the presence of a contaminant that prevented the dye from filling the pores. Therefore, the rinsing and cleaning process should be done with utmost care to have a uniform coating.
Can markings or engraving be done on color-anodized parts?
Yes, markings or engraving can be done on color-anodized parts. For example, laser engraving on color-anodized parts provides elegant markings. You can even physically scrap and mark on color-anodized aluminum parts with accurate tools.