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xometry.proBlack anodizing refers to the process of anodizing an aluminium part and then dying it black using a suitable dye. This must not be confused with black oxide coating which is unsuitable for aluminium parts. Anodized aluminium has improved wear and corrosion resistance, while black anodized parts have the additional benefits of being light absorbent as well as being able to effectively radiate heat. This article will describe what black anodizing is, how it works, as well as the various pros and cons.
Black anodizing refers to the electrochemical process of blackening an aluminium part by first performing standard anodizing to create a semi-porous aluminium oxide layer. The oxide layer is then dyed black using one of three dyeing techniques: organic dyes, inorganic dye, or electrolytic dyeing using metal salts.
The metals that can be black anodized are aluminium, magnesium, and titanium. Aluminium, however, is the most common material used for black anodizing. It must be noted that not all grades of aluminium can be black anodized. Only aluminium series 5, 6, and 7 can be anodized.
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 coating thickness of 25 microns or more is recommended for optimal colourfastness. Once anodized, the part will have a semi-porous surface structure. These pores are utilised to hold the black dye.
There are three types of dyeing techniques, as described below:
After dyeing, the pores on the anodic layer must be sealed to prevent colour leaching. Sealing is typically done via hydration of the coating, which closes the pores off by reaction with the anodic layer. Wherever there is a pore, AI2O3H2O is formed; this is less abrasion-resistant than the normal aluminium oxide surface. Alternatively, chemical sealing can be put into effect, for example by using nickel acetate or dichromate sealing.
The time required to black anodize a part depends on the desired anodic layer thickness, the dyeing method, and the post-dye sealing technique. In general, it takes five minutes to anodize 2.5 microns. So to anodize the minimum recommended black anodized thickness of 25 microns will take around an hour. The dyeing process can take up to 20 minutes. However, these time frames also depend on the cleaning and rinsing processes.
Black anodized aluminium has all the benefits of standard anodized aluminium with the additional benefits afforded by the black colouring. The black anodized aluminium is used in applications where the black coating is beneficial. Here are some applications examples:
An anodized coating can last between 10 and 20 years depending on the application, the thickness of the coating, and whether or not the relevant sealing steps were performed after anodizing. When metal salts are used for dyeing, the colour will not fade. However organic dyes will fade over time.
The benefits of black anodizing include increased wear resistance and corrosion resistance. This finish ensures that anodized aluminium parts will not be prone to rust. Rust is typically used to describe the formation of a destructive flaking oxide layer on ferrous metals. When an oxide layer forms on aluminium, it adheres to the metal surface and prevents further oxidation of the base material.
However, black anodizing has a few additional specific benefits described below:
While black anodizing stands as a robust finishing process with notable advantages, it is important to acknowledge its inherent limitations. Navigating beyond its sleek black aesthetic and corrosion-resistant properties, certain constraints shape the boundaries of its applicability.
In conclusion, black anodizing emerges as a versatile and essential process in the manufacturing landscape. Its dual advantages of enhanced corrosion resistance and an appealing black finish make it a preferred choice in industries ranging from aerospace to robotics.
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