You may not be familiar with the term Black Anodizing. This term is specific to industries like satellite components, and optical equipment manufacturers. The black anodizing process involves the formation of an Aluminium oxide layer on the surface of Aluminium through an anodizing process followed by coloration. In the black anodizing process, the Aluminium oxide layer is made porous by hard anodizing or sulphuric acid anodizing, and the coloration process is carried out by the introduction of dyes or pigments into the porous oxide layer. The result of the black anodizing process is a blackened metal surface. However, the exact shade and the appearance of black can vary depending on factors like the type of dye used, the quality of the Aluminium substrate, and the anodizing process parameters.
What Metal Can be Anodized Black?
Apart from Aluminium, Magnesium, and Titanium can also be black anodized. However, Aluminium is the most preferred metal for black anodization. But not all grades of Aluminium can be black anodized. Aluminium series 5,6, and 7 are suitable for black anodization.
How Does Black Anodizing Work?
The black anodizing process first follows the standard type II or type III anodizing process. Whatever the process, type II or type III, the objective is to build a thin coating on the Aluminium part that needs to be black anodized. If the type II anodizing process is followed, the Aluminium is placed in a sulphuric acid solution to build up a layer of Aluminium oxide on the surface. The surface thickness of 25 microns or more is ideal for high-quality color fastness. The anodization process will turn the surface into a semi-porous structure. These pores will then absorb black dye to give the surface a black appearance.
The type III anodizing process also called hard anodizing forms an extremely hard, abrasion-resistant, porous oxide on Aluminium. The operating parameters of the type III anodizing process are different than the type II anodizing process. The type II anodizing process, for example, is carried out at room temperature while the type III is carried out at controlled temperature.
The anodizing process is followed by the dying process. There are three types of dying techniques available. These are
organic pigments:
organic pigments are one type of dyes. These dyes are extracted from animals or vegetables, and then dissolved in warm water. The solution containing organic dyes is placed in the dye bath followed by the immersion of black anodized parts. The organic dyes then filled the semi-porous structure of the anodic layer. Unlike lighter colors, black dying requires a prolonged time of dying. Parts dyed with organic dye are discouraged for outdoor use.
Inorganic Pigments
These pigments are insoluble in water. Coatings formulated with inorganic pigments exhibit superb resistance to light-induced fading. In the black anodizing process, parts are manufactured with Inorganic salts of Cobalt Sulfide.
Electrolytic Dyeing
parts first undergo a normal anodizing process followed by another electrolytic process. The second electrolytic process is carried out by immersing the parts into the solution of heavy metal salts. The anodic layer of heavy metal salt functions as a cathode and a stainless steel electrode is set up in the electrolysis bath. The resultant product of the second electrolysis process or metal salts deposited beneath the porous Aluminum oxide layer. The heavy metal salts are Cobalt, Nickel, or Tin which can produce different colors. The electrolytic dying process is protected by copyright rules and produces colors with superb color fastness.
Following the standard anodizing procedure, components undergo an additional electrolysis step known as electrolytic dyeing. This entails immersing the parts into a specialized electrolyte bath infused with heavy metal salts. Here, the anodized layer serves as the cathode while a stainless steel electrode is introduced into the bath. Within this environment, metal salts are systematically deposited into the micro-pores of the anodic layer. Utilizing nickel, cobalt, and tin salts enables the creation of a diverse range of colors through a unique and proprietary technique. This meticulous process ensures the resulting parts exhibit exceptional color fastness, meeting high-quality standards.
Black Anodizing Vs Black Oxide Coating, What is the difference?
Manufacturers have a contrasting view over the effectiveness of black anodizing, and black oxide coating process. A closer look at both of these processes is required to find which one is better.
One significant difference between black oxide coating, and black anodizing is the former process does not rely on an electrochemical process. The black oxide coating process is about covering materials with an oxide layer through chemical conversion. The black oxide coating is ideal for materials like stainless steel, copper, copper-build alloys, silver solder, zinc, and ferrous materials. The effects of black anodizing on metals are light corrosion resistance, decreased light reflection, and improved elegance. The process of black oxide coating varies depending on the temperature applied. These are cold, mild, and hot. The black anodizing, on the other hand, is specific for Aluminium materials. The process makes Aluminium more durable, and beautiful. The black anodizing process leaves the metal surface non-corrosive which withstands weather elements for a long time. The anodizing process produces the best result when the surface subject to anodized is clean, dry, and there is no trace amount of contaminants.
What are the benefits of black anodizing
While the standard anodizing makes materials more resistant to wear and corrosion, the black anodizing process takes the product quality to the next level. However, black anodizing offers a few additional benefits. These are
Heat Emmissibility
Due to the metallic character of Aluminum, components made from this metal have low emissivity. The black anodizing process is an effective way to dye back these components to increase their emissivity. The application of the black dying process can be extended to the manufacturing of satellite components and heat sinks.
No outgassing
The black anodized parts prevent any gas leakage. Utilizing this property, sensitive space-based equipment is protected with black anodized parts.
Color Fastness
When parts undergo the black anodizing process followed by dying with inorganic or metal salts, obtain excellent color fastness. It means they can retain color even exposure to UV radiation.
What are the limitations of Black anodizing
Surface cracking
The black anodized parts in the anodic coating layer develop cracks when they undergo a thermal cycling process. These cracks happen due to the uneven thermal expansion of the anodic coating and substrate.
Limited Materials
While there are different grades of Aluminum are available, not all grades can be black anodized. The aluminum series 5,6, and 7 are ideal for black anodized while the series 6 is the most preferred grade.
Color Fading
Exposure to UV light makes organic dyes unstable. Color will not last long if anodized parts are dyed with organic color.
Does black anodized aluminum scratch?
Due to the formation of a thicker layer of oxide, the black anodized parts do not corrode, rust, or scratch.
How thick is black anodizing?
A recommended black anodizing finish is 25 microns or more for optimal color fastness.
