Table of Contents
- 1 What is the basic aluminum anodizing process?
- 2 Different types of aluminum anodizing process
- 3 The steps of anodizing colors on aluminum
- 4 Anodized aluminum vs Painted aluminum
- 5 Benefits of anodizing aluminum parts
What is the basic aluminum anodizing process?
Anodizing is an electrochemical process for the passivation of aluminum. This process is difficult to compare with any other surface treatment, as aluminum plays an active role during anodizing. As part of this process, a thin layer of aluminum oxidation is created, fully integrated with the outer surface of the profiles, thus offering remarkable features, such as: aesthetics, shade stability, durability, ease of maintenance, health & safety.
The ultimate goal is to ensure the maximum possible wear resistance, while maintaining a nice aesthetic result. What happens (with physics terms) is the use of aluminum as an anode, while an acidic electrolyte is usually used as cathode. This phenomenon is called “cell electrochemical reaction”. More precisely, the whole process is performed by electrolysis (direct current), in a sulfuric acid bath, under strict conditions of control of the concentrations of chemical components, temperature and density of the foam. The result of electrolysis is the oxidation of the aluminum surface, in a completely controlled way. The anodic coating is transparent (like glass), while not being continuous and presenting pores.
The anodizing thickness corresponds to that of the aluminum oxide. Depending on the use and the environment in which the construction will be placed, a minimum allowed anodizing thickness is required. The final appearance of anodized aluminum products can be colorless (natural color) or colored, with specific aluminum anodized colors
Different types of aluminum anodizing process
The aluminum anodizing process involves four types, which use different electrodes, electrolytes and form of energy, resulting in a coating of different intensity, in each case.
Chromic acid anodize
This is the “light” type, using aluminum as anode and chromic acid as electrolyte, in the whole electrochemical process. Then, as the current flows through the chromic acid, positively charged ions are thrown from the anode, resulting in the formation of small grooves on the surface, which under the influence of oxygen, will cause the formation of an oxide layer. The ultimate goal, is to achieve resistance to thermal effects, as well as anti-corrosion properties. During chromic acid anodize process gray color appears, while when the aluminum is painted, color’s absorption is lower.
Boric-Sulfuric acid anodize
This is an option, which is preferred regarding the safety of employees’ health, while at the same time it is eco-friendly. Compared to the first type of anodizing, it is a more efficient method, and is usually applied to aircraft components.
Sulfuric acid anodize
This type of anodizing uses aluminum as an anode, but chromic is now replaced by sulfuric acid, the effect of which is more intense. The consequence of this change is a larger particle ejection than the corresponding type 1, ensuring a deeper groove and a thicker oxidized layer. Of course, the durability of aluminum products that undergo this type of treatment is positively affected.
That is the method of choice when talking about heavy aluminum products. In such case, sulfuric acid is used (as in the 3rd type), the difference, however, lies in the application of a higher voltage.
The steps of anodizing colors on aluminum
Anodizing is an electrochemical treatment in which the surface of aluminum is transformed, while artificial oxidation that protects it from corrosion is created. The formation of the anodic coating consists of the following steps: preparation, anodizing, coloring, and sealing.
The surfaces to be anodized are subjected to mechanical or chemical grinding treatment, with suitable abrasive materials or chemical reagents. Such a treatment aims to produce a glossy or matte surface appearance. Then, the aluminum parts are subjected to neutralization.
This process uses the electrolytic principle (passing a direct current through a sulfuric acid bath), under strictly controlled conditions, in terms of chemical concentrations, temperature, current density. The results of electrolysis, under controlled conditions, cause oxidation of the aluminum surface. The anodic coating is transparent and looks like glass. In addition, the anodic coating is not continuous and shows pores on its surface.
Τhe coloring with anodized aluminum colors is achieved by depositing colored elements in the pores of the anodic coating (electrolytic coloring). This process takes place after the anodizing phase, and before sealing. The coloring is done by immersing the products in a bath containing metal ions (e.g. tin, cobalt, nickel). The metal ions are deposited in the pores of the anodic coating. Thus, any color can be achieved on the surface of aluminum.
Sealing the pores is one of the main processes, to ensure proper protection of aluminum. At the points of the pores, the thickness of the anodizing is very small (2-3 microns), and therefore the protection at these points is weak. The sealing process is achieved by hydrating the aluminum. The pores are sealed by the resulting dilation. In addition, if the electrolytic dyeing process has preceded, the dyes are trapped inside the pores and thus the color stability is ensured over time. The pigments are visible because, as already mentioned, the anodic coating is transparent like glass.
Sealing the pores is achieved in two ways:
1. Hot sealing is done by immersing the aluminum pieces in deionized water of 96oC, at least for a time of 2min, for each small anodizing.
2. Cold sealing is done by immersing in a bath containing nickel fluoride salts, under strictly controlled conditions of concentrations of ingredients, pH, temperature (25-30oC). The completion of the sealing is achieved by keeping the pieces in a bath of 60oC sulfate, for a time of 0.8-1.2min, for each small anodizing.
Anodized aluminum vs Painted aluminum
Anodizing and electrostatic painting are key determinants of aluminum, especially when used in architectural applications. Anodizing and electrostatic painting, on the one hand, improve the anti-corrosion behavior of aluminum. On the other hand, they do not achieve the decorative appearance required in architectural applications.
One of the most commonly posed questions is: “which method is best”? No direct answer can be given. Both methods have advantages and disadvantages. The aluminum manufacturer should know and expose the particular characteristics of each method to the customer, who has the ultimate responsibility for the choice. In order for the customer to be properly guided, he must always keep in mind the following:
* Both methods, anodizing and painting, are unreservedly recommended for the surface treatment of aluminum intended for architectural applications.
* Aluminum manufacturers, who are responsible to their customers for delivering the right products, should try to use products that have been processed in Qualanod-certified anodizing plants, or Qualicoat-certified paint plants.
* The assembly and handling of anodized or painted aluminum profiles should be done according to the rules known to all, but in a few cases; e.g. stainless steel screws must always be used during assembly or installation.
* Aluminum, as material, has many advantages but it certainly has disadvantages, too. However, it has been proven that aluminum is the most suitable material for architectural applications. Any problems that occur in its constructions, after research and the daily experience gained, always find their solution and are treated effectively.
Benefits of anodizing aluminum parts
Aluminum anodizing is a process that ensures the benefits of durability and high performance. You can see a relevant list below.
Excellent aesthetic result
Using aluminum anodized colors of all shades (bronze, brown, black, etc.) with a perfect finish on the metal surface of aluminum, ensures an excellent aesthetic result. There is always the possibility of differentiation into darker or lighter shades. Also the surface can be glossy or matte, depending on the customer’s desire (paint thickness up to 20-22μm).
Durability and cost efficiency
Aluminum anodized products are corrosion resistant and have low maintenance costs. As a result, they are more energy efficient, while in the long run, they amortize all their initial processing costs, due to the absence of the need for frequent maintenance or replacement.
Color stability and aesthetics
Products processed with anodized aluminum colors are resistant to harmful ultraviolet sunlight, as they do not degrade or crumble. In addition, a variety of colors and gloss is offered, allowing the starting material (aluminum) to retain its classic metallic appearance.
Ease of maintenance
The durability of the anodized surfaces should not prevent their frequent gentle cleaning, which in fact enhances it. Surfaces that have undergone this electrochemical treatment are impervious to scars, structural wear, and the use of surface contaminants.
Health and safety
It is a method that does not affect the health of the professionals who apply it, due to the fact that it is essentially intended to enhance the natural oxidation process. Therefore, no harmful toxic substances or by-products are produced.
Anodized aluminum is eco-friendly and completely recyclable.
The natural beauty and glow of the anodizing has proven to stand the test of time. Even after 20 years, with proper maintenance, your installation will look as nice as the day you installed it. A simple rinse with lukewarm water and a mild liquid soap is sufficient, as the anodized aluminum can be cleaned regularly. For small parts, use a soft cloth scrub. When there is greasy residue, you may need to use a soft cloth dampened with white spirit to clean it. After cleaning, it is necessary to rinse well with water, especially when there are notches, to ensure that all residues have been removed. The frequency with which the cleaning could be done, should be between once a month up to once every six months, depending on the application and the environment where the installation is exposed.