During production, the finishing process is always essential. It is usually the last stage of the production process. In this stage, you have to put some things into place to improve the finished product’s durability and aesthetics.
In Aluminum production, the finishing process is usually done by anodizing the aluminum material. This anodizing process is one of the most used processes for surface finishing of aluminum materials since it offers quality, durability, and beauty to the finished aluminum product.
However, since organic dyes are used for this anodizing process, they fade as time runs, usually when exposed to ultraviolet light. To maintain the aluminum product for an extended period, we must learn how to minimize the rate at which the anodized color fades. But before we go into the methods or practices usually put in place to reduce the fading of anodized colors, we need to learn about the anodizing process briefly.
What is Anodizing Process?
The anodizing process is an electrochemical finishing process done on an aluminum product that involves using a wear-resistant oxide layer in coating the surfaces of aluminum products. This process is done to improve the properties of the finished aluminum products ranging from durability, aesthetics, resistance to wear and corrosion, and generally the quality of the finished product.
The electrochemical process involves the aluminum part as the anode, an inert material as the cathode, and an acidic electrolyte. Below are the chemical reactions at the electrode;
- Anode: 2Al + 3H2O = Al2O3 + 6H+ + 6e–
- Cathode: 6H+ + 6e– = 3H2
- Resulting anodizing reaction: 2Al + 3H2O = Al2O3 + 3H2
The Different Types of Anodizing Process
There are three types of anodizing due to the differences in their coating intensity. The differences are caused by the electrodes, electrolytes, and energy used in the anodizing process. Listed below are the three types of the anodizing process;
- Type 1 Anodizing Process: Also known as the ‘light’ type, the type 1 anodizing process involves using chromic acid as the electrolyte and the aluminum part as the anode. This process consists of forming microscopic grooves on the surface of the anode, which is caused by the positive particles ejected from the anode when current is being passed to the electrolyte. These grooves are then oxidized to form an oxidized layer on the anode, the aluminum product. Aluminum materials that go through this process perform better in heat and have good wear and corrosion resistance than aluminum products with unfinished surfaces.
- Type 2 Anodizing Process: Unlike Type 1, sulfuric acid is used instead of chromic acid as the electrolyte. Since sulfuric acid is more potent, it tends to eject more positive particles on the surface of the anode compared to what is seen during the Type 1 process. Therefore, this makes the microscopic grooves formed to be more profound and increases the thickness of the oxide layer formed on the surface of the aluminium anode. These properties explain the ability of the Type 2 Anodizing Process to retain paint better than that of the Type 1 process.
- Type 3 Anodizing Process: This process is usually used for heavy aluminum products; it is the ideal process. It involves the introduction of higher voltage electricity and a strong acid, such as sulfuric acid.
Steps Involved in Minimizing Fading of Anodized Colors
When coloring an anodized aluminum surface, organic dyes are commonly used, and the fact that all dyes fade to an extent when exposed to UV light. Because of this fact stated, certain practices are put in place to minimize the fading of these anodized colors as it is seen as an unwanted effect.
Every dye has its unique chemical properties and resistance to ultraviolet light fading rates. The procedure of determining the UV rating of each dye is called the ISO 2135 procedure. The rating is usually numerical and ranges from 5 to 8, noted on a technical datasheet. In this procedure, a UV chamber is calibrated to determine the duration of an exposure cycle. The dyed panel is then exposed to this cycle for 75 per cent of the original color fades off. So, if a dye rates 6, the dye takes twice as many cycles to fade compared to the dye rated 5. A dye rated 7 took twice as many cycles to fade as the rated 6. This cycle continues; those that rate above eight are denoted by either 8* or 8+.
If the color you decide to use has dyes with higher ratings, this will ensure a higher fade resistance to your finished surfaces, as you can always change the dye to one with a higher rating. Black dyes are suitable for this as highly rated black dyes are very much available. Other decorative colors also have highly rated dyes, but the availability is somewhat limited.
When we talk about saturation, this means the amount of dye absorbed by a coating. Deeper shade surfaces tend to be the best as lighter shade surfaces fade faster. For a deeper shade surface, the dye is to be absorbed as much as possible into the surface’s pores. The most fade-resistant color coatings are usually thick and fully saturated with dye. Type 2 coatings are generally preferred during coloring since they have more porous surfaces. Type 3 coatings are usually with relatively low surface area, which will not allow adequate saturation compared to Type 2.
When dye baths are prepared and left for a while, these baths can be contaminated, reducing the quality of the dye bath, which can be referred to as the declination of the activity of the dye. The dye activity impacts maximum saturation. When it takes a more extended immersion period to achieve the color target, you will know that the dye activity has slipped, and full saturation might not occur. Therefore, it is required to maintain the dye activity to get maximum saturation. To maximize the dye activity, one needs to make sure the dye concentration, pH, and temperature are meant to be. The addition of dye buffers helps keep the pH steady and boost the activity of the dye.
The third step in achieving a high fade-resistant color is seeking a high-quality seal. Seals quality are usually impacted by silicate or phosphate contamination, temperature, pH, and immersion time. Generally, nickel seals are preferred for many dyes, and this is because nickel ions bond well to dye molecules, giving more protection against UV radiation.
In conclusion, the anodized coloring of finished aluminum products is a crucial aspect of aluminum products production as it helps to improve the quality of the aluminum products in terms of their aesthetics, durability and likes. However, due to UV light impact, anodized colors fade, but with the above steps listed, one can strengthen the span of the anodized color to minimize the fading.