Everything you need to know about Anodizing Aluminum

How are anodic coatings applied?

The anodizing processes involve submerging the aluminum component into an acid electrolytic bath and then passing an electrical charge through the medium. A cathode is located on the outside of the tank, while the aluminum serves as an anode (hence the term anodizing). As the current moves through the bath, oxygen ions are released from the acid electrolyte and join with the aluminum substrate producing the aluminum oxide layer. It is important to note that, unlike a paint or plating process, the anodic layer is fully integrated into the underlying substrate actually forming into and out of the substrate at the same time.

Types of Anodizing Process

There are three types of the anodizing process, each being different in terms of coating intensity. The difference is due to the electrodes, electrolytes, and energy used in each process.

· Type 1 Anodizing Process
The type 1 anodizing process is also known as the “light” type, and it involves the use of chromic acid as the electrolytes and the aluminum part as the anode. When current is made to pass through the electrolyte, positive particles from the anode are ejected, forming microscopic grooves on the surface. The microscopic grooves are then oxidized to form an oxide layer. Compared to normal aluminum products without finishes, products made via this process are better in heat and corrosion resistance.

· Type 2 Anodizing Process
The type 2 anodizing process uses sulfuric acid instead of chromic acid. Sulfuric acid is more potent, leading to a better ejection of positive aluminum particles than what is seen in type 1’s. Therefore, the microscopic groove formed is deeper, and the oxide layer is thicker. These two properties are responsible for the better paint retention properties exhibited by Type 2 aluminum parts.

· Type 3 Anodizing Process
The type 3 anodizing process is the ideal one for making heavy aluminum products. Unlike the other types of the anodizing process, it uses a higher voltage and a strong acid (sulfuric acid).

How do you color anodized aluminum?

There are four ways to color anodized aluminum:

Dye: The freshly anodized part is immersed in a liquid solution that contains dissolved dye. The porous anodic coating absorbs the dye. The intensity of color is related to the thickness of the anodic film, the dye concentration, immersion time, and temperature, among other things.
Electrolytic Coloring : This method involves the immersion of the aluminum parts in a solution containing some metallic salts. On filling the pore, they provide a coating strong enough to resist UV rays. However, there is a limitation on the number of anodizing colors you can use, with bronze or black color being the most common.
Integral Coloring: This so-called one-step process combines anodizing and coloring to simultaneously form and color the oxide cell wall in bronze and black shades, while more abrasion resistant than conventional anodizing.
Interference Coloring: An additional coloring procedure, recently introduced, involves modification of the pore structure produced in sulfuric acid. Pore enlargement occurs at the base of the pore. Metal deposition at this location produces light-fast colors ranging from blue, green and yellow to red. The colors are caused by optical-interference effects, rather than by light scattering as with the basic electrolytic coloring process.

Anodizing color VS Painting color

When deciding on an anodized or painted finishes there are a few things to consider. One thing to factor is color choice. Anodizing has very limited color options. Painting has a clear advantage over anodizing as there is limited color availability (clear, champagne, light bronze, medium bronze, dark bronze, and black). Paint has an almost unlimited color pallet and with few exceptions, you can match almost any color.

Anodizing is also very difficult to maintain color consistency, otherwise known as the checker boarding effect. To properly anodize material with consistency you need proper equipment and expertise and even that is no guarantee. For example, different lots of aluminum can anodize in different shades. The checker boarding effect is visible when the final product is installed on the job site.These different shades are clearly visible from ground level. Overall, paint is much easier to maintain color consistency. With the wide variety of color options paint allows you to differentiate your project from others by creating your own signature look.

Painted and anodized products have their distinct advantages and disadvantages. When trying to determine what application fits your job the best please contact IN3DTEC and we will help you through the process to get the best finish possible.