Why You Want to Anodize Aluminum
There are several good reasons to consider anodizing aluminum parts. When aluminum is anodized, a layer of oxide is produced and forms a layer on the surface of the aluminum. This layer is so thin that it does not even materially affect the dimensions of precision machined parts. Because the oxide layer is harder than aluminum, it protects the metal underneath, making it scratch-resistant and increasing corrosion resistance.
The oxide layer is also rougher than the surface of aluminum, so anodized aluminum can be painted or stained. In fact, you can add any color you like to your anodized aluminum parts. Coloring is accomplished by using dye during the anodizing process or by painting afterward.
Benefits of Anodized Aluminum Parts
Anodized aluminum parts offer many benefits both aesthetically and in terms of the mechanics of the part itself.
Visually, the effect is very impressive and the finish is permanent. It’s wear-resistant, so it’ll never scratch or fade, and it’ll never need touch-ups. Plus, it’s environmentally friendly.
Mechanically, the anodizing process makes the surface of the part extremely durable. Oxides are extremely hard and provide excellent protection against wear and corrosion. Thermal insulation properties are also excellent.
Overall, the process makes precision machined parts look better and last longer.
How to do aluminum anodizing
Aluminum is a relatively active metal with a standard potential of -1.66v. It can naturally form an oxide film with a thickness of about 0.01 to 0.1 microns in the air. This oxide film is amorphous, thin, porous, and has poor corrosion resistance. However, if aluminum and its alloys are placed in a suitable electrolyte, and the aluminum product is used as the anode, an oxide film will be formed on the surface under the action of external current. This method is called anodizing.
By selecting different types and concentrations of electrolytes and controlling the process conditions during the oxidation process, anodized films with different properties can be obtained, with thicknesses ranging from tens to hundreds of microns, etc., which have been significantly improved and improved. The electrolyte used in the anodization of aluminum and aluminum alloys is generally an acidic solution with medium solubility, with lead or aluminum as the cathode, which only conducts electricity. When aluminum and its alloys are anodized, the following reactions occur at the anode.
2Al ā> 6e- + 2Al3+
The following reactions occur at the cathode.
6H2O +6eā> 3H2 + 6OH-
At the same time, the acid chemically dissolves the aluminum and the resulting oxide film, and the reaction is.
2Al + 6H+ ā> 2Al3+ +3H2
Al2O3 + 6H+ ā> 2Al3+ + 3H2O
The growth process of the oxide film is a process of continuous formation and continuous dissolution of the oxide film.
The first section a (curve section ab): the non-porous layer is formed. Within a few seconds to tens of seconds at the beginning of electricity, a dense oxide film with high insulating properties is immediately formed on the surface of the aluminum, with a thickness of about 0.01-0.1 microns. This is a continuous non-porous film layer called as Non-porous layer or barrier layer, the appearance of this film prevents the passage of current and the continued thickening of the film layer. The thickness of the nonporous layer is directly proportional to the formation voltage and inversely proportional to the dissolution rate of the oxide film in the electrolyte. Therefore, the voltage in segment ab of the curve shows a sharp increase from zero to the maximum value.
The second section b (curve section bc): the formation of porous layer. As the oxide film forms, the dissolution of the film by the electrolyte begins. Due to the uneven formation of the oxide film, holes will be dissolved first in the thinnest part of the film. The electrolyte can pass through these holes to reach the fresh surface of the aluminum. The electrochemical reaction can continue, the resistance decreases, and the voltage decreases (the highest value is 10 -15%), a porous layer appears on the membrane.
The third section c (curve section cd): the porous layer becomes thicker. After about 20 seconds of anodization, the voltage enters a relatively stable and slowly rising stage. This shows that while the non-porous layer continues to dissolve to form a porous layer, new non-porous layers are constantly growing. That is to say, the formation speed and dissolution speed of the non-porous layer in the oxide film have basically reached a balance, so the non-porous layer The thickness no longer increases and the voltage changes very little. However, the formation and dissolution of the oxide film at the bottom of the hole did not stop at this time, they continued, and as a result, the bottom of the hole gradually moved into the metal matrix. As the oxidation time continues, the holes deepen to form pores, and the film layer with pores gradually becomes thicker. When the film formation rate and dissolution rate reach a dynamic balance, the thickness of the oxide film will not increase even if the oxidation time is extended, and the anodizing process should be stopped at this time. Aluminum and its alloys are anodized using direct current and alternating current in dilute sulfuric acid electrolyte to obtain a colorless and transparent oxide film with a thickness of 5-20 microns and good adsorption.
Sulfuric acid anodizing process is simple, the solution is stable, easy to operate, has a wide range of allowable impurity content, low power consumption, and low cost. It is almost suitable for the processing of aluminum and various aluminum alloys, so it has been widely used.