Why Aluminum Wheel Spacers (or Adapters) Are Anodized? (I)
In the market, there are alloy aluminum wheel spacers (or wheel adapters) are just with the machined surface, but most are treated further by anodizing.
Aluminum and its alloys are easy to be oxidized rapidly when exposed to the normal atmosphere. Thin oxide film will be formed quickly on the surface of the aluminum and its alloys because of the natural inactivation. Comparatively, it takes the longest time before the industrial pure Al (99.5% purity) is oxidized. With the adding of the other elements, the oxidization takes less time. This is a rule of corrosion resistance of Al. Because of the high content of Fe and Si, corrosion could be even observed by eyes on 7075 (AlZn5.5MgCu) after machining under some specific circumstances. 0.5nm to 10nm thick oxidized film can be formed on alloy 6061 (AlMg1SiCu) in 3 to 5 days in the dry atmosphere. And the thickness of this kind of natural and transparent amorphous oxide film could even reach 200nm with longer time.
Since naturally the oxidized film can be formed, the question is why still to make alloy aluminum wheel spacers (or adapters) anodized, not to leave the surface be just after CNC machining?
The answer is that the protection by the natural oxidized film is too limited. Firstly, the natural oxidized is too thin. Normally the thickness of the natural oxidized film is 4nm to 5nm and it could not get thicker when coming to the max. thick of 200nm; Secondly, this nanometer level film is porous, unconsolidated, uncontinuous and uneven, by which the surface could not be well sealed. In additional, the film is not crystal structure and the hardness (refers to the cross section micro-hardness) is insufficient. Therefore, the natural oxidized film could not really protect the substrate under it well.
Anodizing can make up the deficiency mentioned in the above. According to the national standard, anodizing or anodic oxidation is defined as the process of forming oxide film on the surface of aluminum and its alloy (when used as anode) by electrolysis under the action of external current in electrolyte, which is called electrochemical oxidation or anodic oxidation.
1. Electrolyte (bath solution). The electrolyte is normally acidic and there are various kinds such as sulfuric acid, oxalic acid, chromic acid, phosphoric acid and their mixtures. For wheel spacers (or wheel adapters), normally sulfuric acid is used as electrolyte for anodizing.
2. In the anodizing process, both DC and AC power are adopted, but the most is DC. The characteristics of oxide film produced by different currents are also different. Of course, there are also constant pressure and constant flow processes.
3. Oxide film. The oxide film produced in anodic oxidation is not the cover of external elements on the surface of aluminum products, but the electroosmosis phenomenon appears on the surface of aluminum under the influence of current and bath solution. Under the action of electroosmosis, the oxide film grows towards the interior of the substrate and gets thicker. Its advantage is that it has little impact on the original size of the finished product.
Anodizing brings below advantages comparing with the natural oxidization.
1. Better corrosion resistance. The film produced by anodizing is thicker and more uniform than that formed by natural oxidization. Sufficient thickness is the guarantee of natural erosion resistance. When the film thickness reaches 15-20μm, it can be free of red spots and rust for 100 hours during the neutral salt spray test. And the uniformity of the film by anodizing forms a complete closed loop on the surface of the product, which can well avoid the spot corrosion that is most likely to occur in aluminum alloy corrosion.
2. Better hardness and wear resistance. The hardness of aluminum anodic oxide film is much higher than that of aluminum substrate. The hardness of aluminum substrate is generally around HV100, and the hardness of ordinary anodic oxide film is around HV300, while the hardness of hard anodic oxide film can reach around HV500, and the hardness of micro arc oxide film is HV800-1000 or even higher. Although wear resistance and hardness are not the same, there is usually a certain correlation. The harder and more wear-resistant surface ensures the possibility of the film to exist for a longer time.
Here, the hardness discussed refers to the micro-hardness of the cross section, which is different from the conventional hardness (Brinell hardness HB) adopted in testing the hardness of aluminum and its alloys. In addition, the hardness of the film by anodizing can only improve the surface hardness of the aluminum product, but cannot improve the overall hardness of the substrate.
3. Decoration. Aluminum anodizing film can not only maintain and protect the metallic luster of aluminum surface, but also can be dyed by various processes, thus presenting the various colors.
Except for the above stated three points, anodic oxidation also has such characteristics as below but little to do with the wheel spacers (or wheel adapters).
1. Electrical (thermal) insulation (the breakdown voltage of specially prepared high insulating oxide film can reach 200V/flange).
2. Various functional materials can be obtained by depositing various functional particles in micropores based on the porous nature of the oxide film.