Oxidation treatment-a brief description of hard anodizing and factors affecting the oxide film layer

Hard anodized films generally require a thickness of 25-150um. Most hard anodized films have a thickness of 50-80um and a hard anodized film with a film thickness of less than 25um. They are used for zero key applications such as tooth keys and spirals. For parts, the thickness of the anodized film for wear or insulation is about 50um. Under certain special process conditions, it is required to produce a hard anodized film with a thickness of 125um or more. However, the thicker the anodized film, the greater the thickness of the outer layer. The lower the micro-hardness, the greater the roughness of the film surface. The hard anodizing bath is generally sulfuric acid solution and sulfuric acid added organic acids, such as oxalic acid and sulfamic acid. In addition, a hard anodizing treatment can be achieved by lowering the temperature of anodized aluminum or reducing the concentration of sulfuric acid. For deformed aluminum alloys with a copper content of more than 5% or silicon content of more than 8%, or high-silicon die-cast aluminum alloys, some special measures for anodizing may be considered.

Hard anodizing uses DC power or DC and AC superimposed power. There are also many kinds of solutions, and it is more common to use sulfuric acid hard anodizing treatment.

When using the sulfuric acid hard anodic oxidation method, various factors affecting the oxide film layer should be considered:

The first factor:

The concentration of sulfuric acid oxidation treatment: 200-250 g / L is often used, and the relative density (at room temperature) of the bath is 1.12 to 1.15.

Second factor:

Water: Water is the main component of hard anodizing treatment. Generally, distilled water or cold water is used instead of tap water, because tap water contains chloride ions. When Cl-> 1%, its parts will corrode during oxidation. And white spots appear.

The third factor:

Temperature of oxidation treatment: Temperature is one of the important factors affecting the quality of the oxide film. The temperature is strictly controlled, and the oxide film is thickened, and the hardness is increased, smooth and dense.

The fourth factor:

Current density: Current is also one of the important factors affecting the quality of the oxide film, and it has a greater relationship with the formation rate of the oxide film and the organization of the oxide film. When the current density is too low, the formation rate of the oxide film is slow and the processing time is increased. On the other hand, when it is too high, the solution and the electrode will be overheated due to the Joule effect, which will increase the dissolution rate of the oxide film, reduce the hardness, and cause rough and loose powder .

Fifth factor:

Initial voltage and processing time:

The impact of the initial voltage and time of hard anodizing on the quality of the oxide film is also significant. If the initial voltage is too large, the current will increase, the Joule heat and the generation heat will increase sharply, and the dissolution rate will increase sharply.

For the oxidation treatment time, the thickness of the oxide film generally increases with the extension of the oxidation treatment time, but after a certain period of time, the oxide film does not actually increase without increasing the applied voltage. If you continue to extend the time, the hardness of the oxide film is low, and the powder becomes loose. In contrast, the oxidation treatment time is too short, the thickness of the oxide film is thin and it is not abrasion resistant.

Sixth factor:

Stirring of the oxidation treatment solution:

Stirring speed is related to the formation rate of oxide film (the quality of oxide film)