What is Hard Anodizing？

Hard oxidation is known as hard anodizing. Hard anodizing of aluminum alloys is mainly used for engineering or military, and it is applicable to both deformed aluminum alloys and may also be used for pressure casting alloy parts components. Hard anodized films are required to be 25-150um thick generally, and most hard anodized films are 50-80um thick. Hard anodized films with a thickness of less than 25um are used for parts used in applications such as tooth keys and threads, and the thickness of the anodized film for wear resistance or insulation is about 50um, and under certain special process conditions, hard anodized films with a thickness of 125um or more are required.

However, it must be noted that the thicker the anodic oxide film, the lower the microhardness of the outer layer can be, and the roughness of the film surface increases. The tank solution for hard anodizing is generally a sulfuric acid solution as well as sulfuric acid with organic acids such as oxalic acid and sulfamic acid. In addition, hard anodizing treatment can be achieved by lowering the anodizing temperature or reducing the sulfuric acid concentration. For deformed aluminum alloys with copper content greater than 5% or silicon content greater than 8%, or high silicon pressure casting aluminum alloys, perhaps some special measures for anodizing should also be considered. For example, for 2XXX series aluminum alloy, in order to avoid the aluminum alloy being burned during the anodizing process, 385g/L sulfuric acid plus 15g/L oxalic acid can be used as the electrolytic bath solution, and the current density should also be increased to more than 2. 5A/dm.

What is Type 3 hard anodizing?

The following types of hard anodizing are widely used：　

(1) Sulfuric acid hard anodizing method. 　

(2) Oxalic acid hard anodizing method. 　

(3) Mixed acid hard anodizing 　　

Principle of hard anodizing: The principle of hard anodizing of aluminum alloy by simple sulfuric acid is not different from that of ordinary anodizing, but there are some attached reactions in case of mixed-acid hard oxidizing.

Reaction essence：　

1. Cathodic reaction: 4H2+4e=2H2↑ 　　

2. Anodic reaction: 4OH--4e=2H2O+2O↑ 　　

3. Aluminum oxidation: the oxygen precipitated on the anode is in the atomic state, which is more active than the oxygen in the molecular state and more likely to react with aluminum: 2A1+3O→A12O3 　　

4. oxidation in the anode film dissolution of the dynamic equilibrium: oxide film with the increase in time, the current increases and promote the thickening of the oxide film. At the same time, due to the duality of the chemical properties of (Al2O3), i.e. it is a basic oxide in acidic solution and an acidic oxide in alkaline solution. Undoubtedly, in sulfuric acid solution, the oxide film solution dissolves, and only when the generation rate of the oxide film is greater than its dissolution rate, the oxide film may thicken, and when the dissolution rate is equal to the generation rate, the oxide film no longer thickens. When the oxidation speed is excessively larger than the dissolution speed, the surface of aluminum and aluminum alloy parts are prone to generate the oxide film with powder.

What us hard anodizing production requirements?

In order to obtain a hard anodized film of satisfactory quality and to guarantee the required dimensions of the part, it must be machined according to the following requirements. 　　

Rounding of sharp corners 　　

The processed parts are not allowed to have sharp corners, burrs and other kinds of sharp angular places because of hard oxidation, the general anodizing time is very long, and the oxidation process (A1 + O2 → A12O3 + Q) itself is an exothermic reaction. And because the general parts of the angular places are often more concentrated parts of the current, so these parts are most likely to cause local overheating of the parts, so that the parts are burned. Therefore, all corners of aluminum and aluminum alloy should be chamfered, and the chamfered y radius should not be less than 0.5 mm. 　　

Surface finish 　　

After hard anodizing, the surface finish of the parts is changed, for the rougher surface, after this treatment can appear to be flatter than the original, and for the original finish of the higher parts, often after such treatment, showing the surface finish brightness is reduced instead, the reduction in the range of about 1 to 2 levels. 　　

Margin of part size 　　

Because of the high thickness of the hard oxide film, aluminum parts that need further processing or parts that need to be assembled later should be left with a certain amount of machining allowance in advance, as well as designated clamping parts. 　　Because of hard anodizing, to change the size of the parts, so in the machining, to predict in advance, the possible thickness of the oxide film and size tolerance, and then in the determination of the actual size of the parts before anodizing, so that after processing, in line with the specified tolerance range. 　　

Generally speaking, the increase in size of the part is roughly half of the thickness of the generated oxide film.

Special Tool 　　

Because the hard anodized parts in the oxidation process, to withstand very high voltage and high current, must make the fixture and parts can maintain extremely good contact, otherwise it will be caused by poor contact and breakdown or burn parts contact parts of the problem. Therefore, it is required to design and manufacture special fixtures for different shapes of parts, and the specific requirements of the parts after oxidation. 　

Parts local protection 　　

If in the same part, both ordinary anodic oxidation and hard anodic oxidation parts due to the part according to the degree of finish and precision to arrange the specific process. Usually the first ordinary anodizing, hard anodizing, the surface does not need to be hard anodized to insulate, insulation method with a spray gun or brush, will be prepared nitrocellulose or perhydroethylene glue coated on the surface does not need to be treated, insulation layer to be coated thin and uniform, each layer should be dry at low temperature for 30 ~ 60 minutes a total of 2 ~ 4 layers can be coated.