All You Need to Know About Hard Anodising

In this article Modo Rapid will introduce hard anodizing in terms of process methods, principles, process requirements, processes, characteristics, and process influencing factors. We hope it will be useful for product development, mechanical designers and those interested in hard anodizing.

Hard anodising aluminum part

what is Hard Anodising?

Hard anodizing is an electrochemical process that forms a hard, protective oxide layer on the surface of aluminum. The oxide layer is much harder and more corrosion resistant than plain aluminum and can withstand greater mechanical stress than conventional anodizing. Hard anodizing also increases the wear resistance of the part and is ideal for applications where high levels of abrasion or corrosion resistance are needed.

The main purpose of hard anodizing of aluminum alloys is to improve various properties of aluminum and aluminum alloys, including corrosion resistance, wear resistance, weather resistance, insulation and adsorption. It is applicable to both deformed aluminum alloys and may also be used for die-cast aluminum alloy parts.

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Process Method
Process requirements
Process characteristics
Process influencing factors


Hard anodizing is the full name of hard anodizing treatment. Hard anodized film generally requires a thickness of 25-150um, and most hard anodized The thickness of hard anodic oxide film is 50-80um, and the thickness of hard anodic oxide film with film thickness less than 25um is used for parts used in applications such as tooth bonding and screwing, and the thickness of anodic oxide film for wear resistance or insulation is about 50um. Under some special process conditions, it is required to produce hard anodic oxide film with thickness of 125um or more, but it must be noted that the thicker the anodic oxide film is, the microhardness of its outer layer can 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 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 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.

Hard anodising aluminum part

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Process Methods

There are many electrolytic methods for hard anodizing, such as sulfuric acid, oxalic acid, propylene glycol, sulfosalicylic acid, and other inorganic salts and organic acids. The power sources used can be divided into DC, AC, AC/DC superimposed, pulse and superimposed pulse power sources, etc. Currently, the following hard anodizing methods are widely used.

(1) Sulfuric acid hard anodizing method;

(2) Oxalic acid hard anodizing method.

(3) Mixed acid hard anodizing

Among them, the sulfuric acid method is the most widely used hard anodizing method.

Hard anodising workshop


Hard anodizing principle

There is no essential difference between the principle of pure sulfuric acid type aluminum alloy hard anodizing and ordinary anodizing, but there are some additional reactions in case of mixed acid type hard anodizing. Reaction essence

1 . Cathodic reaction:

4H+ +4e=2H2↑

2. Anodic reaction:


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 reacts more easily with aluminum:


4 . Oxidation in the dynamic equilibrium of the anode film dissolution: the oxide film increases with the increase of energizing time, and the current increases to promote the oxide film thickening. 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 is likely to 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.

Hard anodizing principle

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Process Requirements

Hard Anodizing Process Requirements

In order to obtain a good quality hard anodized film with the required dimensions of the part, it must be processed according to the following requirements.

Folded Sharp Corner Rounding
Sharp corner beveling

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 (Al + 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.

custom hard anodizing aluminum part

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Folding surface finish
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 high finish parts, often after such treatment, the surface finish brightness is reduced, the reduction in the range of about 1 to 2 levels.

Hard anodising aluminum part

Folding size margin
Dimensional allowance of the part

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.

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Folding Fixtures
Specialized Fixtures

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

Folding local protection
Local protection

For example, in the same part, both ordinary anodic oxidation and hard anodic oxidation parts should be arranged according to the finish and precision of the part. Usually the first ordinary anodizing, in the hard anodizing, the surface does not need to be hard anodized to insulate, insulation method with a 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.

Process Features

Hard anodizing process characteristics

The electrolyte for hard anodizing is electrolyzed at a temperature of about -10℃~+5℃. Since the oxide film layer generated by hard anodizing has a high resistance, it will directly affect the current strength of the oxidation. In order to obtain a thicker oxide film, it is necessary to increase the external voltage, the purpose of which is to eliminate the influence of high resistance, while maintaining a certain current density, but the current will produce intense heat generation, coupled with the generation of oxide film will release a lot of heat, so that the temperature of the electrolyte around the part rises sharply, the temperature rise will accelerate the dissolution of the oxide film, so that the oxide film can not be thickened. In addition, the heating phenomenon is most serious in the contact between the film and the metal, if not solved in time, the local surface of the processed parts will be burned due to the rise in temperature.

The solution:is to use a combination of cooling equipment and agitation. The cooling equipment makes the electrolyte forcibly cool down, and the stirring is to make the temperature of the electrolyte uniform throughout the tank, in order to facilitate the obtaining of higher quality hard oxide film.

Hard anodising aluminum part

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Process Influencing Factors

Influence of various factors on the hardness and growth rate of oxide films.

The ability to produce a good quality hard oxide layer on the surface of aluminum and aluminum alloys depends mainly on the concentration of the electrolyte composition, temperature, current density, and the composition of its raw materials.

Folded Electrolyte Concentration
When using sulfuric acid electrolyte for hard anodizing, it is generally within the concentration range of 10%~30%. When the concentration is low, the hardness of the oxide film is high, especially for pure aluminum is more obvious, but the exception is for aluminum alloy with high copper content (CY12). Because the aluminum alloy with higher copper content is easy to generate the compound of CuAl2, this compound dissolves faster in oxidation, and it is very easy to burn the aluminum parts. Therefore, it is generally not suitable to use low concentration of sulfuric acid electrolyte, and must be oxidized in high concentration (H2SO4 in 300~400g/L) or treated by AC/DC superposition method.

Hard anodising aluminum part

Effect of folding temperature on film layer
The temperature of electrolyte has a great influence on the wear resistance of the oxide film. Generally speaking, if the temperature decreases, then the wear resistance of the anodic oxide film of aluminum and aluminum alloy increases, which is caused by the decrease in the dissolution rate of the electrolyte for the film, in order to obtain a higher hardness of the oxide film. We should master the temperature within ±2℃ for hard anodizing treatment.

custom hard anodizing aluminum part

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