Aluminum alloys are the relatively soft metals of various alloys. Due to their light weight, aluminum alloys are ideal for machining. Although it has the advantages of light weight and easy processing, it is difficult to use for mechanical parts that require high strength. That’s why we use hard anodized coatings to strengthen aluminum metal. This process improves hardness and corrosion resistance. And this effect can be properly controlled. Therefore, the application of aluminum alloys can also be extended to a variety of different mechanical parts.
What Is Hard Anodizing Finishing?
Hard coat anodizing is a surface treatment. We use organic acids in electrolytic cells to create aluminum oxide films. The hard anodizing process is characterized by a harder and better wear resistance than films treated by conventional methods. Generally, no sealing treatment is carried out. If corrosion resistance is required, the entire sealing process can be used, but with reduced wear and corrosion resistance.
The Thickness of Hard Anodized
Generally, the thickness of hard anodized film is required to be about 25um to 150um. The maximum thickness of hard anodized film is about 50um to 80um. However, some machine parts require a smaller thickness of 25um. For example, helical and spline gears. Parts requiring good insulation or abrasion resistance benefit from the 50um film thickness. Thicknesses of 125um or greater are ideal for special applications. But this does not mean that the thicker the hard anodized film, the better. But the thicker the anodized film, the lower the microhardness of the outer layer, and the rougher the film surface.
The Difference Between Hard Anodized And Anodized
1. Thickness and appearance
The significant difference between anodizing and hard anodizing is thickness. Hard-anodized aluminum is thicker than standard anodized aluminum, which gives the outer layer of hard-anodized aluminum components a higher scratch point resistance. It also has a more uniform surface than standard anodized aluminum.
This step is important for standard anodized aluminum. Usually, small holes are formed in ordinary aluminum. Pores need to be repaired for better results. However, hard-anodized aluminum is not required, which has a thicker oxide layer that increases the wear resistance of the part.
3. Creation condition
At thicknesses over 25 microns, hard anodized overlays are delivered at lower temperatures and higher current thicknesses.
Standard anodizing should be possible in the electrolyte of chromium corrosion or sulfuric acid corrosion, while hard anodizing mainly uses sulfuric acid corrosion arrangement and sulfuric acid corrosion with addition of natural etchants, such as oxalic acid etchant, sulfamic acid etchant, etc.
Hard-anodized aluminum products are suitable for applications and conditions that require an uncompromising, wear-resistant surface, such as hydraulics, cylinders, some food cookers, frying pans and structural exteriors. In modern or commercial applications, hard anodizing is easier to track than in shopping supplies. Standard sulfuric acid anodized aluminum is suitable for the engineering, aerospace and automotive industries.
Since hard anodized aluminum is non-conductive, areas that need to be conductive will require selective masking. Anodizing is integrated with the part and can be used in salt/corrosive environments. Regular anodizing has good wear resistance, while hard anodized aluminum products have excellent wear resistance.
Processing Methods of Hard Anodizing Coating
There are many methods of hard anodizing electrolysis. For example, sulfuric acid, oxalic acid, propylene glycol and mineral acids. We classify power sources as DC, AC or AC/DC. Other forms are superimposed, pulsed and superimposed pulsed power. Below are the widely used types of hard anodizing.
Sulfuric acid hard anodizing
Oxalic acid hard anodizing
Mixed acid hard anodizing
Five Tips For Hard Anodizing
In order to obtain a good quality hard anodized film, as well as to ensure the required size of the part, the following tips are recommended.
Hard Anodizing – Chamfering
Hard anodized parts are not allowed to have sharp corners, burrs and other sharp features. On the one hand, the normal anodizing time is very long. The anodizing process itself is an exothermic reaction. Corners, on the other hand, are usually places where the current is relatively concentrated. These locations are the most likely to cause local overheating of the part, causing the part to burn out. Therefore, the edges and corners of all-aluminum parts should be chamfered. The chamfer radius should be greater than 0.5mm.
2. Surface roughness
After hard anodizing, the surface roughness of the part changes. For rougher surfaces, it will appear smoother than the original after hard anodizing. And for parts with smoother surfaces, it usually gets worse after hard anodizing. And the range of reduction is about 1 to 2 grades.
3. Size allowance
Due to the large thickness of the hard anodized film, if the aluminum parts are further processed or need to be assembled, a certain machining allowance should be reserved in advance. And the clamping position should also be specified.
During the hard anodizing process, the dimensions of the part change. The conceivable thickness variations and dimensional tolerances of the anodized film should be considered before machining. The actual dimensions of the part before anodizing should then be determined. Make them hard anodized to the specified tolerances.
4. Fixture Design
Hard anodized parts need to withstand high currents and currents during the oxidation process. So we have to be careful to put the parts together. Otherwise, poor contact may cause electric shock, wear or burn the contact parts of the parts. Therefore, after the parts are hard anodized, it is necessary to design and manufacture special fixtures for parts with different shapes and specific requirements.
5. Partial Protection
If there are both conventional anodized areas and hard anodized areas on the same part, specific procedures should be arranged according to the surface finish and precision of the part. Conventional anodizing is usually done first, followed by hard anodizing. Surfaces that do not require hard anodizing should be insulated. Insulation can be achieved by spray gun or brush. Will be coated with prepared nitrocellulose binder or peroxyethylene binder. On surfaces that do not require hard anodizing, the insulating layer should be applied thinly and evenly. Each layer should be dried at low temperature for 30-60 minutes. And 2 to 4 layers should be fully applied.
Advantages of hard anodizing
- The surface hardness can reach HV300kg/mm² or more.
- The thickness of the anodic oxide film is greater than 20um.
- It has high corrosion resistance and high wear resistance in the atmosphere, and is also an ideal insulating film. Hard anodizing has good insulating properties (breakdown voltage up to 2000V) and can be firmly bonded to the base metal.
- Strong adhesion. 50% of the formed hard anodized film penetrates into the interior of the aluminum alloy, and 50% adheres to the surface of the aluminum alloy.
- Non-toxic: the anodic oxide film and the electrochemical process used to produce the anodic oxide film are harmless to the human body.
- It is a good substitute for traditional hard chrome plating. Compared with the hard chrome plating process, it has the advantages of low cost, firm film bonding, and convenient treatment of plating solution and cleaning waste liquid… In many industries, more and more requirements for processing convenience, product light weight, and environmental protection are required. The product adopts aluminum alloy and hard anodizing to replace the traditional stainless steel spraying and electroplating.