The oxide film obtained through hard anodizing is 50% absorbed into the interior of the aluminum alloy and 50% adhered to its surface. Therefore, after hard anodizing, the external dimensions of the product increase while the internal holes decrease, whereas for ordinary anodizing, the external dimensions decrease while the internal holes increase. The color of hard anodizing is usually black or the natural color of the oxide film, which is determined by the composition of the aluminum alloy material. For the same type of aluminum profiles, the colors of the oxide films produced by different manufacturers may vary. Even for the same type of material from the same aluminum alloy manufacturer, the colors of the oxide films may also differ. The internal components of the products are generally coated with the natural color of the oxide film. 

Hard anodizing is a thick film anodizing method, which is a special surface treatment process for aluminum and aluminum alloys. Through this process, the thickness of the anodized film can reach about 250 micrometers. On pure aluminum with a density of 1500 kg/mm2, a micro-hardness anodized film can be obtained, while on aluminum alloys with a density of 400 - 600 kg/mm2, a micro-hardness anodized film can be obtained. The anodized film has poor heat conductivity, a melting point of 2050°, a large resistivity, and after sealing treatment (immersion in insulation or paraffin), the breakdown voltage can reach 2000 volts. It has high corrosion resistance and high wear resistance in the atmosphere, and is also an ideal insulation film. It has good insulation properties and strong adhesion to the base material, among other advantages. Therefore, it has been widely applied in the defense industry and the manufacturing of mechanical parts. 
Hard anodizing is mainly applied to aluminum and aluminum alloy parts that require high wear resistance, high heat resistance, and good insulation performance, such as cylinder inner walls, pistons, steam plugs, cylinders, bearings, aircraft cargo floor, rolling bars and guides, water conservancy equipment, steam impellers, leveling machines, gears and seat cushions, etc. Compared with the hard chromium electroplating process, it has the advantages of low cost, strong adhesion of the film layer, and convenient treatment of electroplating solution and cleaning waste liquid. However, the disadvantage of the film obtained through this process is that when the film thickness is large, the mechanical fatigue strength index of aluminum and aluminum alloys is affected. 
Advantages of aluminum alloy hard anodizing: 1. The surface hardness of the anodized aluminum alloy can reach around HV500; 2. The thickness of the oxide film is 25-250 μm; 3. The adhesion is strong. According to the anodizing characteristics, the generated oxide film penetrates 50% into the interior of the aluminum alloy and adheres 50% to the surface of the aluminum alloy; 4. Good insulation: The breakdown voltage can reach 2000V; 5. Good wear resistance: For aluminum alloys with copper content above 2%, the wear index under large wear is 3.5mg/1000 revolutions. The wear index of all other alloys shall not exceed 1.5 mg/1000 revolutions; 6. Non-toxic: The anodic oxide film and electrochemical process used for production of the oxide film are harmless to the human body. 
Therefore, in many industries, in order to facilitate processing, reduce product weight and protect the environment, some parts of certain products are made of aluminum alloy instead of stainless steel, and the traditional spraying and electroplating processes are used for hard oxidation.