Exploring hard anodizing: processes, advantages and techniques
Definition and principle of hard anodizing
Concepts and categories
Hard anodizing is a special surface treatment process, mainly used in aluminum and aluminum alloy materials. It is designed to form a hard, wear-resistant, corrosion-resistant and good insulating and heat-resistant oxide film on the surface of the material by means of electrolytic oxidation. This process is suitable for aluminum and aluminum alloy parts that require high wear resistance, heat resistance and good insulation properties, such as the inner walls of various cylinders, pistons, steam plugs, cylinders, bearings, aircraft cargo hold floors, roller bars and guides, hydraulic equipment, steam impels, levelers, gears and cushion parts.
Basic principles
In the hard anodizing process, the cathode reaction is as follows: 4H+ + 4e- = 2H2↑; The anode reaction is: 4 oh < sup > - < / sup > - 4 e < sup > - < / sup > = 2 h < sub > 2 < / sub > O + O < sub > 2 < / sub > write; The oxidation reaction of aluminum is: 2Al + 3O → Al2O3.
The thickening of oxide film is a dynamic process. The oxide film thickens gradually with the increase of electrification time and current. This is because the resulting oxygen atoms are more reactive than oxygen in the molecular state and are more likely to react with aluminum to form an oxide film. At the same time, however, the resulting oxide film dissolves in an acidic solution. Only when the formation rate of the oxide film is greater than the dissolution rate, the oxide film can continue to thicken. When the two speeds are equal, the thickness of the oxide film does not increase. If the oxidation rate is too much greater than the dissolution rate, the surface of aluminum and aluminum alloy is easy to form a powdery oxide film.
In order to obtain a thicker oxide film, it is necessary to apply a higher external voltage to overcome the high resistance of the oxide film. However, this will lead to an increase in current, resulting in a severe heating phenomenon, and the heat released when the oxide film is generated will make the electrolyte temperature around the parts rise sharply. The increase of temperature will accelerate the dissolution of oxide film and affect the thickening effect of oxide film. Therefore, it is usually necessary to use a combination of cooling equipment and agitation to maintain the temperature stability of the electrolyte in order to obtain a high-quality hard oxide film.
Process steps for hard anodizing
Pretreatment
Pretreatment is a crucial step before hard anodizing. First, the surface of aluminum or its alloys needs to be degreased, usually using alkaline solutions to remove oil from the surface. The surface oxides and impurities are then removed by pickling, which commonly includes a mixture of nitric acid and phosphoric acid. This step provides a clean surface for the subsequent anodizing process, ensuring the quality and adhesion of the oxide film.
Anodizing
In a specific electrolyte, such as a sulfuric acid solution or a solution of sulfuric acid added with organic acids, an electrolytic reaction is performed to form an oxide film. In this process, parameters such as current density, voltage and temperature need to be strictly controlled. Generally speaking, the current density is 2-6 A/dm2, the voltage is between 40-90 V, and the temperature needs to be adjusted according to the specific process requirements. At the same time, it is necessary to ensure that the position of the parts in the electrolyzer is uniform to obtain an oxide film of uniform thickness.
Rinsing
The role of rinsing is to remove impurities and residues from the electrolyte attached to the surface of the part. It is usually rinsed several times with running cold water to ensure thorough cleaning. This step can reduce the influence of impurities on the subsequent treatment steps and improve the quality and performance of the oxide film.
Protective sealing
The significance of sealing treatment is to fill the micropores of the oxide film, improve the density and corrosion resistance of the film. Common sealing methods include hot water sealing, steam sealing and chemical sealing. For example, hot water sealing is to soak the parts in hot water at 90-100 ° C for a period of time, so that the micropores of the oxide film are filled with hydration products. Chemical sealing is the use of specific chemical agents to treat the surface of the oxide film to form a protective layer.
Film formation
The film forming process usually involves baking at high temperatures. The closed parts are put into a high temperature furnace and baked under certain temperature and time conditions. This causes the oxide film to undergo further structural changes, forming a more dense and hard surface coating. The temperature is generally between 150-200 ° C, and the baking time depends on the film thickness and material.
Post-processing
The surface treatment of the sample after film formation, such as polishing can improve the surface finish, coloring can give the part a specific color, ceramic can further enhance the hardness and wear resistance of the surface. Polishing can be done by mechanical polishing or chemical polishing. Coloring is achieved by soaking in a specific dyeing solution. Ceramic processing requires special processes and equipment to complete.
Advantages of hard anodizing
High hardness and wear resistance
The hard anodized metal surface forms a thick and extremely hard oxide layer, which can reach 400-600HV on aluminum alloy and even exceed 1500HV on pure aluminum. This makes the treated surface have excellent wear resistance and can effectively resist friction and wear, greatly extending the service life of the part.
Excellent corrosion resistance
The oxide layer can effectively resist the erosion of most chemicals, greatly reducing the risk of oxidation and corrosion of the metal surface. Whether in acidic, alkaline or neutral environments, it can maintain good stability and provide reliable protection for parts.
Good electrical insulation
The oxide film has good electrical insulation performance, its resistivity is large, and the breakdown voltage can reach more than 2000V. This feature makes hard anodizing widely used in electronic, electrical and other fields, and can effectively prevent the occurrence of current leakage and electrical corrosion.
Other advantages
Color stability: The metal surface after hard anodizing can show a variety of colors, such as black, silver, etc., and the color is very stable, not easy to fade, and can maintain long-term beauty.
Aesthetics: The treated surface has good luster and texture, which can improve the grade and aesthetics of the parts.
Heat insulation: The oxide film has certain thermal insulation properties, which can reduce heat transfer in high temperature environments and protect parts and the surrounding environment.
Practical tips for hard anodizing
Current and voltage control
The current density of hard anodizing is generally 2-5 A/dm², and the voltage is more than 25V, and the highest may even reach 100V. During operation, the current and voltage should be adjusted according to the requirements of the material of the aluminum alloy, the shape of the part and the oxide film. For more complex parts or the need for thicker oxide film, the current density and voltage can be appropriately increased, but attention should be paid to gradually increasing the voltage to avoid excessive initial voltage resulting in a decline in the quality of the oxide film, soft film, dull, powder, no wear and other problems.
Power supply selection
Pulse power supply or special waveform power supply is a more suitable choice for hard anodizing. Especially for high copper aluminum alloy or high silicon cast aluminum alloy, ordinary DC anodizing is often poor. The pulsed power supply provides more precise current control, which helps to improve the quality and uniformity of the oxide film.
Tank temperature management
The temperature of the bath has an important effect on the quality of the hard anodic oxide film. In general, the temperature of the tank for hard anodizing should be controlled below 5 ° C, because the lower the temperature, the higher the hardness of the oxide film generated. In order to achieve and maintain low temperatures, it is usually necessary to have refrigeration equipment, such as chiller. At the same time, pay attention to stirring the tank liquid to ensure uniform temperature.
Tank liquid concentration adjustment
Taking sulfuric acid anodizing as an example, the tank concentration of hard anodizing is generally less than 15%. The quality of the oxide film can be optimized by adjusting the concentration of the bath. When it is necessary to obtain a thicker and higher hardness oxide film, the tank concentration can be appropriately reduced. However, too low a concentration may affect the growth rate of the oxide film, so it needs to be balanced according to specific needs.
Adding organic acids
Adding oxalic acid, tartaric acid and other organic acids to the bath can improve the hardness of the film. When choosing organic acids, its compatibility with sulfuric acid and its impact on the properties of oxide films should be considered. The amount added should be determined according to experiment and experience, too much or too little may affect the oxidation effect.
Applications of hard anodizing
Aerospace
In the aerospace field, hard anodizing plays an indispensable role. Aircraft engine parts, such as turbine blades, pistons, etc., after hard anodizing treatment, can withstand the extreme conditions brought by high temperature, high pressure and high-speed operation, significantly improve the wear and corrosion resistance of parts, to ensure flight safety. Some key parts of the fuselage structure, such as connectors and support parts, also use hard anodizing processes to enhance their strength and durability, reducing the weight of the fuselage while ensuring structural stability.
The automobile industry
Hard anodizing is widely used in automobile industry. Automotive engine parts, such as valves, camshafts, etc., can effectively resist wear and corrosion after treatment, and extend the service life of the engine. The hydraulic cylinder is hard anodized to maintain good sealing and wear resistance in high-pressure working environments. In addition, some components of the braking system also benefit from this process, improving the overall performance and safety of the car.
Electronic field
In the field of electronics, hard anodizing plays an important role. The shell of electronic devices, such as the metal shell of mobile phones and computers, has good wear resistance and corrosion resistance after treatment, and can also provide a certain electromagnetic shielding effect. The heat sink adopts hard anodizing process, which can not only increase the surface area and improve the heat dissipation efficiency, but also prevent oxidation and corrosion, and ensure the long-term stability of heat dissipation performance.
Military industry
The performance of parts in the military industry is very high, and the hard anodizing process has been applied in it with remarkable results. The components of the weapon, such as the barrel and bolt, are treated to maintain good performance in harsh operating environments. Some key components of radar equipment, such as antenna and waveguide, are improved by hard anodizing treatment to improve their weather resistance and electromagnetic properties, and ensure the reliability and stability of the equipment.Get a quote