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How to remove acid from blind holes after aluminum alloy anodize

The overall strategy for acid removal in blind holes after aluminum alloy anodize

The residual acid solution in blind holes during the anodize process of aluminum alloys is a problem that needs to be solved. Due to the special structure of blind holes, acid solution is prone to residue. If not thoroughly removed, it can absorb moisture and become an acidic solution in humid environments, corroding parts and affecting the service life of blind hole parts. The following will introduce methods for removing blind hole acid from aluminum alloys after anodize from multiple aspects.

1、 Aluminum alloy anodize and blind hole related situations

（1） Overview of Aluminum Alloy anodize Aluminum alloy anodize is a common surface treatment method, mainly including anodizing and other processes. For example, chromic acid anodizing is the process of using aluminum alloy parts as anodes, placing them in a chromic acid solution of a specified concentration (30-40g/L chromic anhydride), and subjecting them to electrical treatment at a temperature range of 38-42 ℃ to generate a layer of elastic, gray to dark gray, and opaque oxide film on their surface, with a thickness of 0.5-1.5 μ m. This oxide film has almost no voids and has a certain degree of corrosion resistance, but if the acid residue in the blind hole is not treated properly, it can still cause harm to the parts.

2） The particularity of blind holes refers to holes that are closed at one end. During the anodize process of aluminum alloys, the acid solution inside the blind hole is difficult to clean like it is on an open surface. Due to factors such as capillary action and liquid surface tension, acid is prone to accumulate inside blind holes, and as the anodize process progresses, the concentration of acid inside blind holes may change, increasing the difficulty of cleaning.

2,  Techniques for Removing Acid from Blind Holes

（1） Water injection cleaning method

1. Basic principle: Water injection cleaning is a relatively effective method. For example, using a medical syringe to inject water for cleaning, the needle is inserted into the bottom of the blind hole, and through multiple injections, the newly injected cleaning water is pushed out from the bottom of the blind hole to the mouth, pushing out the cleaning water that was previously injected into the blind hole. This utilizes the fluidity and pressure difference of water to continuously replace the acid solution inside the blind hole.

2. Improvement and optimization: Initially, there were some limitations to the use of medical syringes, such as short needles, narrow aperture, limited depth of insertion into blind holes, low flow rate and slow speed of injection water, and limited syringe capacity, requiring multiple injections and unable to achieve uninterrupted continuous cleaning. To improve cleaning efficiency, a discarded signature pen tip that removes metal pen tips can be used instead of a needle tip installed at the front end of the syringe. Then, the back end of the syringe can be flattened and connected with a plastic hose. The plastic hose can be connected to tap water, which can increase the amount of flushing water and achieve continuous cleaning of blind holes with flowing water.

（2） Auxiliary tool cleaning

1. Combine water pipe flushing with cotton swab wiping. First, use a water pipe to flush the blind hole area of the component. But this method has problems. The water injected in the early stage fills the blind hole and makes it difficult for it to flow out smoothly, resulting in the inability of the later flushing water to enter the blind hole and push out the previous flushing water. The residual chromic anhydride in the blind hole cannot be thoroughly cleaned. So after flushing the water pipe, use a cotton swab to wipe each blind hole again. The hole wall that can be touched by a cotton swab can be wiped multiple times to remove the chromic acid inside the hole. However, this method is labor-intensive and difficult to ensure that every blind hole is thoroughly wiped clean.

2. With the assistance of other tools, specialized cleaning tools such as slender brushes can be made according to the size and shape of blind holes. For some larger blind holes, the brush can penetrate deep into the inside of the blind hole to carry out the acid solution. When using a brush, it is important to choose the appropriate material to avoid scratching the oxide film. At the same time, the cleaning of the brush requires the use of appropriate cleaning solution to improve the cleaning effect.

3、 Chemical agents for acid removal in blind holes of aluminum alloy anodize

（1） Neutralizing agent

1. Neutralization principle: Neutralizing agents can undergo neutralization reactions with acids to remove acidic solutions. For example, some neutralizing agents containing alkaline components can neutralize acidic substances in blind holes into salts and water. Like sodium hydroxide solution (to be used with caution to prevent excessive corrosion of aluminum alloys), when it comes into contact with residual acid in blind holes, it undergoes acid-base neutralization reactions, such as reacting with sulfuric acid to produce sodium sulfate and water (H2SO4+2NaOH=Na2SO4+2H2O).

2. Special neutralizing agents · There are also neutralizing agents on the market specifically designed for removing acid from aluminum alloy after anodize. These neutralizing agents may have some other characteristics in addition to neutralizing acid solution. For example, some neutralizing agents can thoroughly open the oxide film while removing acid, making it dye uniformly and non corrosive to the material. Neutralizing agents such as LQ-310N have such characteristics and perform well in acid removal, effectively ensuring yield.

（2） Sealing agent related chemicals

1. The relationship between sealing agents and acid removal: In aluminum alloy anodizing, the use of sealing agents is also related to acid removal. Some sealing agents may react with residual acid in blind holes during the sealing process. For example, high-temperature sealing agents (containing nickel) in aluminum anodizing are composed of nickel salts, buffering agents, ash suppressants, surfactants, accelerators, wetting agents, and other compounds. During use, although its main purpose is to seal the hole, it may also affect the presence of acid in the blind hole to a certain extent. If there is acid residue in the blind hole before sealing, it may affect the sealing effect, and some components in the sealing agent may react with acid, reducing the corrosion risk of acid on aluminum alloy.

2. Other sealing agents and other agents such as those containing rare earth metal salts may play multiple roles in surface treatment of aluminum alloys. The anodize of rare earth metal salts has become a research hotspot due to its excellent corrosion resistance, stable solution, and pollution-free coating. These agents may indirectly affect the treatment of acid in blind holes during the formation and stabilization of aluminum alloy oxide films, such as by changing the structure and properties of the oxide film to reduce the possibility of acid penetration and residue.

4、 Physical methods for acid removal from blind holes in aluminum alloy anodize

（1） Mechanical grinding

1. Impact on surface: Mechanical grinding is a commonly used surface pretreatment method. Grinding can increase the surface roughness of aluminum alloy, which helps to obtain a large area of pore surface and thus change the wettability of the material itself. In terms of blind hole acid removal, although mechanical grinding cannot directly remove the acid solution inside the blind hole, it can change the surface state around the blind hole. For example, the ground surface may make it easier for blind holes to exchange liquids with the outside world. In the subsequent cleaning process, the cleaning solution is more likely to enter the interior of the blind hole, which is beneficial for the removal of acid. At the same time, the reaction activity between the ground surface and the acid solution may change, which to some extent affects the residual state of the acid solution in the blind hole.

2. Precautions for operation: When performing mechanical grinding, it is necessary to control the grinding parameters, such as grinding force, speed, and time. If the grinding force is too strong, it may damage the oxide film on the surface of the aluminum alloy and affect the corrosion resistance of the aluminum alloy; If the grinding time is too long, it may also cause excessive wear on the surface of the aluminum alloy, affecting the dimensional accuracy of the parts. In addition, choosing the appropriate grinding tool is also very important. It is necessary to choose the appropriate grinding wheel or grinding paper according to the material of the aluminum alloy and the shape of the parts.

（2） Ultrasonic cleaning

1. Ultrasonic cleaning principle: Ultrasonic cleaning uses the cavitation effect of ultrasound in liquid to clean the dirt on the surface and inside of objects. In blind hole acid removal, when aluminum alloy parts with blind holes are placed in ultrasonic cleaning equipment containing cleaning solution, ultrasonic waves will generate a large number of tiny cavitation bubbles in the cleaning solution. These cavitation bubbles generate high temperature, high pressure, and strong shock waves during their growth and collapse. This shock wave can penetrate the interior of blind holes, thoroughly mixing the acid and cleaning solution inside the blind holes, and can shake off the acid attached to the blind hole wall. Meanwhile, ultrasonic vibration can also accelerate the speed of acid diffusion from blind holes to external cleaning solution, thereby improving cleaning efficiency.

2. Advantages and limitations of ultrasonic cleaning: The advantage of ultrasonic cleaning is that it can penetrate deep into blind holes for cleaning, and has a good cleaning effect on blind holes with complex shapes that are difficult to clean using conventional methods. Moreover, the ultrasonic cleaning speed is relatively fast, which can complete the cleaning task in a shorter time. However, ultrasonic cleaning also has certain limitations, such as for some blind holes with deep depths and small apertures, ultrasonic energy may not fully reach the bottom of the blind hole, resulting in unsatisfactory cleaning effect. In addition, the cost of ultrasonic cleaning equipment is relatively high, requiring specialized equipment and operational skills.

5、 Case study on acid removal from blind holes after aluminum alloy anodize

（1） Case study of blind hole cleaning after chromic acid anodizing

1. Problem description: When chromic acid anodizing of aluminum alloy parts is used for industrial mass production, it is clamped with a lifting device and operated with a crane. For parts with blind holes, especially small-sized blind holes, the chromic anhydride solution that enters the blind holes during anodizing is difficult to clean during the later cleaning process. Even if the blind hole containing parts are visually cleaned and dried, the surface of the parts will have a uniform and smooth chromic acid anodized film after blowing and drying treatment. However, after being stored for several days, especially when the humidity is high, brown red chromic anhydride flow marks will seep out from the blind holes, contaminating the film layer on the surface of the parts. This not only affects the appearance quality of the parts, but also corrodes the surface and blind holes of the parts.

2. Cleaning process and effect: Initially, the method of flushing with water pipes and wiping with cotton swabs was used. Although the phenomenon of blind hole color leakage has been greatly improved, there are still some parts that are not thoroughly cleaned. Later, the method of using medical syringes to inject water for cleaning was adopted. The needle was inserted into the bottom of the blind hole and injected multiple times, causing the newly injected water to push out the previously injected water. This fundamentally solved the problem of uncleanliness in blind hole cleaning. However, the efficiency of the injection method for medical syringes is relatively low, and improvements have been made by using discarded signature pen tips instead of needles, connecting plastic hoses with tap water, improving cleaning efficiency and effectiveness, completely solving the problem of residual chromic anhydride in blind holes, and ensuring the quality of chromic acid anodized parts.

（2） Case study of blind hole cleaning after sulfuric acid anodizing

1. Problem description: After sulfuric acid anodization, the residual sulfuric acid solution in the blind hole of aluminum parts with small deep threads will damage the anodized film around the blind hole, causing corrosion to the threads and affecting the dyeing of the workpiece, resulting in discoloration, unevenness, and other phenomena. And over time, the residual sulfuric acid in the blind hole reacts with aluminum to form white powdery sulfates, seriously affecting the appearance quality of the product.

2. Cleaning process and effect: Ultrasonic cleaning is used, which utilizes the cavitation effect of ultrasound to fully mix the sulfuric acid solution inside the blind hole with the cleaning solution, and shake off the sulfuric acid residue on the blind hole wall. At the same time, dilute ammonia solution is used for neutralization to neutralize the residual sulfuric acid in blind holes and reduce their corrosion on blind holes and threads. And rubber plugs were used to block blind threaded holes, and dilute nitric acid solution was used for surface adjustment. The final results showed that this method can significantly improve the appearance quality of the workpiece. After batch production verification, the results can meet the appearance quality requirements.