Analysis of common problems in anodic oxidation of aluminum

Aluminum alloy of different materials, bare aluminum and pure parts or different sizes and specifications of aluminum and aluminum alloy parts, generally should not be treated with the same tank oxidation. If the surface of the metal phase is uneven, tissue segregation, segregation of micro impurities or improper heat treatment caused by uneven structure of each department, it is easy to produce selective oxidation or selective dissolution. In general, when the concentration of Cl- in water reaches 25mg/L, it will have a harmful effect on the anodizing treatment of aluminum alloy. This kind of failure often occurs in the practice of flow acid anodic oxidation process, which seriously affects the quality of aluminum alloy anodic oxidation. Retractor should choose copper or copper alloy material. Such failures are rare but do occur. If impurity content exceeds allow content, can produce harmful effect, can branch or replace sulfuric acid solution completely, ability ensures aluminum alloy sulfuric acid anodized quality effectively. Sulfuric acid anodic oxidation should choose softened water, deionized water or distilled water, Ccl-≤15mg/L in the electrolyte, total minerals ≤50mg/L. If the temperature of the electrolyte exceeds 30℃, the oxide film will produce loose powder, the film quality is poor, and the phenomenon of "burning" occurs when the severity is severe. For example, the surface of cast aluminum alloy, its non-machined surface should generally be sandblasted or shot peened to remove its original oxide film, sand, etc.

(4) The water quality used in sulfuric acid anodic oxidation electrolyte and the harmful impurities in the electrolyte must be strictly controlled. In the process of anodic oxidation of sulfuric acid, compressed air should be used to stir, and refrigeration device should be equipped. To reduce or avoid anodic oxidation fault and improve product quality, effective measures should be taken from the micro point of view. If the used special or general fixture is used again during anodizing treatment, the oxide film on its surface must be completely removed to ensure good contact. If the contact surface is too small, it will lead to burning erosion anodized parts. Tap water should not be used to prepare anodic oxidation solution of sulfuric acid, especially turbidity tap water with high content of Ca2+, Mg2+, SiO32- and Cl- should not be used. In general, the temperature should be controlled in 13 ~ 26℃, the quality of the oxide film is better. Therefore, in the process of anodizing, it is necessary to use stirring or cooling devices to keep the temperature of the electrolyte in a certain range.

(3) The temperature of sulfuric acid anodic oxidation solution must be strictly controlled, and the best temperature range is 15 ~ 25℃. Anodic oxidation film thickness is generally 3 ~ 15μm, aluminum alloy sulfuric acid anodic oxidation process simple operation, electrolyte is not disorderly, the cost is not high, is a mature process method, but in the sulfuric acid anodization process is often inevitable various faults, affecting the quality of the oxide film. Anodic oxidation of aluminum and aluminum alloy with sulfuric acid oxidation film has a higher adsorption capacity, easy to seal holes or coloring treatment, more improve its corrosion resistance and appearance. In addition, when the electrolyte temperature is constant, the anode current density must also be limited, because the anode current dense through high temperature rise, the oxide film is easy to loose powder or sand, the quality of the oxide film is very adverse. It is of great practical significance to summarize and analyze the causes of failure and take effective preventive measures to improve the quality of sulfuric acid anodic oxidation of aluminum alloy.

Aluminum alloy sulfuric acid anodizing treatment is widely used and mature corrosion protection decoration treatment process, as long as the strict implementation of the process premise, really operating, sulfuric acid anodizing oxidation film quality is completely guaranteed.

(4) the oxidation film of aluminum alloy after anodic oxidation of sulfuric acid accidentally occurs is dull and dull, and sometimes there is dot erosion. When it is severe, the black dot erosion is obvious, leading to the scrap of parts and causing greater losses. Aluminum alloy with high silicon content (especially cast aluminum) should be activated by nitric acid mixed acid solution containing about 5% hydrofluoric acid to effectively maintain a good activated surface and ensure the quality of oxide film. The contact area is too small, the current density is too large, will produce overheating easy to burn parts and fixtures. Conductive rods should be made of copper or copper alloy and have sufficient contact area. If the local silicon content segregation in aluminum alloy, it often leads to local no oxide film or black freckle stripes or local selective dissolution to produce holes.

Because the resistance of the anodic oxidation film of aluminum alloy is very large, a large amount of joule heat will be generated in the anodic oxidation process. The higher the tank voltage is, the greater the heat will be generated, resulting in the rising temperature of the electrolyte. In addition, if the electrolyte has suspended impurities, dust or copper and iron and other metal impurities ion content is too high, often make the oxide film black freckles or black stripes, affect the corrosion protection function of the oxide film. In addition, under the premise of constant temperature control, attention should also be paid to effectively control the anode current density to better ensure the quality of oxide film.

Because the insulation of aluminum oxide film is good, so aluminum alloy parts must be firmly mounted on the general or special fixture before anodizing treatment, in order to ensure good electrical conductivity. This kind of failure mostly occurs in summer, especially in anodized sulphuric acid tank without cooling device. After processing 1-2 tank parts, the phenomenon of loose powdering will occur, which significantly affects the quality of oxide film.

(3) After sulfuric acid anodizing treatment of aluminum alloy, the oxide film is loose powder or even touched off, especially after filling and sealing, the surface of the workpiece is covered with severe powder layer, and the corrosion resistance is poor. - some process test and production practice have proved that adding suitable amount of carboxylic acid in sulfuric acid anodizing electrolyte or glycerol can effectively reduce the adverse impact of the reaction heat effect and can decrease the thickness of the oxide film and hardness progress under the premise of anodic oxidation of the electrolyte temperature promise to limit under the condition of ensure the quality, progress in production efficiency.

Sulfuric acid solution in the process of anodic oxidation, will produce oil foam and suspended impurities, should be removed as scheduled. In the process of aluminum alloy anodic oxidation, halfway back to the electric power, often can make the oxide film, and middle power failure parts in cleaning tank for too long, high acid through cleaning the sink and water quality not net, containing suspended solids, mud and so on, tend to electrochemical etching in the aluminum alloy parts, such as black dot erosion.

The above reasons are generally related to the composition, microstructure and phase evenness of aluminum and aluminum alloy, or to some metal ions or suspended impurities dissolved in the electrolyte.

1 Common faults and analysis

The anodic oxidation of aluminum takes aluminum or aluminum alloy as anode and lead plate as cathode to electrolysis in sulfuric acid, oxalic acid, chromic acid and other aqueous solutions, so as to make the surface of natural oxide film.

(2) Anodized parts treated with the same tank, some have no oxidation film or film layer thin or incomplete, and some have burning erosion at the contact point of fixture and parts.

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(1) Aluminum alloy products after sulfuric acid anodizing treatment, local no oxidation film, black spots or stripes visible to the naked eye, oxide film has a bulge or hole phenomenon.

Aluminum alloy sulfuric acid anodizing oxidation film quality, corrosion protection function of the merits and disadvantages mainly depends on the composition of aluminum alloy, film thickness and anodizing process prerequisite, such as temperature, current density, the use of water quality and anodizing after the filling and sealing process.

(2) The mounting fixture material must ensure good conductivity, generally choose duraluminum-alloy rod, plate to ensure that there is a certain elasticity and strength. Sulfuric acid anodic oxidation is the most widely used treatment. No oxide film or film layer is not complete, mainly because the fixture and the workpiece contact is not good, conductive poor or because the oxide film layer on the fixture is not completely removed. The chemical composition, microstructure and metallicity of aluminum and aluminum alloy affect the nature and function of oxide film.

(1) For different aluminum alloys, such as forging forming, calendering forming or machining forming or heat treatment welding and other processes, according to the actual situation to choose the appropriate pretreatment method. Other common harmful impurities in sulfuric acid positive plate oxidation solution include Cu2+, Fe3+, Al3+ and so on.

For lap, spot welding or riveted aluminum alloy assembly, for the aluminum alloy parts that are easy to form air bags in the process of anodizing, from the quality consideration, sulfuric acid anodizing process is generally not allowed.

2 Measures to prevent faults

This kind of failure often happens accidentally and has special reason to cause. Cl-(including other halogen elements) can destroy the oxide film naturally, or even no oxide film at all. The oxide film of pure aluminum or aluminum magnesium alloy is easy to be born, and the quality of the film is better. The fixture should not only ensure enough conductive contact area, but also reduce the clamp mark as far as possible. And aluminum silicon alloy or aluminum alloy with higher copper content, the oxide film is more difficult to natural, and natural film dark, gray, luster is not good. Sometimes add tap water to the electrolyte, the water bleaching powder treatment and Cl- content exceeds the standard or sometimes the HCl container is not thoroughly cleaned and brilliant sulfuric acid, will make the anodic oxidation electrolyte mixed with excessive amount of Cl-, resulting in the anodic oxidation of aluminum alloy parts point erosion to scrap products and so on. At the contact point between fixture and parts, it is necessary to ensure the free flow of current and reduce the contact marks between fixture and parts as far as possible. In the absence of refrigeration device, adding 1.5% ~ 2.0% of the carboxylic acids, such as malonate or oxalic acid and lactic acid, to the sulfuric acid electrolyte can make the temperature range of the anodic oxidation solution over 35℃ and avoid or reduce the looseness or powder of the oxide film.