After the die-casting aluminum alloy shell was cleaned, there were massive black spots and punctate white spots on the surface. What are the differences?

Analysis and Prevention of Anodic Oxidation Failure of Aluminum Alloy

1. Common faults and analysis

(1) After anodic oxidation with sulfuric acid, the aluminum alloy product has no local oxidation, showing black spots or stripes visible to the naked eye, and the oxide film has bulges or holes. Although this kind of fault is rare, it also happens.

The causes of the above failures are generally related to the composition, microstructure and phase uniformity of aluminum and its alloys, or to some metal ions or suspended impurities dissolved in the electrolyte. The chemical composition, microstructure and uniformity of metal phase of aluminum and its alloy will affect the formation and properties of oxide film. The oxide film of pure aluminum or aluminum-magnesium alloy is easy to form, and the film quality is also good. However, the oxide film of Al-Si alloy or aluminum alloy with high copper content is difficult to form, and the formed film is dark gray with poor gloss. If the surface metal phase is uneven, microstructure segregation, trace impurity segregation or improper heat treatment, selective oxidation or selective dissolution is easy to occur. If the local silicon content in aluminum alloy segregates, it will often cause local no oxide film or black spot stripes or local selective dissolution to produce holes. In addition, if there are suspended impurities, dust, copper, iron and other metal impurities in the electrolyte with high ion content, black spots or black stripes often appear on the oxide film, which affects the anticorrosion performance of the oxide film.

(2) Anodized parts treated in the same tank have no oxide film or thin or incomplete film layer, and some parts have burning and erosion at the contact between the fixture and the parts. This kind of fault often occurs in the practice of flowing acid anodizing process, which seriously affects the quality of aluminum alloy anodizing.

Because of the good insulation of alumina film, aluminum alloy parts must be firmly hung on general or special fixtures before anodic oxidation treatment to ensure good conductivity. The conductive rod should be made of copper or copper alloy, and ensure sufficient contact area. At the contact between fixture and parts, it is necessary to ensure the free passage of current and minimize the contact trace between fixture and parts. If the contact area is too small and the current density is too high, the parts and fixtures will overheat and be easily burned. No oxide film or incomplete oxide film, mainly due to poor contact between fixture and workpiece, poor conductivity or incomplete oxide film removal on fixture.

(3) After anodic oxidation of aluminum alloy with sulfuric acid, the oxide film is loose and powdery, and even falls off when touched by hand, especially after filling and sealing, serious powder layer appears on the surface of the product, and the corrosion resistance is poor. This kind of fault mostly occurs in summer, especially in sulfuric acid anodic oxidation tank without cooling device. 1-2 tank parts will appear loose pulverization after treatment, which obviously affects the quality of oxide film.

Due to the high resistance of anodic oxidation film of aluminum alloy, a lot of Joule heat will be generated during anodic oxidation. The higher the cell voltage, the greater the heat, which leads to the increase of electrolyte temperature. Therefore, in the process of anodic oxidation, stirring or cooling devices must be used to keep the electrolyte temperature within a certain range. In general, the temperature should be controlled at 13 ~ 26℃, and the quality of oxide film is good. When the electrolyte temperature exceeds 30℃, the oxide film will be loose and powdery, and the film quality will be poor, and it will be "burnt" in severe cases. In addition, under the condition of constant electrolyte temperature, the anode current density must also be limited, because the anode current density is too high, the temperature rise is fierce, and the oxide film is easy to be loose and powdery or sandy, which is very unfavorable to the quality of the oxide film.

(4) After anodic oxidation of aluminum alloy with sulfuric acid, occasionally the oxide film is dull, sometimes pitting occurs, and when it is serious, the black pitting is obvious, which leads to the scrapping of parts and heavy losses.

This kind of fault often happens by accident and is caused by special reasons. In the process of anodic oxidation of aluminum alloy, the oxide film is often dull when power is cut off and power is re-supplied. However, if the parts cut off in the middle stay in the cleaning tank for too long, the acidity of the cleaning tank is too high, the water quality is not clean, and there are many suspended substances and mud and sand, which will often lead to electrochemical corrosion of aluminum alloy parts and the appearance of punctate corrosion black spots. Sometimes tap water is added to the electrolyte, bleaching powder is used to treat the water and the Cl- content exceeds the standard, or sometimes the container containing HCl is filled with sulfuric acid without thorough cleaning, which will cause excessive Cl- to be mixed into the anodic oxidation electrolyte, thus leading to pitting corrosion of aluminum alloy parts and product scrap.

2. Measures to prevent failure

The quality and corrosion resistance of sulfuric acid anodizing film on aluminum alloy mainly depend on the composition, film thickness and anodizing process conditions of aluminum alloy, such as temperature, current density, water quality and filling and sealing process after anodizing. In order to reduce or avoid anodic oxidation failure and improve product quality, effective measures should be taken from the details.

(1) For different aluminum alloys, such as casting, calendering, machining or heat treatment, welding, etc., appropriate pretreatment methods should be selected according to the actual situation. For example, for the surface of cast aluminum alloy, the unprocessed surface should be cleaned by sandblasting or shot blasting to remove its original oxide film and sand adhesion. Aluminum alloy with high silicon content (especially cast aluminum) should be etched and activated by nitric acid mixed acid solution containing about 5% hydrofluoric acid, so as to effectively maintain a good activated surface and ensure the quality of oxide film. Aluminum alloy, bare aluminum and pure aluminum parts with different materials or aluminum and aluminum alloy parts with different sizes and specifications are generally not suitable for oxidation treatment in the same tank.

For aluminum alloy components that are lapped, spot welded or riveted, and aluminum alloy parts that are prone to cavitation during anodizing, sulfuric acid anodizing process is generally not allowed for quality reasons.

(2) The material for installing the fixture must ensure good conductivity. Generally, hard aluminum alloy bars are selected, and the plate must ensure certain elasticity and strength. Tractor should be made of copper or copper alloy. Used special or general fixtures, such as anodic oxidation, must be reused, and the oxide film on its surface must be completely removed to ensure good contact. Fixtures should not only ensure sufficient conductive contact area, but also minimize fixture traces. If the contact surface is too small, it will lead to burns and corrosion of anodized parts.

(3) The temperature of sulfuric acid anodizing solution must be strictly controlled, and the optimal temperature range is 65 05 ~ 25℃. In the process of sulfuric acid anodic oxidation, compressed air should be used for stirring and equipped with refrigeration device. Without refrigeration device, adding 1.5% ~ 2.0% propionic acid, oxalic acid, lactic acid and other carboxylic acids into sulfuric acid electrolyte can make the temperature range of anodic oxidation solution exceed 35℃ and avoid or reduce the looseness or pulverization of oxide film. Some process tests and production practices have proved that adding a proper amount of carboxylic acid or glycerol to sulfuric acid anodizing electrolyte can effectively reduce the adverse effects of reaction thermal effect, increase the allowable upper limit of temperature of anodizing electrolyte without reducing the thickness and hardness of oxide film, and improve production efficiency on the premise of ensuring quality. In addition, under the condition of constant temperature control, we should also pay attention to effectively controlling the anode current density to better ensure the quality of the oxide film.

(4) The water quality used in sulfuric acid anodizing electrolyte and harmful impurities in electrolyte must be strictly controlled. Tap water is not suitable for preparing sulfuric acid anodizing solution, especially turbid tap water with high content of Ca2+, Mg2+, SiO 2- and Cl-. In general, when the concentration of Cl- in water reaches 25mg/L, it will have adverse effects on the anodizing treatment of aluminum alloy. Cl- (including other halogen elements) can destroy the formation of oxide film, or even not form oxide film at all. Demineralized water, deionized water or distilled water should be selected for sulfuric acid anodic oxidation, with Ccl-≤ 15mg/L and total minerals ≤ 50 mg/L. ..

In the process of anodic oxidation of sulfuric acid solution, oily foam and suspended impurities will be produced, which should be removed regularly. Other common harmful impurities in sulfuric acid anodizing solution are Cu2+, Fe3+, Al3+, etc. If the impurity content exceeds the allowable content, it will have harmful effects, which can partially or completely replace sulfuric acid solution and effectively ensure the quality of sulfuric acid anodizing of aluminum alloy.

Sulfuric acid anodizing of aluminum alloy is a widely used and mature anticorrosion and decoration treatment process. As long as the process conditions are strictly implemented and carefully operated, the quality of sulfuric acid anodic oxidation film can be completely guaranteed.