Analysis Of Common Problems In Anodizing

The anodic oxidation of aluminum uses aluminum or aluminum alloy as anode and lead plate as cathode to electrolyze in aqueous solution such as sulfuric acid, oxalic acid, chromic acid, etc., to form an oxide film on the surface. Among them, sulfuric acid anodizing is the most widely used. Aluminum and aluminum alloy sulfuric acid anodic oxidation film layer has high adsorption capacity, easy to be sealed or colored, and its corrosion resistance and appearance are improved.

The thickness of the anodized film is generally 3~15μm. The sulfuric acid anodizing process of aluminum alloy is simple to operate, the electrolyte is stable, and the cost is not high. It is a mature process method. However, various failures are unavoidable in the process of sulfuric acid anodization, which affects the quality of the oxide film.

Seriously summarize and analyze the causes of failures and take effective preventive measures, which have important practical significance for improving the quality of aluminum alloy sulfuric acid anodizing.

(1) After the aluminum alloy product is treated with sulfuric acid anodizing, there will be partial non-oxidized film, showing visible dark spots or streaks, and the oxidation film has tympanic tumors or cavities. Although such failures are rare, they also occur.

The causes of the above failures are generally related to the composition, structure and phase uniformity of aluminum and aluminum alloys, or to some metal ions or suspended impurities dissolved in the electrolyte. The chemical composition, structure and uniformity of the metal phase of aluminum and aluminum alloys will affect the formation and performance of the oxide film. The oxide film of pure aluminum or aluminum-magnesium alloy is easily formed, and the quality of the film is also better. For aluminum-silicon alloys or aluminum alloys with a high copper content, the oxide film is more difficult to form, and the resulting film is dark, gray, and poor in gloss. If the surface produces metal phase inhomogeneity, structure segregation, micro-impurity segregation, or uneven structure of each part caused by improper heat treatment, it is easy to produce selective oxidation or selective dissolution. If the local silicon content in the aluminum alloy is segregated, it often results in local no oxide film or black spot streaks, or local selective dissolution to produce holes. In addition, if there are excessively high levels of suspended impurities, dust or metal impurity ions such as copper and iron in the electrolyte, black spots or black streaks will appear on the oxide film, which will affect the corrosion protection performance of the oxide film.

Anodized aluminum parts

(2) Some of the anodized parts processed in the same tank have no oxide film or the film is thin or incomplete, and some have burnt and corrosion at the contact between the fixture and the part. Such failures often occur in the practice of flowing acid anodizing process, which seriously affects the quality of aluminum alloy anodizing.

Due to the good insulation of aluminum oxide film, aluminum alloy parts must be firmly mounted on general or special fixtures before anodizing to ensure good electrical conductivity. Conductive rods should be made of copper or copper alloy materials and ensure sufficient contact area. At the contact point between the fixture and the part, it is necessary to ensure the free passage of current, but also to minimize the contact marks between the fixture and the part. If the contact area is too small and the current density is too large, it will cause overheating and easily burn parts and fixtures. There is no oxide film or incomplete film, which is mainly due to poor contact between the fixture and the workpiece, poor electrical conductivity, or the oxide film on the fixture is not completely removed.

(3) After the aluminum alloy sulfuric acid anodizing treatment, the oxide film becomes loose and powdered and even falls off with the touch of the hand, especially after filling and sealing, a serious powder layer appears on the surface of the part, and the corrosion resistance is poor. This type of failure mostly occurs in summer, especially in sulfuric acid anodizing tanks without cooling devices. After 1-2 tank parts are processed, loose powdering will appear, which obviously affects the quality of the oxide film.

Due to the high resistance of the aluminum alloy anodic oxide film, a large amount of Joule heat will be generated during the anodic oxidation process. The higher the cell voltage, the greater the heat generated, which leads to the continuous rise of the electrolyte temperature. Therefore, in the anodic oxidation process, a stirring or cooling device must be used to keep the temperature of the electrolyte within a certain range. Under normal circumstances, the temperature should be controlled at 13 ~ 26 ℃, the quality of the oxide film is better. If the temperature of the electrolyte exceeds 30°C, the oxide film will be loose and powdered, the quality of the film will be inferior, and the phenomenon of “scorching” will occur in severe cases. In addition, when the electrolyte temperature is constant, the anode current density must also be limited, because the anode current density is too high, the temperature rises sharply, and the oxide film is also easy to loose powder or sand, which is very detrimental to the quality of the oxide film.

(4) Occasionally, the oxide film of aluminum alloy after sulfuric acid anodization is dim and dull, and sometimes pitting corrosion occurs. In severe cases, the black pitting corrosion is significant, resulting in scrapped parts and large losses.

Such failures often occur accidentally and have special causes. In the aluminum alloy anodizing process, the power is cut off and re-powered again, which will often make the oxide film dim and dull, and the parts of the power cut off stay in the cleaning tank for too long, the cleaning tank is too acid, the water quality is not clean, contains suspended solids, There are more mud and sand, which will often cause electrochemical corrosion of aluminum alloy parts and pitting corrosion black spots. Sometimes tap water is added to the electrolyte, the water is treated with bleaching powder and the Cl- content exceeds the standard, or sometimes the HCl-containing container is not thoroughly cleaned and then contains sulfuric acid, which will cause excessive Cl- to be mixed in the anodized electrolyte. As a result, the anodic oxidation of aluminum alloy parts produces pitting corrosion and the product is scrapped.

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