Trouble analysis and treatment method of sulfuric acid anodizing process for aluminum and aluminum alloy in Guangdong mechanical processing plant:
1. Partial dark spots or streaks, as well as tympanic tumors or cavities. After the aluminum alloy product is anodized, there will be local non-oxidized film, showing visible dark spots or streaks, and the oxide film has tympanic tumors or cavities. Although such failures are rare, they also occur. The reason is usually related to the uniformity of the composition, structure and phase of aluminum and aluminum alloys, or to some metal ions or suspended impurities dissolved in the oxidizing solution. 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 easy to form a high-quality film. However, aluminum-silicon alloys or copper-containing high-strength aluminum alloys produce dark and gray oxide films and poor gloss. If the surface phase structure is uneven, structure segregation, impurity segregation or uneven structure caused by improper heat treatment, etc., selective oxidation or selective dissolution is likely to occur, resulting in poor quality of the oxide film. If the local silicon content in the aluminum alloy material is segregated, it often results in partial no oxide film or black spot streaks or partial selective dissolution to produce holes. In addition, if there are excessively high levels of suspended impurities, dust particles, 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 anti-corrosion protection performance of the anodic oxide film.
2. The oxide film is incomplete, there is no film locally or burns the anodized parts processed in the same tank, and some parts have no oxide film or the film is thin or incomplete, and there is burnt and erosion at the contact between the fixture and the part.
Such failures in Guangdong mechanical processing plants often occur in the practice of sulfuric acid anodizing process, which seriously affects the anodizing quality of aluminum alloy parts. The reasons for this failure are: ① Improper clamping position. Due to the improper clamping position, oxygen cannot escape and stay on the local surface during anodization, resulting in stuffiness without oxide film. Care should be taken to choose an appropriate position when clamping, so that the concave surface of the part is inclined upward or upward, so that the failure of no oxide film can be avoided. ②There are attachments on the surface of the part. If there are transparent viscous materials such as adhesive tape on the surface of the part before anodization, it does not fall off during anodization, but falls off when the film is closed. As a result, there is no oxide film at the adsorbent. Care should be taken to remove such substances. ③The composition of the oxidizing solution is too dilute. If the components of the oxidizing solution are too dilute, there will be no oxide film formed where the local current of the oxidized parts is too small. The high sulfuric acid concentration can be analyzed and the problem can be eliminated.

Due to the good insulation of aluminum oxide film, aluminum alloy parts must be firmly mounted on a general or special fixture before anodizing to ensure good 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.
3. The oxide film is loose and powdered. After sulfuric acid anodizing treatment, the oxide film becomes loose and powdered and even falls off when touched by hand. Especially after the house is sealed, the surface of the part has a serious powder layer, and the corrosion resistance is poor.
During the anodizing process of aluminum alloy in Guangdong mechanical processing plant, the formation and dissolution of the oxide film are carried out at the same time. The chemical dissolution of the oxide film will cause the oxide film to loosen, and even produce loose powdery oxide film. The dissolution rate of the oxide film is less affected by the temperature of the oxidizing solution, and it accelerates as the temperature of the solution rises. Therefore, this type of failure mostly occurs in summer, especially in sulfuric acid anodizing tanks without cooling devices. After processing 1 to 2 tank parts, loose powder 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. In general, the temperature should be controlled between 13°C and 26°C, and 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 film quality will be poor, 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.
In addition, the anodic oxidation time is long and the solution concentration is concentrated, which will also produce a loose anodic oxide film. When the current density is too high or the voltage is too high, the generated Joule heat will heat the aluminum alloy parts, causing the temperature of the solution around the parts to rise, thereby accelerating the chemical dissolution of the oxide film, causing the oxide film to become loose or produce a powdery oxide film. Therefore, strict control of process specifications is an important factor to prevent the anodic oxide film from being loose. In the aluminum alloy anodizing process, water washing and other processes may bring chloride ions to the oxidation solution, and sulfuric acid may also contain chloride ions, which may cause the chloride ion content in the oxidation solution to increase, thereby causing the oxide film to loosen.
After adding sulfuric acid to the oxidation tank, the uneven mixing of the solution will also cause the anodized parts in the same tank. Some oxide films are too thin, and some oxide films are loose and have powdery ash. The measures to eliminate the above faults are: Strictly regulate the operation of the process. If the electrolyte temperature is too high, try to reduce the temperature, stir or activate the cooling equipment, and control the temperature difference within (20±2)℃. If the oxidation treatment time is too long, the anodic oxidation time should be shortened; if the current density is too high, the current density should be reduced. After adding sulfuric acid to the solution, it should be fully stirred by compressed air and energized; when the solution. When the chloride ion content is too high, trouble shooting can be done by diluting the solution or partially replacing the solution.
4.　The oxide film is dull and dull. Produce pitting corrosion The aluminum alloy sulfuric acid anodic oxide film is dim and dull, sometimes pitting corrosion occurs, and serious black pitting corrosion occurs.
Such failures in Guangdong machinery processing plants are often accidental and caused by special reasons. During the aluminum alloy anodizing process, the power is cut off and re-energized, and the double-layer oxide film is often formed, which will make the oxide film dim; and the parts of the power outage stay in the cleaning tank for too long, the cleaning tank is too acidic, and the water quality is not clean. , Containing more suspended solids, often cause pitting corrosion, black spots and other failures of aluminum alloy parts. The increase of aluminum ions dissolved in the sulfuric acid solution will also cause the transparency of the oxide film to deteriorate. Generally, the aluminum content in the sulfuric acid solution is about 109/L. Excessive Cl- ions mixed in the aluminum oxidation solution can also cause pitting corrosion and other failures due to anodization of aluminum alloy parts.
5.　The anodic oxide film is thin or has red ash, and the film has poor corrosion resistance. The reasons for this failure are many, including the following aspects.
(1) The sulfuric acid content and operating conditions do not meet the process specifications. If the anodic oxidation current density is low, the voltage is low, and the oxidation time is short, the oxide film formed on the surface of the aluminum alloy is thinner. If the concentration of the solution is low, there are few ions in the solution, and the conductivity of the solution is small, to achieve the same current density, the cell voltage of the solution with low concentration is higher than that of the cell with high concentration, while sulfuric acid anodizing generally controls the voltage , So when the concentration of the solution is low, the current density will be small under the specified voltage, and the oxide film will be thinner. When the temperature of the solution is low, the viscosity of the solution is large, the ion movement is slow, and the conductivity of the solution is small. Under a specified voltage, the current density is small, and the oxide film formed is of course thin. On the contrary, the oxide film is thick, but the dissolution of the oxide film is also accelerated, and a loose powdery oxide film will be formed.
(2) The parts are installed too loosely, and the clamp is in poor contact with the conductive rod. Checking the clamping situation of the aluminum alloy parts will find that the parts are too loosely clamped and the conductivity is poor, which will also cause the anodic oxide film to become thin. In addition, the old oxide film on the fixture is not completely removed, and the parts are clamped on the fixture with a thin oxide film, which will also cause poor conductivity, and the oxide film formation speed will slow down and the oxide film will become thinner.
(3) The anodic oxidation solution contains too much copper and iron ions. Because the anodizing process includes the surface dissolution of aluminum alloy and the formation of oxide film, for example, some high-strength aluminum alloy materials contain copper, manganese, iron, silicon and other elements. During the anodizing process, it is possible for copper and iron plasma to dissolve into the solution. As the use time of the anodizing solution increases, the content of copper ions in the solution will increase, which will affect the thickness of the oxide film or the appearance of dust on the surface.
The measures taken by the Guangdong machinery processing plant to eliminate this failure are: strictly control the oxidation process specification; pay attention to the removal of the residual oxide film on the surface of the hanger, and ensure the contact between the hanger and the parts. The problem of excessively high content of copper and iron ion impurities in the plating solution can be solved by increasing the area of the cathode with low-current electrolysis or renewing the oxidizing solution.
6.　The parts are burnt at the contact point of the fixture, and there are electric shock burns in the parts of the parts. The cause of this failure is that the aluminum alloy parts and the cathode are in short circuit, and the parts are in contact with each other and the parts are in contact with each other. Therefore, it is necessary to strengthen the contact inspection between the anodized parts and the fixture. The fixture must be cleaned before use and must be clamped with the parts to ensure good electrical conductivity of the parts. When placing parts in the oxidation tank, pay attention to the distance from the cathode to avoid contact. In addition, it is necessary to prevent short circuits between parts in contact with each other and increase the distance between parts.
7. The oxide film has black spots (black streaks) or reticulated foam-like patterns. The main reasons for this failure are: after the parts are chemically degreasing, the parts are oxidized when the light is omitted, so that the water glass left on the parts forms silicic acid. Result: The chemical degreasing liquid is not clean during heat treatment; there are oil stains or other pollutants on the surface of the parts; there are suspended impurities in the oxidation solution; the parts are sealed and cleaned before the oxidation is cleaned; the surface of the solution has suspended impurities, copper and Too much iron impurities, etc.
To eliminate this kind of fault, it is necessary to thoroughly carry out pre-treatment; strengthen the cleaning of oxidized parts, and clean them immediately after oxidation to avoid the treatment liquid or impurities remaining on the surface of the oxide film; remove or update part of the oxidizing solution to reduce the contamination of impurities in the solution.
8. Oxidized parts are easily stained with fingerprints and watermarks after being filled with hot water, and the oxide film is light and whitish after being filled with potassium dichromate. The reasons for this failure are: the temperature of the filling sealing solution is too low and the filling time is short; the pH value of the solution is improper ; Too much aluminum hydroxide in the sealing solution, too high SO42- content; too high anodizing temperature causes the oxide film to be too thin, and the film pores are open.
The troubleshooting of such faults is mainly aimed at adjusting the pH value of the closed filling solution, strictly controlling the solution composition and process conditions; checking and correcting the SO42- content and aluminum hydroxide concentration in the closed solution; controlling the anodizing process conditions and increasing the oxidation time.
9. The redness or blueness of the film after anodic oxidation is black and the main reasons for the redness or blueness of the film after the oxidation is black are:
(1) Poor adsorption performance of the oxide film layer. When the black oxide film was dyed with acid black ATT dye, there was a problem of redness and blueness. Acid black ATT dye is composed of 70% acid blue black l0B and 30% acid orange II molecular structure, so when the anodic oxide film has small pores and poor absorption performance, the small molecule acid orange II is easily absorbed by the oxide film , Acid Orange II in the film exceeds the proportion of the dye, so the color is red.
(2) The influence of the surface condition of the sheet and extruded parts. Before anodizing, the parts should be alkali-etched to remove the surface crust to obtain a uniform black color, otherwise, redness or flowery black will be obtained. For aluminum alloy parts with high silicon content, it can be immersed in 40mL/L～50mL/L nitric acid and 10mL/L hydrofluoric acid solution for about 20s to dissolve impurities, leaving a thin layer of pure aluminum on the surface. To improve the performance of the anodic oxide film, make it have good adsorption performance, and get a uniform black.
(3) Improper pH value of the dyeing solution. The pH value of the dyeing solution has a greater impact on the dyeing of the oxide film, because the absorption properties of various dyes have their own different pH values. For example, when the pH value is close to 7, the adsorption performance of acid blue black 10B is good, while the adsorption performance of acid orange II is relatively poor, so the color is bluish. When pH=4.5, the adsorption capacity of acid blue black 10B is poor, and acid orange II The adsorption capacity is strong, so the color is reddish.
The methods of Guangdong mechanical processing plant to eliminate this failure include: strengthen the treatment before oxidation; appropriately extend the anodic oxidation time to increase the pores of the oxide film, so that the large molecule acid blue black 10B can be absorbed by the oxide film smoothly; control the dyeing solution The pH value should avoid the value around 7 or 4.5, and it is generally better to control around 3.6 or 5-6.