The anodization of aluminum uses aluminum or an aluminum alloy as an anode, and the lead plate as a cathode is electrolyzed in an aqueous solution such as sulfuric acid, oxalic acid, and chromic acid to form an oxide film layer on the surface thereof. Among them, the anodizing treatment of sulfuric acid is the most widely used. Aluminum and aluminum alloy sulfate anodic oxidation oxide film has a higher adsorption capacity, easy sealing or L coloring treatment, and more to improve its corrosion resistance and appearance. Anodized film thickness is generally 3 ~ 15μm, aluminum sulfate anodizing process is simple, stable electrolyte, the cost is not high, is a mature process, but in the sulfate anodization process will inevitably occur in a variety of failures, affect the oxidation Film quality. Seriously summarizing and analyzing the causes of failures and adopting effective preventive measures have important practical significance for improving the quality of aluminum sulfate anodizing.
1 Common Faults and Analysis
(1) After the aluminum alloy products are anodized by sulfuric acid, localized non-oxidizing touches occur, showing black spots or streaks that are visible to the naked eye, and the oxide film has bulging or cavitation. Although such failures are rare, they also occur.
The above causes of failure are generally related to the composition, structure, and phase uniformity of aluminum and aluminum alloys, or to the dissolution of certain metal ions or suspended impurities in the electrolyte. The chemical composition, microstructure, and metal phase uniformity of aluminum and aluminum alloys can affect the oxide film formation and performance. Pure aluminum or aluminum-magnesium alloy oxide film is easy to produce, and the quality of the film is also better. Aluminum-silicon alloys or aluminum alloys with high copper content do not easily form oxide films, and the resulting films are dark, gray, and have poor gloss. If the surface produces metal phase non-uniformity, tissue segregation, micro-impurity segregation, or uneven heat treatment caused by various parts of the tissue, etc., it is easy to produce selective oxidation or selective dissolution. If the local silicon content in the aluminum alloy is segregated, local oxidation-free films or black spot streaks or local selective dissolution of holes are often caused. In addition, if the electrolyte contains suspended impurities, dust, copper or iron and other metal impurities, ion content is too high, the oxide film will often appear black spots or black streaks, affecting the oxide film's corrosion protection performance.
(2) Anodized parts treated with the same groove, some non-oxidized film or film layer is thin or incomplete, and some have burning and erosion phenomena at the contact between the jig and the part. Such failures often occur in the process of flowing acid anodizing, which seriously affects the quality of the anodized aluminum alloy.
Because of the better insulation of the aluminum oxide film, the aluminum alloy parts must be firmly attached to general purpose or special fixtures before anodizing to ensure good electrical conductivity. Conductive rods should use copper or copper alloy materials and must ensure sufficient contact area. At the contact between the fixture and the part, it is necessary to ensure the free passage of current, and to minimize the contact marks between the fixture and the part. If the contact area is too small and the current density is too high, overheated parts and jigs may burn. Non-oxidized film or incomplete film, mainly due to poor contact between the fixture and the workpiece, poor conductivity or due to the oxide film on the fixture is not completely removed.
(3) After the anodizing treatment of aluminum alloy sulfuric acid, the oxide film is loosely powdered and even dropped off by hand. Especially after the filling and sealing, the surface of the workpiece has a severe powder layer and the corrosion resistance is poor. This type of failure occurs mostly in the summer, and in particular, sulfate anodizing tanks without a cooling device often treat 1-2 flute parts and loose pulverization occurs, significantly affecting the quality of the oxide film.
Due to the large resistance of the aluminum alloy anodic oxidation film, a large amount of Joule heat is generated during the anodic oxidation process. The higher the cell voltage, the greater the heat generation, and the resulting rise in the electrolyte temperature. Therefore, in the anodizing process, stirring or cooling devices must be used to keep the electrolyte temperature within a certain range. Under normal circumstances, the temperature should be controlled at 13 ~ 26 °C, the quality of the oxide film is better. If the temperature of the electrolyte exceeds 30°C, the oxide film will be loosely powdered and the quality of the film will be poor, and the phenomenon of "burning" will occur when the temperature is high. 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, the oxide film is also easy to loose in the form of powder or sand, the quality of the oxide film is very unfavorable.
(4) Occasionally, after the anodizing of aluminum sulfate anodized, the oxide film is dull, sometimes with pitting corrosion. In severe cases, black spot corrosion is significant, resulting in scrapped parts and causing considerable losses.
Such failures are often accidental and have special causes. In the process of anodizing aluminum alloy, power is cut off and power is supplied again, which often makes the oxide film dark and dull, and parts that have been suspended halfway stay in the cleaning tank for too long, the acidity of the cleaning tank is too high, the water quality is not clean, and contains suspended solids. Silt, etc., often cause electrochemical corrosion of aluminum alloy parts, spot corrosion black spots and so on. Sometimes tap water is added to the electrolyte. The water is treated with bleaching powder and the container with excessive Cl- content or sometimes HCl has not been thoroughly cleaned and filled with sulfuric acid. This will cause the anodic oxidation electrolyte to contain excessive amounts of Cl-, resulting in aluminum. Anodic oxidation of alloy parts produces spot corrosion to make the product discarded.
2 Prevention of failures
The quality of the aluminum sulfate anodized oxide film is good or bad. The quality of the corrosion protection depends on the composition of the aluminum alloy, the thickness of the film, and the anodizing process conditions such as temperature, current density, water quality, and filling after anodization. Closed process and so on. To reduce or avoid the anodization failure to improve the quality of products to start from the fine point, take effective measures.
(1) For different aluminum alloys, such as casting, calendering, or machining, or heat-treating, etc., select the appropriate pre-treatment method according to the actual situation. For example, the non-machined surface of the cast aluminum alloy should generally be sandblasted or shot blasted to remove its original oxide film, sticky sand, etc. For aluminum alloys with high silicon content (especially cast aluminum), it should be etched and activated by nitric acid mixed acid solution containing about 5% hydrofluoric acid to effectively maintain a good activated surface and ensure the quality of the oxide film. Aluminum alloys of different materials, bare aluminum and pure parts, or aluminum and aluminum alloy parts of different sizes and sizes are generally not suitable for oxidation treatment in the same tank.
For lapped, spot welded or riveted aluminum alloy assemblies, for anodized aluminum alloy parts that are not easily removed during the anodizing process, it is generally not permissible to use anodization of sulfuric acid in terms of quality.
(2) The mounting fixture material must ensure good electrical conductivity. Generally, hard alloy rods are used. The panel must ensure certain elasticity and strength. The hook should use copper or copper alloy material. Used special or general-purpose fixtures, such as those re-used during anodization, must be completely removed from their surface oxide film to ensure good contact. Fixtures must not only ensure sufficient conductive contact area, but also minimize fixture impressions. If the contact surface is too small, it can lead to erosion of the anodized parts.
(3) The temperature of the sulfuric acid anodizing solution must be strictly controlled. The optimum temperature range is 15 to 25°C. Sulfuric acid anodizing process requires the use of compressed air mixing, and should be equipped with refrigeration equipment. In the case of no refrigeration device, the addition of 1.5% to 2.0% of tricarboxylic acid or carboxylic acid such as oxalic acid, lactic acid, etc. to the sulfuric acid electrolyte can make the anodizing solution temperature range exceed 35°C to avoid or reduce the loosening of the oxide film. Powdering. Some process tests and production practices have confirmed that the addition of an appropriate amount of carboxylic acid or glycerol in the anodizing electrolyte of sulfuric acid can effectively reduce the adverse effects of the reaction heat effect, and can improve the anodized electrolysis without reducing the thickness and hardness of the oxide film. The temperature of the liquid is allowed to an upper limit, and the production efficiency is improved on the premise of ensuring the quality. In addition, under the condition of constant temperature control, it is necessary to pay attention to the effective control of the anode current density in order to better ensure the quality of the oxide film.
(4) The quality of the water used in the anodizing of the sulfuric acid electrolyte and the harmful impurities in the electrolyte must be strictly controlled. Preparation of sulfuric acid anodizing solution should not use tap water, especially can not use turbid water containing Ca2+, Mg2+, SiO32- and Cl- high content of tap water. Under normal circumstances, when the concentration of Cl- in the water reaches 25 mg/L, it will have an adverse effect on the anodizing treatment of the aluminum alloy. Cl- (including other halogen elements) can damage the oxide film and even form oxide films. Sulfuric acid anodization should use softened water, deionized water or distilled water, Ccl-≤15mg/L in the electrolyte, and total minerals ≤50mg/L.
Sulfuric acid solution will produce oily foam and suspended impurities during the anodizing process and should be periodically removed. Other harmful impurities commonly found in sulfuric acid plate oxidation solutions are Cu2+, Fe3+, and Al3+. If the impurity content exceeds the allowable content, harmful effects may occur, and the sulfuric acid solution may be partially or completely replaced in order to effectively ensure the quality of anodized aluminum alloy sulfate.
Aluminum sulfate anodizing treatment is a widely used and mature anti-corrosion protection decoration treatment process. As long as the strict implementation of process conditions and careful operation, the quality of the anodized oxide film can be guaranteed.
1 Common Faults and Analysis
(1) After the aluminum alloy products are anodized by sulfuric acid, localized non-oxidizing touches occur, showing black spots or streaks that are visible to the naked eye, and the oxide film has bulging or cavitation. Although such failures are rare, they also occur.
The above causes of failure are generally related to the composition, structure, and phase uniformity of aluminum and aluminum alloys, or to the dissolution of certain metal ions or suspended impurities in the electrolyte. The chemical composition, microstructure, and metal phase uniformity of aluminum and aluminum alloys can affect the oxide film formation and performance. Pure aluminum or aluminum-magnesium alloy oxide film is easy to produce, and the quality of the film is also better. Aluminum-silicon alloys or aluminum alloys with high copper content do not easily form oxide films, and the resulting films are dark, gray, and have poor gloss. If the surface produces metal phase non-uniformity, tissue segregation, micro-impurity segregation, or uneven heat treatment caused by various parts of the tissue, etc., it is easy to produce selective oxidation or selective dissolution. If the local silicon content in the aluminum alloy is segregated, local oxidation-free films or black spot streaks or local selective dissolution of holes are often caused. In addition, if the electrolyte contains suspended impurities, dust, copper or iron and other metal impurities, ion content is too high, the oxide film will often appear black spots or black streaks, affecting the oxide film's corrosion protection performance.
(2) Anodized parts treated with the same groove, some non-oxidized film or film layer is thin or incomplete, and some have burning and erosion phenomena at the contact between the jig and the part. Such failures often occur in the process of flowing acid anodizing, which seriously affects the quality of the anodized aluminum alloy.
Because of the better insulation of the aluminum oxide film, the aluminum alloy parts must be firmly attached to general purpose or special fixtures before anodizing to ensure good electrical conductivity. Conductive rods should use copper or copper alloy materials and must ensure sufficient contact area. At the contact between the fixture and the part, it is necessary to ensure the free passage of current, and to minimize the contact marks between the fixture and the part. If the contact area is too small and the current density is too high, overheated parts and jigs may burn. Non-oxidized film or incomplete film, mainly due to poor contact between the fixture and the workpiece, poor conductivity or due to the oxide film on the fixture is not completely removed.
(3) After the anodizing treatment of aluminum alloy sulfuric acid, the oxide film is loosely powdered and even dropped off by hand. Especially after the filling and sealing, the surface of the workpiece has a severe powder layer and the corrosion resistance is poor. This type of failure occurs mostly in the summer, and in particular, sulfate anodizing tanks without a cooling device often treat 1-2 flute parts and loose pulverization occurs, significantly affecting the quality of the oxide film.
Due to the large resistance of the aluminum alloy anodic oxidation film, a large amount of Joule heat is generated during the anodic oxidation process. The higher the cell voltage, the greater the heat generation, and the resulting rise in the electrolyte temperature. Therefore, in the anodizing process, stirring or cooling devices must be used to keep the electrolyte temperature within a certain range. Under normal circumstances, the temperature should be controlled at 13 ~ 26 °C, the quality of the oxide film is better. If the temperature of the electrolyte exceeds 30°C, the oxide film will be loosely powdered and the quality of the film will be poor, and the phenomenon of "burning" will occur when the temperature is high. 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, the oxide film is also easy to loose in the form of powder or sand, the quality of the oxide film is very unfavorable.
(4) Occasionally, after the anodizing of aluminum sulfate anodized, the oxide film is dull, sometimes with pitting corrosion. In severe cases, black spot corrosion is significant, resulting in scrapped parts and causing considerable losses.
Such failures are often accidental and have special causes. In the process of anodizing aluminum alloy, power is cut off and power is supplied again, which often makes the oxide film dark and dull, and parts that have been suspended halfway stay in the cleaning tank for too long, the acidity of the cleaning tank is too high, the water quality is not clean, and contains suspended solids. Silt, etc., often cause electrochemical corrosion of aluminum alloy parts, spot corrosion black spots and so on. Sometimes tap water is added to the electrolyte. The water is treated with bleaching powder and the container with excessive Cl- content or sometimes HCl has not been thoroughly cleaned and filled with sulfuric acid. This will cause the anodic oxidation electrolyte to contain excessive amounts of Cl-, resulting in aluminum. Anodic oxidation of alloy parts produces spot corrosion to make the product discarded.
2 Prevention of failures
The quality of the aluminum sulfate anodized oxide film is good or bad. The quality of the corrosion protection depends on the composition of the aluminum alloy, the thickness of the film, and the anodizing process conditions such as temperature, current density, water quality, and filling after anodization. Closed process and so on. To reduce or avoid the anodization failure to improve the quality of products to start from the fine point, take effective measures.
(1) For different aluminum alloys, such as casting, calendering, or machining, or heat-treating, etc., select the appropriate pre-treatment method according to the actual situation. For example, the non-machined surface of the cast aluminum alloy should generally be sandblasted or shot blasted to remove its original oxide film, sticky sand, etc. For aluminum alloys with high silicon content (especially cast aluminum), it should be etched and activated by nitric acid mixed acid solution containing about 5% hydrofluoric acid to effectively maintain a good activated surface and ensure the quality of the oxide film. Aluminum alloys of different materials, bare aluminum and pure parts, or aluminum and aluminum alloy parts of different sizes and sizes are generally not suitable for oxidation treatment in the same tank.
For lapped, spot welded or riveted aluminum alloy assemblies, for anodized aluminum alloy parts that are not easily removed during the anodizing process, it is generally not permissible to use anodization of sulfuric acid in terms of quality.
(2) The mounting fixture material must ensure good electrical conductivity. Generally, hard alloy rods are used. The panel must ensure certain elasticity and strength. The hook should use copper or copper alloy material. Used special or general-purpose fixtures, such as those re-used during anodization, must be completely removed from their surface oxide film to ensure good contact. Fixtures must not only ensure sufficient conductive contact area, but also minimize fixture impressions. If the contact surface is too small, it can lead to erosion of the anodized parts.
(3) The temperature of the sulfuric acid anodizing solution must be strictly controlled. The optimum temperature range is 15 to 25°C. Sulfuric acid anodizing process requires the use of compressed air mixing, and should be equipped with refrigeration equipment. In the case of no refrigeration device, the addition of 1.5% to 2.0% of tricarboxylic acid or carboxylic acid such as oxalic acid, lactic acid, etc. to the sulfuric acid electrolyte can make the anodizing solution temperature range exceed 35°C to avoid or reduce the loosening of the oxide film. Powdering. Some process tests and production practices have confirmed that the addition of an appropriate amount of carboxylic acid or glycerol in the anodizing electrolyte of sulfuric acid can effectively reduce the adverse effects of the reaction heat effect, and can improve the anodized electrolysis without reducing the thickness and hardness of the oxide film. The temperature of the liquid is allowed to an upper limit, and the production efficiency is improved on the premise of ensuring the quality. In addition, under the condition of constant temperature control, it is necessary to pay attention to the effective control of the anode current density in order to better ensure the quality of the oxide film.
(4) The quality of the water used in the anodizing of the sulfuric acid electrolyte and the harmful impurities in the electrolyte must be strictly controlled. Preparation of sulfuric acid anodizing solution should not use tap water, especially can not use turbid water containing Ca2+, Mg2+, SiO32- and Cl- high content of tap water. Under normal circumstances, when the concentration of Cl- in the water reaches 25 mg/L, it will have an adverse effect on the anodizing treatment of the aluminum alloy. Cl- (including other halogen elements) can damage the oxide film and even form oxide films. Sulfuric acid anodization should use softened water, deionized water or distilled water, Ccl-≤15mg/L in the electrolyte, and total minerals ≤50mg/L.
Sulfuric acid solution will produce oily foam and suspended impurities during the anodizing process and should be periodically removed. Other harmful impurities commonly found in sulfuric acid plate oxidation solutions are Cu2+, Fe3+, and Al3+. If the impurity content exceeds the allowable content, harmful effects may occur, and the sulfuric acid solution may be partially or completely replaced in order to effectively ensure the quality of anodized aluminum alloy sulfate.
Aluminum sulfate anodizing treatment is a widely used and mature anti-corrosion protection decoration treatment process. As long as the strict implementation of process conditions and careful operation, the quality of the anodized oxide film can be guaranteed.