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What is anodic oxidation of aluminum oxide?

- Mar 26, 2018 -

Aluminum is more active metal, the standard potential 1.66 v, can naturally formed in the air layer thickness is about 0.01 ~ 0.01 microns of oxide film, this layer of oxide film is crystallized, thin and porous, poor corrosion resistance.However, if aluminum and its alloys are placed in an appropriate electrolyte, the anode is made of aluminum, and the surface is generated by an external current, which is called anodic oxidation.By choosing different types, different concentration of electrolyte, and control oxidation process conditions, can be obtained with different properties, thickness of about a few to several hundred microns of anodic oxide film, its corrosion resistance, abrasion resistance and decoration and so on all have obviously improved and improve.The electrolyte used for anodic oxidation of Al and aluminum alloys is generally an acidic solution of medium solubility. Lead ACTS as a cathode and only conducts electricity.Anodic oxidation on aluminum and its alloys, the anode in the following response: 2 al - > e - 6 + 2 al3 + in the cathode of the following reaction: 6 h2o + e -- -- -- - > 3 h2 + 6 oh - and acid oxide film of aluminum and generating chemical dissolved, its response is: 2 + al + 6 h - > 2 h2al2o3 al3 + + 3 + 6 h + - > al3 + 2 + 3 h2o oxide film growth process is oxidation membrane formation and melting process.In the first paragraph, a (curve ab) : no hole formation.Power on the first few seconds to a few seconds, the aluminum surface immediately generate a layer of dense, high insulation performance of the oxide film, thickness is about 0.01 ~ 0.01 microns, is a continuous, imperforate film layer, called nonporous layer or barrier layer, the appearance of the film hinder of the current through and continue to thickening of the film.The thickness of the non-porous layer is proportional to the forming voltage and inversely with the dissolution rate of the oxide film in the electrolyte.Therefore, the voltage of the curve ab is shown to increase from zero to maximum.Section b (curve BC) : formation of the porous layer.With the formation of the oxide film, the dissolution of the electrolyte to the membrane begins.Due to the generation of oxidation film was not uniform, in the film the thinnest place will be the first to be dissolved out of holes, the electrolyte can reach the surface of the aluminum fresh through these holes, electrochemical reaction to proceed, resistance is reduced, the voltage drops (drop for maximum of 10 ~ 15%), membrane appears on the porous layer.Section 3 c (curve CD segment) : the porous layer thickens.After the anodic oxidation of 20 seconds, the voltage enters a relatively stable and slow ascent stage.Showed no hole in being dissolved to form porous layer at the same time, the new nonporous layer in growth, that is to say, oxide film forming speed and dissolution rate of non-porous layer in basically achieved the balance, so no hole layer thickness increases, voltage change is very small.However, at this time, the formation and dissolution of the oxide film at the bottom of the hole did not stop, and they continued to continue, resulting in the gradual movement of the bottom of the hole to the metal matrix.With the continuation of oxidation time, the pore deepens into the pores, and the pores are gradually thickened.When the film formation velocity and dissolution rate reach the dynamic equilibrium, the thickness of the oxide film will no longer increase even when the oxidation time is prolonged, and the anodic oxidation process should be stopped.The growth process of anodic oxidation characteristic curve and oxide film is shown in the figure below.The anodic oxidation treatment of aluminum and its alloys in dilute sulfuric acid electrolyte can be obtained from 5 ~ 20 micron thick, with good adsorption of colorless transparent oxide film.Sulfuric acid anodic oxidation process is simple, stable, easy to operate, allowing a wider range of content of impurities, less power consumption, low cost, and almost can be applied to aluminum and various aluminum alloy processing, so has been widely used in China.Anodic oxidation process in the following table for several typical: formula and technological conditions of dc ac method 1 2 3 sulfuric acid (g/L) 50 ~ 200, 160 ~ 170, 100 ~ 100 aluminum ion Al3 + (g/L) < < < 15 20 to 25 temperature (15 ~ 25 ℃) 0 ~ 3 15 ~ 25 anode current density (A/dm2) 0.8 ~ 1.5 0.8 2 ~ 4 ~ 6 voltage (V) 18 ~ 25 16 18 ~ 20 ~ 30 time (min) 20 ~ 40 60 20 ~ 40 mixing compressed air compressed air compressed air cathode/anode area of 1.5:1, 1.5:1 1:1 are the major factors affecting the quality of oxide film: (1) sulfuric acid concentration, usually by 15% ~ 20%.Concentration increases, the dissolution rate of membrane increased, the growth speed of membrane, membrane of high porosity, adsorption capacity is strong, flexible, good dyeing (easy to dye the dark), but the hardness, wear resistance of slightly;While reducing the concentration of sulfuric acid, the growth rate of oxide film is accelerated, the pores of membrane are less, the hardness is high and the wear resistance is good.Therefore, when used for protection, decoration and pure decorative processing, the upper limit of allowable concentration, i.e. 20% concentration of sulfuric acid, is used to make electrolyte.Electrolyte temperature: the electrolyte temperature has a great influence on the quality of the film.As the temperature rises, the dissolution rate of the film increases and the film thickness decreases.When the temperature is 22 ~ 30 ℃, the resulting film is soft, good adsorption ability, but wearability is poor;When the temperature is greater than 30 ℃, the film became loose and uneven, and sometimes even discontinuous, and the hardness is low, thus lost the use value;When the temperature between 10 ~ 20 ℃, the generated porous oxide film, adsorption ability, and full of elasticity, suitable for dyeing, but the film hardness is low, poor wear resistance;When the temperature is lower than 10, the oxide film thickness increases, the hardness is high, the wear resistance is good, but the porosity is low.Therefore, the temperature of electrolyte must be strictly controlled in production.In order to make a thick and hard oxide film, the operating temperature must be reduced. In the process of oxidation, the compressed air is used to stir and the temperature is relatively low, and the hard oxidation is usually carried out at about zero degree.The current density: within a certain limit, the current density increases, the membrane growth rate increases, the oxidation time is shortened, the formation of the membrane is more porous, and it is easy to stain, and the hardness and wear resistance rise.If the current density is too high, it will cause overheating of the parts surface and temperature of the local solution, and the dissolution rate of the film will increase, and the possibility of burning the parts will be increased.The current density is too low, the film growth rate is slow, but the film is dense, and the hardness and wear resistance are reduced.Oxidation time: the choice of oxidation time depends on the electrolyte concentration, temperature, anode current density and required film thickness.Under the same conditions, when the current density is constant, the growth rate of the membrane is proportional to the oxidation time.But when the film growth to a certain thickness, due to the membrane resistance increases, affect the conductive ability, and because the temperature rise, the dissolution rate of membrane increases, so the film growth rate will decrease gradually, until the last no longer increases.Stirring and moving: it can promote the convection of electrolyte, strengthen the cooling effect, ensure the uniformity of solution temperature, and do not cause the degradation of the oxide film due to the local heating of the metal.Impurities in the electrolyte: the possible impurities in the electrolyte used in aluminum anodizing are Cl,F,NO3,Cu2+,Al3+,Fe2+, etc.Among them, Cl,F, and NO3 have increased porosity and surface roughness and porosity.If the content exceeds the limit value, it may even cause corrosion perforation of the manufacturing parts (Cl should be less than 0.05g/L, and F should be less than 0.01g/L);When the Al3+ content in the electrolyte exceeds a certain value, the surface of the workpiece tends to appear white or macular white, and the adsorption performance of the membrane is reduced, and the dystoiesis is difficult (Al3+ should be less than 20g/L).When Cu2+ content reaches 0.02g/L, dark streaks or black spots appear on the oxide film.Si2+ often exists in the electrolyte with the suspended state, so that the electrolyte is slightly turbid and adsorbed on the membrane with a brown powder.All landowners aluminum alloy components: in general, the quality of the other elements make the film of aluminum metal drops, and didn't get the pure aluminium oxide layer with the thickness, hardness too low, different composition of aluminum alloy, in the anodic oxidation process should pay attention to not with groove.Chromic acid anodizing is the technology of anodizing aluminum and its alloys with 5-10% chromate electrolyte.The oxide film obtained by this method has the following characteristics: it is relatively thin (compared with sulfuric acid and oxalate oxide film), about 2 ~ 5 microns, which can maintain the original accuracy and roughness of the workpiece.It has high soft elasticity, almost no stoma, and has a stronger corrosion resistance than sulfuric acid anodized film.It is not transparent, the color is gray to dark gray, even rainbow color, so it is not easy to dye;Due to the lack of pores, the membrane layer can be used without sealing.The binding force between the two is good, so it is often used as the base of paint;Compared with sulfuric acid anodization, the cost is higher and the use is limited.Several kinds of chromic acid anodic oxidation process are listed in the table below: formula and technological conditions 1 2 3 chromate (90 ~ 100 g/L) 30 ~ 50 to 55 35 temperature (℃) 37 to 39 40 + 2 + 2 + 2 current density (A/dm2) 0.3 ~ 2.5 0.3 0.7 0.2 ~ 0.6 voltage (V) 0 ~ 40 0 ~ 40 0 ~ 40 oxidation time (min) 35 to 60 60 aluminum cathode materials and graphite oxalic acid anodic oxidation is made of 2% ~ 10% oxalic acid electrolyte with dc or ac oxidation process.When using direct current for anodic oxidation, the film hardness and corrosion resistance as - H2SO4 anodic oxide film, and the oxide film of aluminium with oxalic acid solution and the solubility of small, so can be obtained than a thick layer of oxide film in sulfuric acid solution;If it is oxidized by alternating current, it can be soft and elastic.The film layer of oxalate anodizing is generally 8 ~ 20 microns, the thickest can reach 60 microns.As long as the change in the process of oxidation process conditions (such as oxalic acid concentration, temperature, current density, waveform, etc.), you can get silvery white, golden yellow to brown, such as decorative membrane layer, no longer need to dyeing processing.Oxalate anodized electrolyte is very sensitive to chloride ion, and its mass concentration exceeds 0.04g/L film layer, which can cause corrosion spots.The mass concentration of trivalent aluminum ions is not allowed to exceed 3g/L.However, the oxidation cost of oxalic acid anode is high, and the energy dissipation is high (because the resistance of oxalate electrolyte is greater than that of sulfuric acid and chromic acid), the solution is toxic and the electrolyte stability is poor.The oxalic acid anodic oxidation process is shown in the following table.Formula and technological conditions 1 2 3 oxalic acid (g/L) to 30 + 3 + 5 to 50 + 50 to 10 temperature (℃) 18 + 3 to 30 + 3 to 30 + 3 current density (A/dm2) 1 ~ 2 1 ~ 2 2 ~ 3 voltage (V) 110 ~ 120 30 ~ 35 to 40 ~ 60 oxidation time (min) 120 30 ~ 60 30 ~ 60 carbon cathode materials carbon power dc dc ac Add some material in the electrolyte, which at the same time in the formation of the oxide film was adsorbed in the membrane layer, thus to obtain smooth, luster, uniform opaque like enamel and enamel colour and lustre of the oxide film, called "porcelain anodic oxidation film" or "ceramic oxide film.The film has good elasticity, good corrosion resistance and a plastic appearance after dyeing.The film thickness is about 6 ~ 25 microns.It's porcelain oxidation of two methods: (1) in the sulfuric acid or oxalic acid solution with some rare metal elements (such as titanium, thorium, etc.) of salt: oxidation process, because the salt hydrolysis to produce hair color material deposition in oxidized film pore, shaped like enamel coating, high hardness, can keep parts of high precision and high bright and clean degree, but this is expensive, solution using cycle is short, strict process conditions.The mixture of chromic anhydride and boric acid is the anodic oxidation solution: simple composition, low cost and good elasticity of the oxide film, but the hardness is lower than the previous one, which can be used for general decorative porcelain oxidation surface treatment.The oxidation solution and process conditions of porcelain anode are shown in the table below.Formula number The electrolyte composition Mass concentration/g · L / ℃ temperature current density/A/dm2 / V voltage time/min said Ming chromic anhydride oxalic acid boric acid 35 ~ 405 ~ 1 125 ~ 7 45 ~ 55 0.5 ~ 1.0 25 ~ 40 40 ~ 50 1. The film for the milky white, can be dyed 2. The film thickness 10 to 16 mu m3. Suitable for decorative parts commonly 2 chromic acid boric acid 30 ~ 40 ~ 50 401 ~ 3 starting: 2 ~ 3 termination: 0.1 ~ 0.6 gradually increases from 90 to 110 40 ~ 80 booster time < 5 keep 35 to 55 total time: 40 ~ 60 1. The solution has good stability, convenient operation, 2. Membrane layer for grey 3. The thickness of 10 ~ 15 mu m4. Suitable for general decorative parts, low cost 3 titanium potassium oxalate boric acid citric acid oxalate 35 ~ 458 ~ 458 ~ 51 ~ 24 to 28 starting 1.5:2 ~ 3 termination: 0.6 ~ 1.2 increased 90 ~ 110, 90 ~ 110 booster time 5 ~ 10:25 ~ 30 total time: 40 ~ 60 1. The film as the gray, high hardness (2) film thickness 8 ~ 16 microns (3) is suitable decoration to the wear resistance of high precision parts 4. The high cost, short service life of solution note: pure aluminum cathode materials available, lead plate or stainless steel plate.In the oxidation solution, the changes of various components will determine the color of the oxide film: if the chromic anhydride is increased, the color of the film will be transformed to the opaque gray direction;With the increase of boric acid, the color of the film is transformed to the milky white direction.However, with the increase of oxalic acid, the color of membrane is transformed to yellow.

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