I. Introduction
    Aluminum has a face-centered cubic structure and has good electrical and thermal conductivity, second only to Au, Ag, and Cu. It has good ductility and high plasticity, and can be machined in various ways.
    The chemical nature of aluminum is lively, and a dense oxide film with a thickness of about 5 nm is immediately formed on the surface of aluminum in dry air, so that aluminum will not be further oxidized and can withstand water; however, aluminum powder mixed with air is extremely flammable; molten aluminum can react with Water reacts violently, and can reduce many metal oxides to corresponding metals at high temperature; aluminum is amphoteric, which is easily soluble in strong bases and dilute acids. Aluminum has good corrosion resistance in the atmosphere, but the strength of pure aluminum is low, and only through alloying can various aluminum alloys be used as structural materials.
    The outstanding features of aluminum alloys are small density and high strength. Al-Mn and Al-Mg alloys formed by adding Mn and Mg to aluminum have good corrosion resistance, good plasticity and high strength. They are called rust-proof aluminum alloys and are used to manufacture fuel tanks, containers, pipes and rivets. Wait. The strength of the hard aluminum alloy is higher than that of the rust-proof aluminum alloy, but the corrosion resistance is reduced. Such alloys include Al-Cu-Mg series and Al-Cu-Mg-Zn series. The newly developed high-strength duralumin has a further increase in strength, and its density is 15% lower than that of ordinary duralumin. It can be extruded and can be used as a motorcycle frame and rim and other components. Al-Li alloy can be used to make aircraft parts and high-end sports equipment under load. Because adding 3% to 5% (mass fraction) of metal lithium lighter than aluminum to aluminum, it is possible to manufacture an aluminum-lithium alloy with a strength 20% to 25% higher than that of pure aluminum and a density of only 2.5 t/m3. This alloy is used in large passenger aircraft, which can reduce the weight of the aircraft by more than 5t without reducing the number of passengers.
    Aluminium and its alloys are placed in an appropriate electrolyte as an anode for energization treatment. This process is called anodization. After anodizing, the aluminum surface can produce an oxide film with a thickness of several to hundreds of microns. The surface of this layer of oxide film is porous honeycomb. Compared with the natural oxide film of aluminum alloy, its corrosion resistance, abrasion resistance and decoration are obviously improved and improved. Using different electrolytes and process conditions, anodized films with different properties can be obtained.
    As early as 1896, Pollak proposed direct current electrolysis in a solution of boric acid or phosphoric acid to obtain a patent for a "fortress" type oxide film. By the 1920s, this process was used industrially to manufacture electrolytic capacitors.
    The initial commercial application of anodizing is chromic acid anodizing. When GDBengough and JMStuart studied chromium plating on aluminum, they found that an anodic oxide film was formed on the aluminum surface due to the wrong connection. The composition of the electrolyte at the time was 250g/L chromic acid and 2.5g/L sulfuric acid. Later, people further research found that this oxide film can be dyed with ink or dye, the thickness of the oxide film is 3 ~ 5μm, and the working voltage is about 50V. This process is first used in the aircraft manufacturing industry, for the bottom layer of paint, to prevent cracks and improve corrosion resistance.
    In 1927, Kujirai and Ueki of Japan first used oxalic acid electrolyte for anodization, which can obtain an oxide film of more than 15mm, but the working voltage is higher than that of sulfuric acid anodization. This technique was first popularized in Japan and later spread to Germany, and was gradually adopted by Europeans for decoration of storefronts and buildings.
    In 1927, Gower, Stafford O'Brien and Partners4 published a patent for sulfuric acid anodization, the current density of oxidation is 0.7 ~ 1.3A/dm2, this current density has been used until now. Compared with oxalic acid and chromic acid anodizing, sulfuric acid anodizing has lower working voltage, lower electrolyte cost, simpler operation, and stronger decorative oxide film, so this process is quickly improved and popularized. At present, more than 95% of anodization is carried out in sulfuric acid. If not specified, anodization usually refers to sulfuric acid anodization.
    2. Pre-treatment
    1. Degreasing and degreasing
    Aluminum and aluminum alloys used to be a degreasing process using steel in the early stage, that is, the bath solution was Na2Co3, Na2SiO3, Na3PO4 solution, the operating temperature was 40～70℃, and the time was 5～15min. This process has stable performance and long life, but the cost of the bath is high and it is not easy to clean, and it is basically not used now.
    In the 1960s, people used NaOH or Na2CO3 plus Na3PO4, complexing agents, nonionic surfactants, and anionic surfactants to degrease at room temperature for 3 to 5 minutes. This process has high oil removal efficiency, low cost, and energy saving, but the bath liquid is prone to flocculent precipitation, and the complexing agent and surfactant are easily brought into the subsequent tank to form pollution. A few factories are still using it.
    Since the 1980s, acidic degreasing baths have been gradually popularized. The baths are H2SO4 or H2PO4, plus HF, Fe3+, H202, NO2- and nonionic surfactants, and operate at room temperature for 3 to 5 minutes. This process has high efficiency and does not pollute the subsequent tank. It is a better degreasing process and is now more and more widely used. Typical process:
    H2SO410g/L
    H3PO410g/L
    HNO310g/L
    HF10g/L
    Surfactant 2g/L
    Sodium persulfate 5g/L
    Temperature 45～70℃
    Time 10～120s
    2. Alkali corrosion
    Before anodic oxidation of aluminum and its alloys, the dense but uneven natural oxide film needs to be removed. For high-silicon aluminum alloys, a mixed solution of HN03 and HF is used at room temperature for 3 to 5 minutes. Toxic gases are generated during work. Generally not used for other aluminum alloys. Other aluminum alloys use an alkaline bath based on NaOH solution. The concentration of NaOH is 30-70g/L, the operating temperature is 40-80℃, and the time is 3-10min. The alkaline corrosion process has simple maintenance, low cost, good corrosion effect, and easy removal of processing stripes on the surface of the aluminum alloy. It is an important supporting process for anodizing. At present, it is very rare to use NaOH solution alone as alkali corrosion. Sodium gluconate, sodium citrate and other complexing agents are often added to prevent the precipitation of Al(OH)3 in the bath, which is called "longevity alkali". Sulfide is added to prevent the displacement reaction of heavy metals on the surface of aluminum alloy and eliminate "flow marks". Some also add fluoride and nitrate to produce matte sand surface effect.
    3. Electrolytic polishing
    After mechanical polishing, it is often difficult to maintain the gloss after polishing. This is mainly due to the uneven surface pressure of mechanical polishing. Anodizing produces a matte surface, so mechanical polishing cannot be directly used for bright anodizing.
    In 1934 in the United Kingdom and the United States, the electrolytic polishing processes "Brytal" and "Alzak" were invented almost simultaneously. The anode was electrolyzed in alkaline (Brytal process) or fluoroboric acid (Alzak process) solutions for 10 to 20 minutes. Surface, and continue to be maintained after anodizing. Electrolytic polishing utilizes the effect of current to cause the aluminum alloy to undergo electrochemical reactions, and dissolves in varying degrees on the uneven parts of the aluminum alloy surface, resulting in a smooth mirror effect on the surface of the aluminum parts. Electrolytically polished aluminum parts can still retain most of their gloss after subsequent anodizing treatment. Electrolytically polished aluminum parts have good corrosion resistance, even if they are not anodized, they will not corrode in the atmosphere for a long time and maintain the original luster. During the Second World War, the demand for aluminum reflectors was great. During this period, aluminum electrolytic polishing developed rapidly.
    The high-purity aluminum sheet (99.99%) is electrolytically polished to obtain a mirror effect with a reflectivity close to 100%. The higher the purity of the aluminum sheet, the higher the reflectance. Commonly used processes are: phosphoric acid-chromic acid type, phosphoric acid-sulfuric acid, chromic acid type, perchloric acid-acetic acid type, phosphoric acid-sulfuric acid-glycerol type, fluoroboric acid type, sodium carbonate-sodium phosphate type, potassium hydroxide-chromic acid type. The operating temperature is from room temperature to 90℃, the current density is 10～20A/dm2, and some processes are up to 150A/dm2.
    The main pollutant of electrolytic polishing is hexavalent chromium. In recent years, with the strengthening of environmental awareness, electrolytic polishing treatment fluids containing no chromic acid have gradually become popular. The main problem with electrochromic polishing solutions that do not contain chromic acid is that the polishing of some alloys is not bright enough. In addition, after the aluminum alloy is polished in this treatment liquid, the brightness will drop rapidly under the condition of no electricity, and it needs to be taken out from the treatment liquid immediately and washed with water, which has a certain impact on the wide application of this process. Later, someone added some kind of additives to this treatment liquid to alleviate these two problems.
    The electrolytic polishing process is mature and has little pollution, but the working current is large and the cost is high. It is currently used for the processing of high-brightness and highly decorative aluminum parts.
    4. Chemical polishing
    Chemical polishing was invented in the late 1940s, when Henley was doing electrolytic polishing of phosphoric acid-sulfuric acid type. When there was no power, he found that the corrosion of aluminum parts had a bright effect. Then he carefully studied this phenomenon and got the earliest chemical polishing process: phosphoric acid 75% (volume), 25% (volume) sulfuric acid, the operating temperature is 90 ~ 100 ℃. Later, it was found that adding 10% nitric acid to the above process can obtain a particularly bright effect. At this time, chemical polishing was only applied in industry, and corresponding patents were successively published. Bright anodizing has entered the market steadily, replacing some nickel-chromium plating processes on steel or copper.
    Chemical polishing does not require electricity or special fixtures, and the operation is simple, but it requires good heating and ventilation equipment. The use of high-purity aluminum can achieve the effect of 100% reflectance, and ordinary aluminum alloy can also achieve decorative gloss. Because chemical polishing is less expensive than electrolytic polishing, most bright anodizing is supported by chemical polishing. The most commonly used chemical polishing process is: phosphoric acid 75% (volume fraction), nitric acid 150% (volume fraction), sulfuric acid 10% (volume fraction), operating temperature 90 ~ 110 ℃, time 0.5 ~ 3min. Some processes only have phosphoric acid and nitric acid, and some add acetic acid, chromic acid or hydrofluoric acid. Add a small amount of drilling salt, nickel salt, copper salt can increase the brightness of polishing. The biggest disadvantage of chemical polishing is that it produces NOX toxic gas. The yellow NOX gas is a strong carcinogen and is a "Huanglong" that is difficult to sway in chemical polishing workshops. Many methods have been used to solve this technical problem. Tajima of Japan uses a "cage-shaped" compound to absorb toxic gases to obtain a new technology for yellow-smoke-free chemical polishing; Germany uses 16% (volume fraction) for aluminum parts with a purity of 99.99% ) Ammonium bifluoride, 13% (volume fraction) nitric acid, 25g/L dextrin, the operating temperature is low, and there is little gas evolution. Kaiser Aluminum and Chemical Company invented a similar process (volume fraction): 2.5% nitric acid, 0.60%, ammonium bifluoride, 0.6% chromic acid, 0.6% glycerin, 0.05% copper nitrate. There is also an organic sulfide added to the phosphoric acid-sulfuric acid formula instead of nitric acid to obtain a yellow smoke-free polishing process.
    In addition to acidic chemical polishing, there is an alkaline chemical polishing process, but the effect of alkaline chemical polishing is far less than acidic chemical polishing, so it is rarely used. Typical alkaline polishing formulas include: NaOH, NaNO2, NaNO3, Na3PO4, Cu(NO3)2, etc. There is no toxic gas evolution in alkaline chemical polishing.
    3. Anodizing
    1. Sulfuric acid anodizing
    Sulfuric acid anodizing has the following characteristics:
    (1) The bath cost is low, the composition is simple, and the operation and maintenance are simple. Generally, it is only necessary to dilute the sulfuric acid to a certain concentration without adding other chemicals. It is recommended to use chemically pure sulfuric acid, and industrial grade sulfuric acid with less impurities can also be used. , So the cost is particularly low.
    (2) The transparency of the oxide film is high. The sulfuric acid anodized film of pure aluminum is colorless and transparent. For aluminum alloys, as the alloying elements Si, Fe, Cu, and Mn increase, the transparency decreases. Compared with other electrolytes, the color of sulfuric acid anodized film is the lightest.
    (3) High coloring property, transparent sulfuric acid oxide film, strong adsorption of porous layer, easy to dye and color, bright coloring is not easy to fade, and has a strong decorative effect.
    (4) The operating conditions of sulfuric acid anodizing are:
    H2SO4 (volume) 10%～30%
    Temperature ℃18～22
    Al/g.L-1≤20
    Current density/A.dm-20.6～3
    Time/min10～60
    2. Oxidation of oxalic acid and chromic acid
    Oxalic acid anodic oxidation is more commonly used in Japan. The characteristics of oxalic acid oxide film are similar to sulfuric acid oxide film. Porosity is lower than sulfuric acid oxide film. Corrosion resistance and hardness are higher than sulfuric acid oxide film. The cost and operating voltage of oxalic acid bath is higher than that of sulfuric acid, and some alloys have darker oxalic acid oxide film. Both oxalic acid and sulfuric acid anodizing require a good cooling system.
    The operating conditions of oxalic acid anodizing are:
    Oxalic acid (volume fraction) 2%～10%
    Temperature/℃15～35
    Current density/A.dm-20.5～3
    Voltage/V40～60
    The chromic acid anodized film is particularly resistant to corrosion. It is mainly used in the dry aircraft manufacturing industry. The adhesion of chromic acid oxide film and paint is strong. It is also used as the bottom layer of paint. The chromic acid anodized film is gray and opaque, and is generally not used for decoration.
    The operating conditions of chromic acid anodizing are:
    Cr03/g.L-130～100
    Temperature/℃40～70
    Current density/A.dm-20.1～3
    Voltage/VO～100
    Time/min35～60
    3. Hard anodizing
    In the late World War II, in order to increase the hardness and thickness of the anodized film, the temperature of the sulfuric acid oxidation tank was reduced to 0°C, and the current density was increased to 2.7 to 4.0 A/dm2, and a “hard oxide film” of 25 to 50 μm was obtained. . Adding a small amount of sulfuric acid with oxalic acid can obtain a hard oxide film at 5-15℃. Some patents use optimized sulfuric acid concentration, add organic acid or other additives, such as mellitic acid for hard anodizing.
    In Scotland, Campbell invented the use of AC-DC superimposed power supply, electrolyte flow at high speed, 0 ℃, current density of 25 ~ 35A/dm2, to obtain a hard anodized film of 100μm.
    Now, when pulse current is used for hard anodizing, especially high-copper aluminum alloy is generally difficult to hard anodize. Using pulse current can prevent "ablation". There are also many power supplies for hard anodizing, alternating current plus direct current, single-phase or three-phase pulse currents of various frequencies, reverse phase currents, etc. In traditional DC hard anodizing, the current density cannot generally exceed 4.0A/dm2. For single-phase rectified pulse power supplies, the current pulse peak value can be very large, but keeping the thickness of the oxide film uniform is an important issue.
    Because of its unique hardness and wear resistance, the hard anodized film is gradually being adopted by aviation, aerospace, automation, automotive, computer equipment, electronics and other industries. At present, it is being developed to be sealed with polytetrafluoroethylene, molybdenum disulfide and other solid lubricants, so that the hard anodized film has self-lubricating properties