Latest developments in anodizing aluminum and its alloys

I. Introduction
Aluminum is a face-centered cubic structure with good electrical conductivity and thermal conductivity. It is second only to Au, Ag and Cu. It has good ductility and high plasticity and can be processed in various ways.
The chemical nature of aluminum is active. Immediately in the dry air, the surface of aluminum forms a dense oxide film with a thickness of about 5 nm, so that aluminum does not oxidize further and is resistant to water. However, aluminum powder is highly flammable when mixed with air; molten aluminum can water violent reaction, at elevated temperatures can in many metal oxides to the corresponding metal; aluminum is amphoteric, both soluble in alkali, can be dissolved in acid. Aluminum has good corrosion resistance in the atmosphere, but the strength of pure aluminum is low, and only various alloys which can be used as structural materials can be obtained by alloying.
The outstanding characteristics of aluminum alloys are low 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 hard aluminum alloy is higher than that of rust-proof aluminum alloy, but the corrosion resistance is reduced. These alloys are Al-Cu-Mg and Al-Cu-Mg-Zn. The newly developed high-strength hard aluminum further increases the strength, and the density is 15% lower than that of ordinary hard aluminum, and can be extruded and used as a component such as a motorcycle skeleton and a rim. Al-Li alloys are used to make aircraft parts and advanced sports equipment that can withstand loads. Because the lithium metal was added 3% to 5% (mass fraction) is lighter than aluminum in the aluminum, the intensity ratio can be manufactured of pure aluminum by 20% to 25%, a density aluminum-lithium alloy only 2.5t / m3 of. This alloy is used on large passenger aircraft, which can reduce the weight of the aircraft by more than 5 tons, while the number of passengers is not reduced.
The aluminum and its alloys are placed in a suitable electrolyte as an anode for energization treatment, which is called anodization. After anodization, the aluminum surface can form an oxide film having a thickness of several to several hundred micrometers. The surface of this oxide film is porous honeycomb, and its corrosion resistance, wear resistance and decorative properties are significantly improved and improved compared with the natural oxide film of aluminum alloy. Anodized films of different properties can be obtained using different electrolytes and process conditions.
As early as 1896, a DC Pollak proposed solution in phosphoric acid or boric acid solution obtained patent "fortress" type oxide film. In the 1920s, this process was used industrially to make electrolytic capacitors.
The initial commercial applications anodization chromic acid anodizing. When GD Bengough and JM Stuart studied chromium plating on aluminum, it was found that an anodized film was formed on the aluminum surface due to the wrong line. The composition of the electrolyte at that time was 250 g/L chromic acid and 2.5 g/L sulfuric acid. Later, people further studied that the oxide film can be dyed by ink or dye, the thickness of the oxide film is 3 to 5 μm, and the working voltage is about 50V. This process was first used in the aircraft industry for the underlying layers of paint to prevent cracking and improve corrosion resistance.
In 1927, Japan's Kujirai and Ueki first used an anodic oxidation of oxalic acid electrolyte to obtain an oxide film of 15 mm or more, but the working voltage was higher than that of sulfuric acid. This process was first popularized in Japan and later spread to Germany and gradually adopted by Europeans for the decoration of storefronts and buildings.
In 1927, Gower, Stafford O'Brien, and Partners 4 published a patent for sulfuric acid anodization, which oxidized at a current density of 0.7 to 1.3 A/dm2, which has been extended to the present. Compared with oxalic acid and chromic acid anodizing, sulfuric acid anodizing has lower operating 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 the anodization is carried out in sulfuric acid, and anodization is usually referred to as anodization of sulfuric acid unless otherwise specified. [next]
Second, pre-processing
1. Early degreasing and degreasing of aluminum alloy and steel is cast iron degreasing process, namely: bath of _{Na 2 Co 3, Na 2 SiO} 3, Na 3 PO 4 solution, the operating temperature is 40 ~ 70 ℃, time 5 to 15 minutes. This process has stable performance and long life, but the cost of the bath is high and it is not easy to wash. Now it is basically not used.
In the 1960s, NaOH or Na ₂ CO ₃ plus Na ₃ PO ₄ , a complexing agent, a nonionic surfactant, and an anionic surfactant were used to degrease at room temperature for 3 to 5 minutes. The process has high oil removal efficiency, low cost and energy saving, but the bath liquid is prone to flocculation, and the complexing agent and surfactant are easily brought into the subsequent tank to form pollution. A few factories are still in use today.
Since the 1980s, the acid degreasing bath has been gradually popularized. The bath is H ₂ SO ₄ or H ₂ PO ₄ , plus HF, Fe ³⁺ , H ₂ 0 ₂ , NO ₂ ^- and nonionic surfactants. The time is 3 to 5 minutes. This process has high efficiency and does not pollute the subsequent tanks. It is a good degreasing process and is now more and more widely used. Typical process:
H ₂ SO ^₄ 10g/L
H ₃ PO ₄ 10g/L
HNO ₃ 10g/L
HF 10g/L
Surfactant 2g/L
Sodium persulfate 5g/L
Temperature 45～70°C
Time 10~120s
2. Alkaline corrosion of aluminum and its alloys before the need to remove dense but uneven natural oxide film, for high-silicon aluminum alloy, using HN0 ₃ , HF mixed solution, operating at room temperature, time 3 ~ 5min, will produce toxic gases when working . Generally, it is not used for other aluminum alloys. Other aluminum alloys use an alkaline bath mainly composed of NaOH solution. The concentration of NaOH is 30-70 g/L, the operating temperature is 40-80 ° C, and the time is 3-10 min. The alkali corrosion process is simple in maintenance, low in cost, good in corrosion effect, and easy to remove the processing stripes on the surface of the aluminum alloy, and is an important supporting process for anodizing. At present, it is rare to use NaOH solution alone for alkali corrosion, and a complexing agent such as sodium gluconate or sodium citrate is often added to prevent precipitation of Al(OH) _{3 in the} bath, so-called "long-lived base". Sulfide is added to prevent displacement of heavy metals on the surface of the aluminum alloy, eliminating "flow marks". Fluoride and nitrate are also added to produce a matt sand surface effect. [next]
3. After electropolishing mechanical polishing and anodizing, it is often difficult to maintain the gloss after polishing. This is mainly due to the uneven surface pressure of mechanical polishing, and the anodic oxidation produces a matte surface, so mechanical polishing cannot be directly used for bright anodization.
In 1934, in the United Kingdom and the United States, the electropolishing process "Brytal" and "Alzak" were invented almost simultaneously, and the anode was electrolyzed in alkaline (Brytal process) or fluoroboric acid (Alzak process) solution for 10-20 min to obtain higher reflectivity. The surface continues to remain after anodization. Electrolytic polishing uses the action of electric current to cause an electrochemical reaction of the aluminum alloy, which dissolves in different degrees on the uneven surface of the aluminum alloy, so that the surface of the aluminum piece has a smooth mirror effect. The electropolished aluminum parts retain most of the gloss after subsequent anodizing. Electrolytic polished aluminum parts have good corrosion resistance, and can be rusted in the atmosphere for a long time without anodizing, maintaining the original luster. In the Second World War, the demand for aluminum mirrors was large, and the electrolytic polishing of aluminum developed rapidly during this period.
High-purity aluminum sheets (99.99%) are electropolished to give a mirror effect with a reflectance 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 type, 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 ° C, the current density is 10 to 20 A/dm ² , and some processes are as high as 150 A/dm ² .
The main pollutant of electrolytic polishing is hexavalent chromium. In recent years, with the enhancement of environmental awareness, electrolytic polishing liquids containing no chromic acid have gradually become popular. The main problem with electrolytic cleaning solutions that do not contain chromic acid is that some alloys are not sufficiently bright to polish. In addition, after the aluminum alloy is polished in the treatment liquid, the brightness will rapidly drop under the condition of no electricity supply, and it needs to be immediately taken out from the treatment liquid and washed with water, which has a certain influence on the wide application of the process. Later, some additives were added to the treatment solution to alleviate these two problems.
The electropolishing process is mature, the pollution is small, but the working current is large and the cost is high. Currently, it is used for the processing of high-brightness and high-decorative aluminum parts.
4. Chemical polishing
Chemical polishing was invented in the late 1940s. At that time, Henley was doing electrophosphorization of phosphoric acid-sulfuric acid type. When there was no electricity, it was found that the corrosion of aluminum parts had a bright effect. Then he carefully studied this phenomenon and obtained the earliest chemical polishing process: phosphoric acid. 75% by volume, 25% by volume of sulfuric acid, and an operating temperature of 90 to 100 °C. Later, it was found that adding 10% nitric acid to the above process can obtain a particularly bright effect. At this time, chemical polishing is applied in the industry, and the corresponding patents are successively announced. Bright anodized steadily enters the market, replacing some of the steel or copper nickel -chromium plating processes. [next]
Chemical polishing does not require electricity, and does not require special fixtures. It is easy to operate, but requires good heating and ventilation equipment. High-purity aluminum can achieve a reflection rate of 100%, and ordinary aluminum alloys can also achieve decorative gloss. Since chemical polishing is less expensive than electropolishing, most bright anodizing is done with chemical polishing. The most commonly used chemical polishing processes are: 75% phosphoric acid (volume fraction), 150% nitric acid (volume fraction), 10% sulfuric acid (volume fraction), an operating temperature of 90 to 110 ° C, and a time of 0.5 to 3 min. Some processes only have phosphoric acid and nitric acid, and some add acetic acid, chromic acid or hydrofluoric acid. Adding a small amount of diamond, nickel, and copper salts can increase the brightness of the polishing. The biggest disadvantage of chemical polishing is that it produces NOX toxic gases. The yellow NOX gas is a strong carcinogen and is a "yellow dragon" that is difficult to remove in chemical polishing workshops. Many methods have been used to solve this technical problem. Japan's Tajima uses a "cage" compound to absorb toxic gases to obtain a new chemical process without yellow smoke; Germany uses 16% (volume fraction for aluminum parts with a purity of 99.99%). Ammonium hydrogen fluoride, 13% (volume fraction) nitric acid, 25 g / L dextrin, operating temperature is low, and there is little evolved gas. Kaiser Aluminium and Chemicals invented a similar process (volume fraction): 2.5% nitric acid, 0.60%, ammonium bifluoride, 0.6% chromic acid, 0.6% glycerol, 0.05% copper nitrate. There is also an organic sulfur sulfide instead of nitric acid in the phosphoric acid-sulfuric acid formula to obtain a yellow smoke-free polishing process.
In addition to acid chemical polishing, there is an alkaline chemical polishing process, but alkaline chemical polishing is far less effective than acid chemical polishing, so it is less used. A typical alkaline polishing formulation contains: NaOH, NaNO ₂ , NaNO ₃ , Na ₃ PO ₄ , Cu(NO ₃ ) _{2 ,} etc., and alkaline chemical polishing without toxic gas precipitation.
Third, anodizing 1. Sulfuric acid anodized sulfuric acid anodizing has the following characteristics:
(1) The cost of the bath 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. It is not necessary to add other chemicals. It is recommended to use chemical pure sulfuric acid, and industrial grade sulfuric acid with less impurities can also be used. , so the cost is particularly low.
(2) The oxide film has high transparency. The sulfuric acid anodized film of pure aluminum is colorless and transparent. For aluminum alloys, the transparency decreases as the alloying elements Si, Fe, Cu, and Mn increase. The color of the sulfuric acid anodized film is the lightest relative to other electrolytes.
(3) The coloring property is high, the sulfuric acid oxide film is transparent, the porous layer has strong adsorption property, is easy to be dyed and colored, and the coloring is bright and hard to retreat, and has a strong decorative effect.
(4) The operating conditions of sulfuric acid anodizing are:
H ₂ SO ₄ (by volume) 10% to 30%
Temperature °C 18～22
Al/gL ^-1 ≤20
Current density / A.dm ^-2 0.6~3
Time/min 10~60[next]
2. Oxalic acid and chromic acid anodizing oxalic acid anodizing is more common in Japan. The characteristics of oxalic acid oxide film are similar to those of sulfuric acid oxide film. The porosity is lower than that of sulfuric acid oxide film, and the corrosion resistance and hardness are higher than that of sulfuric acid oxide film. The oxalic acid bath cost and operating voltage are higher than sulfuric acid, and some alloys have a darker oxalic acid oxide film. Both oxalic acid and sulfuric acid anodization require a good cooling system.
The oxalic acid anodizing operating conditions are:
Oxalic acid (volume fraction) 2% to 10%
Temperature / °C 15 ~ 35
Current density / A.dm ^-2 0.5~3
Voltage / V 40 ~ 60
The chromic acid anodized film is particularly resistant to corrosion. It is mainly used in the dry-aircraft manufacturing industry. The chromic acid oxide film and the paint have strong adhesion, and are also used as the bottom layer of the paint. The chromic anodized film is gray opaque and generally not used for decoration.
The operating conditions for chromic acid anodization are:
Cr03/gL ^-1 30～100
Temperature / °C 40 ~ 70
Current density / A.dm ^-2 0.1~3
Voltage / V O ~ 100
Time / min 35 ~ 60
3. Hard anodization 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 lowered to 0 ° C, and the current density was increased to 2.7 to 4.0 A/dm ² to obtain 25 to 50 μm. "hard oxide film". A hard oxide film can be obtained at 5 to 15 ° C by adding a small amount of sulfuric acid with oxalic acid. Some patents use hard acid anodic oxidation by optimizing the concentration of sulfuric acid with the addition of organic acids or other additives such as benzene hexacarboxylic acid.
In Scotland, Campbell invented an AC-DC superimposed power supply with high-speed electrolyte flow at 0 ° C and a current density of 25-35 A/dm ² to obtain a 100 μm hard anodized film.
Now, when pulsed current is used for hard anodization, especially high-copper aluminum alloys are generally difficult to hard anodize, and pulse currents can be used to prevent "ablation". There are also many power supplies for hard anodizing, AC plus DC, single or three phase pulse currents, reverse currents, etc. at various frequencies. Conventional DC hard anodization, current density generally can not exceed 4.0A / dm ² , single-phase rectified pulse power supply, the peak pulse of current can be large, but maintaining the uniform thickness of the oxide film is an important issue.
Due to their unique hardness and wear resistance, hard anodized films are gradually being adopted by aerospace, aerospace, automation, automotive, computer equipment, electronics and other industries. At present, it is being developed to be sealed with a solid lubricant such as polytetrafluoroethylene or molybdenum disulfide, so that the hard anodized film has self-lubricating properties and has a wider application prospect.

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