Status and Development Trend of Anodic Oxidation, Coloring and Sealing of Aluminum and Aluminum Alloys

Due to its high strength/weight ratio, ease of molding processing and excellent physical and chemical properties, aluminum and its alloy materials have become the second largest class of metal materials used in the industry after steel. However, aluminum alloy materials have low hardness and poor abrasion resistance, and often suffer from abrasion damage. Therefore, aluminum alloys are often subjected to corresponding surface treatment before use to meet their environmental adaptability and safety, reduce abrasion, and prolong their service life. . In the industry, anodization is more and more widely used to form a thick and dense oxide film on the aluminum surface to significantly change the corrosion resistance of the aluminum alloy and improve the hardness, wear resistance and decorative properties.

Anodizing is a more basic and more versatile method of aluminum alloy surface treatment in the country. Anodizing can be divided into ordinary anodizing and hard anodizing. The color film obtained by electrolytic coloring of aluminum and aluminum alloys has good wear resistance, light resistance, heat resistance and corrosion resistance, and is widely used in the decorative anti-corrosion of modern architectural aluminum profiles. However, the aluminum anodic oxide film has a high porosity and adsorption capacity, and is easily eroded by contaminants and corrosive media. The heart must be subjected to sealing treatment in order to improve corrosion resistance, anti-pollution ability, and fixation of the pigment body.

2 Anodizing of Aluminum and Aluminum Alloys

2.1 Ordinary anodizing

Aluminum and its alloys can form an Al2O3 film on the surface by ordinary anodization. Different Al2O3 films are used because of different anodizing solutions. During anodization, the growth of the oxide film on the aluminum surface involves two processes: the electrochemical generation of the film and the chemical dissolution process. Only when the growth rate of the film exceeds the dissolution rate, the oxide film can grow and thicken. Ordinary anodizing mainly includes anodization of sulfuric acid, anodization of chromic acid, anodization of oxalic acid, and anodization of phosphoric acid. The following describes some common new anodizing processes.

2.1.1 Wide temperature rapid anodization [1]

The temperature of sulfuric acid anodic oxidation electrolyte is required to be below 23°C. When the temperature of the solution is higher than 25°C, the oxide film becomes loose, the thickness is thin, the hardness is low, and the wear resistance is poor. Therefore, the oxidation process is added to the original sulfuric acid solution to the original process. To make improvements, the improved solution formulation is:

Sulfuric acid (ρ=1.84g/cm3) 150-200g/L (better 160g/L) CK-LY additive 20-35g/L (better 30g/L) Aluminum ion 0.5-20g/L (preferred value 5g/L)

CK-LY oxidation additives include specific organic acids and conductive salts. The former can increase the working temperature of the electrolyte, inhibit the chemical dissolution of the anodic oxide film, and have a good effect on inhibiting the loosening of the oxide film at a higher temperature; the latter can Enhances the conductivity of the electrolyte, increases the current density, and accelerates the deposition rate. The additive is soluble in sulfuric acid electrolyte and has complexation effect on the metal ions in the electrolyte, so that the tolerance of aluminum ions in the solution is increased, the life of the oxidation liquid is prolonged, the operating temperature can reach above 30° C., and the ordinary sulfuric acid oxidation process 21 Above the °C, it is necessary to open the chiller; at the same time, the oxidation time is reduced, and a high-quality oxide film can be obtained.

2.1.2 Boric acid-sulfuric acid anodizing [2]

Boric acid-sulfuric acid anodizing is a new thin-layer anodizing process replacing chromic acid anodizing. The composition of boric acid-sulfuric acid anodizing solution is: 45g/L H2SO4+8g/L H3BO3.

Anodized film stripping solution: According to ASTM B137 (U.S. experimental material standard) prescribed solution, namely: 20g/L CrO3+35mL/L H3PO4.

2.1.3 Other aspects of process improvement

Gong Yunlan et al. studied the high-voltage anodization of aluminum in chromic acid [3]. The results showed that the oxide film obtained by high-voltage anodization of chromic acid system is porous, and the membrane pore size is extremely irregular, dendritic, and the concentration of pore size and film thickness. Have influence.

Aluminium samples were anodized in phosphoric acid using a direct-current constant-voltage electrolysis method. The experiment shows that with the increase of the electrolysis voltage, the thickness of the barrier layer, the cell diameter and the pore diameter of the porous layer all increase linearly. The reason is closely related to the ion migration. This technology originated in the 1930s. Since the phosphate oxide film has strong adhesion, it is a good bottom layer for electroplating and painting, so it has been more and more widely used.

2.2 Hard Anodizing of Aluminum and Aluminum Alloys

Aluminium and its alloys, after hard anodizing, can form an oxide film with a thickness of several tens to several hundreds of micrometers on the surface, due to the extremely high hardness of the oxide film (up to 400-6000 kg/mm2 on aluminum alloys. , on pure aluminum up to 1500kg/mm2), excellent wear resistance, heat resistance (oxide film melting point up to 2050 °C) and insulation, greatly improving the physical properties of the material itself, chemical properties and mechanical properties, in defense And machinery manufacturing has been widely used.

2.2.1 Hard Anodizing of Sulfuric Acid

The sulfuric acid composition is simple and stable, easy to operate, and can obtain a hard film of tens to hundreds of micrometers at low temperature oxidation. The main drawback of hard anodic oxidation of sulfuric acid is that it is generally performed at a low temperature and is greatly affected by the composition of the aluminum alloy.

2.2.2 Hard Acid Anodizing at Room Temperature with Mixed Acids

Mixed acid at room temperature Hard anodization refers to the use of sulfuric acid as the main, adding a small amount of diacid such as oxalic acid to obtain a thick film, while expanding the upper limit of the use temperature, which allows the anodizing temperature to be increased to 10-20°C. The characteristics of the obtained oxide film are similar to those of the anodized film of sulfuric acid. Electrolysis at 10-20°C results in a good wear resistance oxide film and high coloring rate; high acid density mixed acid electrolysis is used to prevent dissolution of the oxide film, which can be implemented at higher temperatures and lower production costs. Make the film smoother, smoother, finer, thicker and harder.

2.2.3 Pulsed Hard Anodizing

Pulsed hard anodic oxidation uses intermittent current or alternating high and low currents to oxidize, successfully avoiding scorching and powder. At room temperature, the obtained oxide film is excellent in hardness, corrosion resistance, flexibility, resistance and thickness uniformity. In general DC oxidation, and production efficiency can be increased by 3 times. The performance of the oxide film is shown in Table 1.

2.2.4 hard anodized cast aluminum alloy [4]

The alloy containing more silicon (over 7%) is difficult to anodize in the sulfuric acid system, while the ZL102 alloy contains up to 10%-13% of silicon, and the presence of high silicon can easily cause crystal grain segregation, resulting in Film formation is difficult and film uniformity is poor.

Through the experimental research, Ouyang New Equality Man developed a process recipe for high-silicon aluminum alloy hard anodizing so that the DC power source can successfully produce a good hard oxide film on the ZL102 alloy. The experiment adopts the constant current method and additional air agitation, and the resulting better formula is [4]:

Sulfuric acid (ρ=1.84g/cm3) 15-40g/L

Sulfosalicylic acid 20g/L

Additive MY 2.5-5.0g/L

Current density 3-6A/dm2

60min

Temperature 0°C

Among them, MY is an anionic surfactant and also a complexing agent for Al3+. It can be preferentially adsorbed at high current density and discharge to make the electric field distributed evenly. At the same time, it can also act as a buffer to inhibit the dissolution of the oxide film, thereby obtaining a uniform and smooth oxide film.

Zhou Jianjun et al. performed hard anodic oxidation of copper-containing high-silicon cast aluminum alloys with a DC superimposed pulsed power supply, and studied the effect of power pulse amplitude on the film properties. The better process conditions for the experiment are [5]:

Sulfuric acid (ρ=1.84g/cm3) 120-160g/L

Additive 7-8g/L

Pulse ratio 1.0:1.3

Current density 2.5-3.5A/dm2

Temperature 0°C

50min

Stir compressed air

The results show that increasing the pulse amplitude of the power supply during oxidation can significantly improve the film performance. The use of DC superimposed pulsed hard anodic oxidation can produce oxide films with better properties on difficult-to-oxidize cast aluminum alloys containing copper and high silicon.

2.2.5 Low Pressure Hard Anodizing [6]

The vast majority of aluminum alloy hard anodized parts, especially the sealing surface and sliding parts of the parts, not only require the film layer to have a higher hardness and thickness, but also require a low roughness (Ra 0.08-0.16). By analyzing the surface state of the parts in the oxidation process and measuring the growth rate of the film, Raining found out the main reasons that affect the quality of the oxide film and the surface roughness, and proposed a low voltage hard anodizing process:

Sulfuric acid (ρ=1.84g/cm3) 220-240g/L

T -2-2°C

t 180min

DA 0.8-1.0A/dm2

The final voltage ≤40V

Feeding mode: In the initial 20 min, the current density rises to 0.8-1.0 A/dm2 and is always maintained until the end of oxidation.

In addition, Chengdu Aircraft Industry Corporation evaluated the properties of the anodized aluminum alloy film according to the US military standard MIL-A-8625F and the McDonnell Douglas company standard DPS11.02, and studied the specific material and the applied current density to the film thickness, film formation time, The effect of corrosion resistance, wear resistance and burn rate. The results show that the high quality aluminum alloy anodic oxide film can be obtained under the high current density produced by the AC superimposed power supply.

3 Electrolytic coloring

Electrolytic coloring of anodized aluminum can improve decorative effects and product value. The thickness, uniformity and structure of the oxide film are directly related to the coloration rate and color difference of the electrolysis. During electrolytic coloring, metal ions are reductively deposited on the barrier layer at the bottom of the film pores. Metallic particles develop color due to the scattering of light. The key to depositing metal on the barrier is to activate the barrier. Therefore, the polarity of alternating current should be used to increase its chemical reactivity. Since the barrier layer has a rectifying effect, the alternating current is changed to a direct current, so the negative component of the aluminum side current dominates, and the metal ions entering the film hole are reduced and precipitated.

In the past, most of the aluminum colorings were bronzes, mainly tin salts or nickel-tin mixed salts. In recent years, electrolysis of bronze color will be replaced by titanium, gold, imitation stainless steel, light red, champagne, silver and other light colors. Titanium gold is lively and not glamorous, red in the yellow, is pleasing to the eye, and has the advantages of low cost of coloration and high added value. It has become very clear as the main tone in light colors. Golden yellow, which is mainly salted with silver salts and manganese salts, has a good market in Hong Kong and Vietnam. Manganese salt is vivid and golden, with low cost. However, it is not stable and is not suitable for continuous production. Silver salts can be colored in various colors such as golden yellow, green gold, yellow green and gold earth. The bath is very stable and has potential potential economic benefits. It should be developed and applied.

3.1 Improvement of electrolytic coloring process

3.1.1 bright black process on the surface of aluminum alloy [7]

This process is a binary metal oxide film layer formed by the coloring reaction of tin-copper ions in a colored electrolytic cell. The color is black and bright, and it is a unique aluminum alloy anti-corrosion and decorative material. The electrolytic coloring liquid composition was a mixed solution of 30% SnSO4, 30% NiSO4, and 15% CuSO4. The oxidized aluminum material is an anode, and the graphite electrode is used as a cathode. A 50Hz 220V AC power source is transferred to the electrolytic cell after being adjusted to 8V by a voltage regulator. Electrolysis is performed for 10 minutes to obtain a bright black aluminum alloy surface.

3.1.2 anodized aluminum interference electrolysis coloring process [8]

In the study of light interference electrochromism with tin salts, it has been found that it is difficult to obtain a blue interference color. It is also difficult to obtain a blue color by an ordinary electrolytic coloring method. Zhi Lan et al. conducted research in this regard. The experimental materials are L2 (No. 2 industrial pure aluminum, 99.6% aluminum) and LD31 (equivalent to the United States 6063). The sample size is L250 mm×50mm×1mm, and the LD3125mm×25mm angle material is 1.3mm thick. The surface area is 0.68dm2. Anodizing conditions, H2SO4 (ρ=1.84g/cm3) 180g/L, 18°C, 1.2-1.4A/dm2, 30min, film thickness 12-14μm; DC reaming treatment; tin salt electrolytic coloring: SnSO416g/ L, H2SO414g/L, mixed additive 16g/L, 18-20°C, AC coloring voltage 12-14V, In addition, copper salt and Cu-Ni mixed salt electrolytic coloring are used to obtain yellow red, green and blue more stable interference color.

3.2 The development of a new power source is an important means to open up new processes for electrolytic coloring [9]

It is a new research hotspot to change the waveform of the power source and the way of applying electricity to improve the comprehensive performance of the anodized film and to develop a new process of electrolytic coloring. Commercially available power supplies include pulse, current reversal (commutation), and DC pulse. The functionalized oxidation and coloring compatible micro-arc oxidation power supply is aimed at increasing the oxidation speed, thickness uniformity, hardness, porosity distribution, and improving pore structure morphology. Researching new power sources can overcome the deficiencies and limitations in chemical and electrochemical methods.

4 Closed processing

In order to improve the corrosion resistance, anti-pollution, electrical insulation, and wear resistance of the anodized film, the aluminum and aluminum alloys should be closed after anodizing and coloring. The method is more, and the non-colored oxide film can be closed with hot water, steam, dichromate and organic substances; the colored oxide film can be closed with hot water, steam, inorganic salts and organic substances.

4.1 The main method of closure

4.1.1 Closed by boiling water and steam

With the steam sealing method, all pores can be effectively closed. If the oxidized part is vacuum treated for a period of time before it is closed, the sealing effect is more pronounced. Steam sealing is not characterized by the phenomenon of color diffusion, so it is not appropriate to have "flow color". However, the equipment and the cost of the steam sealing method are higher than the boiling water method, so unless there is a special requirement, it should be sealed with boiling water as much as possible. When closed with steam, the temperature should be controlled at 100-110°C for 30 minutes. The temperature is too high and the hardness and wear resistance of the oxide film are seriously reduced. Therefore, the steam temperature should not be too high.

4.1.2 Dichromate Blocking

This method is suitable for blocking the anodized film layer and the chemically oxidized film layer in the sulfuric acid solution. The oxide film treated by the method is yellow and has high corrosion resistance, but is not suitable for decorative use. The essence of this method is to produce a chemical reaction between the oxide film and the dichromate at a higher temperature. The reaction products, the basic aluminum chromate and the aluminum dichromate, precipitate in the film pores, and the thermal precipitation causes the oxide film. The surface of the layer is hydrated and the sealing effect is enhanced. Therefore, it can be regarded as the double sealing effect of filling and hydration. The commonly used blocking solution is a 5%-10% aqueous solution of potassium dichromate, the operating temperature is 90-95°C, the blocking time is 30 minutes, and there must be no chloride or sulfate in the precipitate.

4.2 Closed Treatment Process Improvements

4.2.1 Research at room temperature closure [10]

Sealing at normal temperature has the advantages of energy saving, short closing time and good sealing effect, and has been widely recognized and accepted.

Blocking liquid formula and process conditions are as follows:

Nickel acetate 5-8g/L

Sodium fluoride 1-1.5g/L

Surfactant 0.3-0.5g/L

Additive A 3g/L

pH 5.5-6.5

T 25-60°C

t 10-15min

The closed membrane obtained by the closed process at room temperature has a compact structure and excellent corrosion resistance. Comparing with the boiling water sealing method, it has the advantages of high speed, energy saving, simple operation and convenient source of raw materials. The longer the closed time, the better its performance.

4.2.2 Hydrolysis salt sealing method [11]

Hydrolysis salt sealing method, also known as passivation. At present, it is widely used in China, and is mainly used for membrane sealing after dyeing. Its sealing mechanism is that the easily-hydrolyzable cobalt salt and nickel salt are adsorbed on the oxide film and then hydrolyzed in the micropores of the anodic oxidation film to produce hydroxide precipitates. Closed. The process recipe is:

NiSO4·7H2O 4-5g/L

CoSO4·7H2O 0.5-0.8g/L

H3BO3 4-5g/L

NaAc·3H2O 4-6g/L

pH 4-6

T 80-85°C

t 15-20min

This method overcomes many of the disadvantages of boiling water seals, and the quality of the seal has reached the national standard.

4.3 Micro-Arc Anodic Oxidation

Micro-arc anodization, also known as micro-plasma oxidation or anodic spark precipitation, is an evolution of anodizing technology that uses higher voltages than conventional anodizing. Micro-arc anodization breaks through the limitations of traditional anodizing, placing metals such as Al, Ti, and Ta or their alloys in the electrolyte, and using electrochemical methods to create spark discharge spots in the micropores on the surface of the material, in thermochemical, plasma The anodic oxidation of the ceramic film layer is produced under the joint action of body chemistry and electrochemistry. During the discharge process, there are about 105 sparks per square centimeter of aluminum anode surface. The instantaneous temperature during discharge can reach more than 8000K, generating a ceramic membrane with properties similar to cemented carbide. This oxide film is extremely hard, wear-resistant and has high insulation resistance. Oxidation in special electrolytes can also create enamel-smooth aluminum surfaces with different shades of patterns, which can be used as both advanced decorative materials and functional films such as automotive piston rings and insulation layers for the electronics industry. Micro-arc anodizing technology uses high-voltage, high-current working methods to obtain more and more extensive applications in the preparation of multi-functional protective coatings, and has broad application prospects in aerospace, aviation, machinery, electronics, textile and other industrial fields. .

4.3.1 Microarc Anodic Ceramic Coatings Performance Study [12]

Lu Lihong et al. used a pulsed power supply to perform micro-arc oxidation on the aluminum alloy (ZL108) substrate for engine pistons.

The process flow is: deoiling → deionized water rinse → micro-arc oxidation → tap water washing → natural drying. The main components of the electrolyte are trisodium citrate and sodium phosphate. Microarc oxidation voltage: The working voltage is adjustable, the initial breakdown voltage is 80V, and the higher operating voltage is 230V. Experiments show that the micro-arc oxidation film surface roughness is higher than the general electroplating layer and anodized layer, much lower than a variety of spray layers. With the increase of current density and strengthening time, the surface roughness of the film increases. At the beginning, as the current density increases, the hardness of the obtained film also increases. After exceeding 8 A/dm2, the hardness of the film tends to be stable. After micro-arc oxidation, wear resistance increased 3-4 times.

4.3.2 Improvement of Micro Arc Anodic Oxidation Technology

4.3.2.1 Micro-arc oxidation self-lubricating ceramic coatings [13]

The weakness of the ceramic layer is its high coefficient of friction and increased wear on the wear parts. One-step electrochemical method was used to study the tribological modification of micro-arc oxidation ceramics. The use of self-made special pulse power supply, the base material for the ZL108, based on the basic micro-arc oxidation electrolyte, dissolved in the appropriate amount of ammonium thiomolybdate and the corresponding additives. Experiments show that after micro-arc oxidation, a self-lubricating ceramic coating is co-generated on the surface of aluminum alloy by one-step method, and the friction coefficient is reduced from 0.8-1.2 of the general micro-arc oxidation coating to 0.2-0.5, and the friction prepared by this process is used. The tribological performance is significantly improved and the service life is extended.

4.3.2.2 Micro-arc oxidation ceramic layer graphite phase

The method of simultaneous deposition of graphite phase in the micro-arc oxidation process can improve the anti-friction properties of the ceramic layer. The wear test was performed on the ceramic layer. The base material was ZL108, the electrolyte used was NaOH solution, and the anti-friction ion added to the original electrolyte was Graphite, while the temperature of electrolysis does not exceed 40 °C. Stir to suspend graphite ions. Experiments show that the method of adding graphite to the electrolyte makes micro-arc oxidation of ZL108, meanwhile, the second phase of graphite is deposited synchronously in the ceramic layer, and the purpose of reducing the friction of the aluminum alloy micro-arc oxidation ceramic layer is achieved.

5 Prospects of Anodic Oxidation Technology

Aluminum and aluminum alloy anodizing technology to improve the oxidation rate and hardness for the development direction. It is recommended to use the EOE-88 series pulse power supply with pulse wave to increase the oxidation speed and comprehensive performance. The output voltage and current have rich pulse components, equivalent to 300 small pulse waves per second superimposed on the DC wave, and the film formation speed fast. For thick-film oxidation, a "fast pulse" power source with a frequency of 3-13.3 Hz can be used, giving full play to the advantages of power saving, speed and hardness enhancement. This type of power supply has no obvious advantage when the oxide film is 12 μm or less.

Composite anodization As a new type of anodizing technology, magnetic powders such as Fe3O4, CrO2, and TiO2 are added to the electrolytes of sulfuric acid, oxalic acid, and trisodium phosphate, respectively, and super-hard powders such as Al2O3, SiC, SiN, and graphite are electrically conductive. The powder (micron) is suspended in the electrolyte and anodized. The process has the advantages of easy operation, simple equipment, low cost, etc. Compared with the conventional anodizing, the oxidation rate, the upper limit of the operating temperature and the performance of the film layer are significantly improved. Japan's Yoshimura Yoshimura first conducted this research, and the results showed that some powders can increase the hardness of the film, some powders can reduce the pressure of the oxidation tank, and some powders can increase the thickness of the film. Recent research results show that: Al2O3 powder can make aluminum in the H3PO4 solution oxide film hardness and corrosion resistance more than doubled, and thus has a broad research future.

The research of additives is very active at present, there are many kinds of additives and the mechanism of action is not the same. The effective role of additives makes it have a huge market potential.

In summary, there are many new processes in the anodizing of aluminum and its alloys, but it is also challenged by various surface treatment methods. It is expected that in the next 10 years, the anodizing technology will still be the main surface treatment method, but Continuous improvement can only dominate for a long time.