What is Anodized Aluminum?
Anodized aluminum is a type of aluminum that has undergone an electrochemical process called anodizing, which increases the natural thickness of the oxide layer on the surface of the metal. This process significantly improves its corrosion and wear resistance properties, in addition to allowing the metal to be colored.
Anodizing Process
During anodizing, aluminum is submerged in an electrolytic bath, usually a sulfuric acid solution, and used as the anode, while metal plates as the cathode surround the aluminum part. When electrical current is applied, oxygen is released onto the surface of the aluminum, forming a layer of aluminum oxide (alumina) that is fully integrated into the underlying structure of the aluminum.
Characteristics of Anodized Aluminum
- Durability : The anodized surface is very hard and resistant to wear and erosion.
- Aesthetics : Anodizing allows coloring by absorption of dyes into the oxide layer or by electrolytic deposition of metals, which gives aluminum an attractive appearance and can be customized in a variety of colors.
- Corrosion Resistance : The oxide layer is impermeable and does not peel or chip, providing excellent protection against corrosion.
- Maintenance : Anodized surfaces do not require painting or frequent cleaning treatments due to their durability and corrosion resistance.
Anodized Aluminum Applications
Due to its improved properties, anodized aluminum is widely used in a variety of applications, including:
- Civil Construction : on building facades, windows, doors and other architectural elements.
- Automotive Components : in parts where durability and resistance are critical.
- Electronics : in casings for computers, smartphones and other devices due to their ability to dissipate heat efficiently.
- Kitchen Utensils : in cookware and utensils due to their food safety and ease of cleaning.
Anodizing transforms common aluminum into an extremely functional and aesthetically versatile material, significantly expanding its range of industrial and commercial applications.
I. Introduction
1. Characteristics of Aluminum and its Alloys
Low density; good plasticity; easy to strengthen; good conductivity; corrosion resistant; recyclable; weldable; easy surface treatment.
2. Briefly describe the characteristics and forms of corrosion in aluminum alloys
1) Corrosion properties:
(1) Acid corrosion: Aluminum exhibits different corrosion behaviors in various acids. In general, a passivation film is formed in concentrated oxidizing acids, which presents excellent corrosion resistance, while in dilute acids pitting corrosion phenomena occur. Localized corrosion;
(2) Alkaline Corrosion: In alkaline solutions, alkali reacts with aluminum oxide to form sodium aluminate and water, which further reacts with aluminum to form sodium hydrogen aluminate. General corrosion;
(3) Neutral Corrosion: In neutral saline solutions, aluminum may be passive or corrode due to the effect of certain cations or anions. Localized corrosion.
2) Forms of corrosion:
Pitting corrosion, galvanic corrosion, crevice corrosion, intergranular corrosion, filiform corrosion and exfoliation corrosion, etc.
Pitting corrosion: The most common form of corrosion, the degree of which is related to the medium and alloy.
Galvanic Corrosion: Contact corrosion, corrosion of dissimilar metals (bimetallic). In an electrolytic solution, when two metals or alloys are in contact (conductors), the corrosion of the more negative metal is accelerated, while the more positive metal is protected from corrosion.
Crevice Corrosion: Occurs when two surfaces are in contact with each other, forming a crevice. The oxygen concentration cell is formed due to the dissolution of oxygen in this area, leading to corrosion within the crack.
Intergranular Corrosion: Related to inadequate heat treatment, alloying elements or intermetallic compounds precipitate along the boundaries of the grains, which act as anodes in relation to the grains, forming a corrosion cell.
Filiform Corrosion: A type of under-film corrosion that develops like a worm under the film. This film may be a paint film or other coatings and generally does not occur under the anodic oxide film. Filiform corrosion is related to alloy composition, pretreatment before coating, and environmental factors including humidity, temperature, and chlorides.
Peeling corrosion: Also known as peeling corrosion.
3. What aspects does aluminum alloy surface treatment technology include?
Surface mechanical pretreatment (mechanical polishing or sanding, etc.), chemical pretreatment or chemical treatment (chemical conversion or chemical coating, etc.), electrochemical treatment (anodizing or electroplating, etc.) and physical treatment (spraying, enamel vitrification, and other physical surface modification techniques), etc.
Enamel vitrification: fusion of a mixture of inorganic substances into glass-like materials with different melting points.
4. What are the characteristics of aluminum alloy anodic oxide film?
Corrosion resistance; hardness and wear resistance; decorative; adhesion of organic coating and galvanized layer; electrical insulation; transparency; functionality.
II. Mechanical pretreatment of aluminum surface
1. The purpose of pre-treatment:
(1) To improve good appearance conditions and surface finish quality.
(2) To improve product quality.
(3) To reduce the impact of welding.
(4) To create decorative effects.
(5) To obtain a clean surface.
2. Polishing operation requirements
(1) Selection of abrasive type and granularity:
This is based on the hardness of the part material, surface condition, and quality requirements; the harder or rougher the surface, the harder and coarser the abrasive used.
(2) Polishing must be carried out in multiple steps and the pressure of the part towards the grinding wheel must be moderate.
(3) A new grinding wheel must be preliminarily scraped to achieve balance before the abrasive adheres.
(4) The abrasive should be replaced regularly.
(5) Alloy materials should be selected according to different needs.
(6) The appropriate grinding wheel speed should be selected, generally controlled at 10~14m/s.
(7) The polishing effect depends on factors such as the abrasive, the rigidity of the grinding wheel, the rotational speed of the grinding wheel, the contact pressure between the workpiece and the grinding wheel, practical experience and qualified techniques.
3. Grinding and Polishing Concepts
Grinding: The operation after the fabric wheel is pasted with abrasive. Purpose: Remove burrs, scratches, corrosion stains, sand eyes, pores and other apparent defects on the surface of the part.
Polishing: The operation after applying polishing paste to a soft cloth wheel or felt wheel.
4. Common problems and solutions:
Common problem: “Scorch” brand.
Cause:
(1) Improper selection of grinding wheel, abrasive and polishing agent;
(2) Inadequate force used in polishing;
(3) Long grinding time;
(4) Overheating during grinding.
Measurements:
(1) Slight alkaline corrosion in dilute alkaline solution;
(2) Mild acid etching: such as chromic acid-sulfuric acid solution or 10% sulfuric acid solution used after heating;
(3) 3% by weight Na2CO3 and 2% by weight Na3PO4, the solution is treated at a temperature of 40 ~ 50 ℃ for 5 minutes, severe cases can be extended to 10 ~ 15 minutes.
After the above cleaning and drying treatment, immediate repolishing should be done with a precision polishing disc or mirror polishing disc.
Prevention:
Use suitable grinding wheels and polishing discs; use appropriate polishing agent; The grinding time between the workpiece and the polishing wheel must be controlled appropriately.
III. Aluminum Chemical Pretreatment
1. What are the methods for degreasing aluminum? What are the principles of these processes?
1) Degreasing methods:
Acid degreaser, alkaline degreaser and organic solvent degreaser. Purpose: To remove oil, grease, dust and other contaminants from the aluminum surface to enable more uniform alkaline washing, thus improving the quality of the anodic oxidation film.
2) Principle
(1) Acid degreasing principle: In an acid degreasing solution based on H2SO4, H3PO4 and HNO3, oils and fats undergo hydrolysis to produce glycerin and corresponding higher fatty acids, achieving the purpose of degreasing.
(2) Alkaline degreasing principle: Alkali reacts with oil to form soluble soap. This saponification reaction removes the bond between the oil and the surface of the aluminum material, achieving the purpose of degreasing.
(3) Organic solvent degreasing principle: Taking advantage of the fact that oils are easily soluble in organic solvents, both saponified and unsaponified oils can be dissolved. This method has strong degreasing ability, is fast and non-corrosive to aluminum, thus achieving the purpose of degreasing.
2. What is the purpose of alkaline washing and what defects does it have? What should be the corresponding countermeasures?
1. Purpose: to remove surface contaminants, completely eliminate the natural oxide film on the aluminum surface, reveal the pure metal base, and prepare for the subsequent main surface treatment process.
2) The three main defects of alkaline washing: rough appearance, stains and streaks.
3) Appearance
(1) Rough appearance: a common problem in the production of alkaline washing blasted aluminum materials, often caused by structural defects in the original aluminum material (large grains or large intermetallic compound precipitates); Improving the quality of the internal structure of the original aluminum material can solve the problem at the source.
Causes: A: The original grain size of the aluminum bar for extrusion is large. B: The heating temperature of the aluminum bar is too high or the extrusion speed is too fast. C: The tonnage of the extruder used is very small. D: Insufficient quenching after extrusion. E: The alkaline washing speed is very fast.
Countermeasures: Use extruded aluminum rods with grain size that meets national standards; control the exit temperature of extruded products; strengthen tempering after extrusion; reasonably control the speed of alkaline washing, etc.
(2) Stains: a fatal defect in aluminum surface treatment: stop subsequent processes or discard as scrap.
Causes:
A: The proportion of recycled aluminum added when melting the cast rods is very high. Al2O3 has a melting point as high as 2050°C, it does not melt during smelting, it only breaks; Erosion during the alkaline washing process leads to corrosive snowflake-like stains. Countermeasures: Control the proportion of recycled aluminum in the anodic oxidation film, it should be less than 10%; refining and removing slag from the melt, the melt should rest for about 25 minutes before smelting, and the melt should be filtered, etc.
B: The chlorine ion content in the water is high. When the quality of aluminum material is poor and the chlorine ion content of the water used is also high, corrosive spots will be revealed during alkali washing or water washing before and after alkali washing. Countermeasures: Improve the quality of the original aluminum material; use tap water that meets national standards; use nitric acid or nitric acid plus sulfuric acid for descaling; adding 1~5g/L of HNO3 to the water tank nickel can also effectively suppress the corrosive effect of chloride ions.
C: Atmospheric corrosion. Aluminum materials placed in coastal atmospheric environments for about 3 days, near smelting furnaces in corrosive atmosphere, rainy weather, etc., often have corrosive marks or stains on the surface. Countermeasures: Reduce the cycle time of the original aluminum material in anodic oxidation; place the original aluminum material with anodic oxidation in a dry and well-ventilated environment; for long-term placement or rainy days, appropriate covering treatment can be carried out on the original aluminum material, etc.
D: “Hot spot” extrusion. The aluminum material contacts the thermally conductive graphite roller on the discharge table, due to different local cooling speeds, the precipitation phase (Mg2Si phase, temperature range 400~250°C) forms on the aluminum material , showing interval spots. Countermeasures: Control the operating speed of the extrusion discharge table (it must be greater than the aluminum extrusion speed); use other heat-resistant materials with low thermal conductivity to replace graphite rollers; borrow the wind-extinguishing force of the cannon; quickly reduce the aluminum material from the extrusion outlet to less than 250 ℃.
(3) Stripes: Defects in alkaline washing stripes caused by improper conditions and operations of the alkaline washing process (the alkaline washing speed is too fast and the transfer speed is too slow). Countermeasures: A: Speed up the transfer. B: Lower the temperature of the alkaline wash bath. C: Reduce the NaOH concentration in the bath. D: The aluminum material is highly compacted and must be reduced accordingly.
3. What is the purpose of dust removal? What are the dust removal methods on the surface of aluminum alloy?
Purpose: To remove surface dust, prevent contamination of the subsequent anodizing bath, and improve the quality of the oxide film.
Methods: Nitric acid dust removal, sulfuric acid dust removal,
4. What are the defects and countermeasures of fluoridated sand surface treatment?
Fluoride sand surface treatment is an acid etching process that uses fluoride ions to produce highly uniform, high-density pinpoint corrosion on the surface of aluminum.
Defects and countermeasures:
(1) The surface has stains: When there are many precipitates in the tank and the concentration of fluoride ions is low, the reaction strength is weak. Precipitates deposit or remain on the surface for a long time, making normal corrosion of fluoride ions difficult.
Countermeasure: Remove excess precipitates in the tank, reduce the density of aluminum, add an appropriate amount of ammonium bifluoride and additives, increase the concentration of fluoride ions, and increase the reaction strength.
(2) The surface is not easy to sand: The liquid in the tank is contaminated by the previous acid degreasing, causing the PH to decrease and the concentration of fluoride ions and additives to be insufficient.
Countermeasure: Adjust the PH value with ammonia or ammonium fluoride and add ammonium bifluoride and additives, etc.
(3) The sand grains on the surface are too coarse: The concentration of fluoride ions in the tank is too high, or the additives are insufficient, or the treatment time is too long.
Countermeasure: Take corresponding measures to control.
(4) Surface gloss varies: The tank process conditions are not properly controlled, or the choice of additives is inappropriate, or there is a problem with the aluminum material.
Countermeasure: Take corresponding measures to control.
(5) Partial areas do not sand: There is a film of compound oxide in the local area.
Countermeasure: Adjust the process flow, such as polishing, polishing, re-acid washing or alkaline washing, etc.
4. Chemical and Electrochemical Polishing of Aluminum
1. Briefly describe the similarities and differences in chemical and electrochemical polishing mechanisms.
1) Chemical Polishing: By controlling the selective dissolution of the aluminum surface, the microscopic protrusions dissolve faster than the recesses, achieving a smooth and shiny surface.
2) Electrochemical Polishing , also known as electropolishing. The principle is similar to chemical polishing, relying on selective dissolution of the protruding parts of the surface to achieve smoothness. The difference is the application of an external current, which reduces processing time.
3) Common point: Both use the same polishing mechanism; Difference: Electrochemical polishing applies a current during the process, while chemical polishing uses chemical oxidants.
2. What are the advantages of chemical and electrochemical polishing?
Compared to mechanical polishing, chemical and electrochemical polishing has the following advantages:
(1) Simple equipment, easy to control process parameters, cost saving and brighter surface;
(2) Capable of processing large components or large quantities of small components as well as complex shaped parts;
(3) Cleaner surface, no residual mechanical polishing dust, with good corrosion resistance;
(4) The mirror reflectivity of the chemically polished surface is higher, the metal texture is better, and no powdery “frost” forms on the surface.
3. Briefly describe the shortcomings and countermeasures of chemical and electrochemical polishing.
1) Chemical polishing defects and countermeasures (taking the phosphoric acid-sulfuric acid-nitric acid process as an example)
(1) Insufficient brightness: Influenced by aluminum composition, nitric acid content, etc.
Countermeasure: Use high-purity aluminum, control the concentration of nitric acid, and make sure the aluminum is dry before polishing.
(2) White deposits: Excessive dissolution of aluminum, which requires control of its content in the bath.
Countermeasure: Adjust the amount of aluminum dissolved in the bath within the normal range.
(3) Rough surface: Very high nitric acid content, excessively intense reaction; or very high Cu content.
Countermeasure: Strict control of nitric acid content; improve the internal quality of the material, reduce the amount of additives, etc.
(4) Transfer corrosion: Occurs when the transition to the rinsing process after chemical polishing is slow.
Countermeasure: Transfer to water to rinse immediately.
(5) Pitting corrosion: Occurs due to the accumulation of gas on the surface forming gas pockets; or due to low nitric acid or Cu content.
Countermeasure: Load workpieces properly, increase workpiece tilt, improve agitation to allow gas to escape. Clean the surface well; control the content of nitric acid, etc.
2) Electrochemical polishing defects and countermeasures (taking the phosphoric-sulfuric-chromic acid process as an example)
(1) Electrical burns: caused by insufficient conductive surface area, poor contact, excessively rapid voltage rise or excessive current density. Countermeasure: Ensure good contact between the workpiece and electrical device, sufficient contact area to accommodate high currents, and avoid excessively rapid voltage rises.
(2) Dark Spots: caused by low current density or uneven local distribution of power lines. Countermeasure: Avoid overload and try to avoid dead zones where power lines cannot reach.
(3) Gas streaks: caused by gas escaping. Countermeasure: Position all part surfaces at an angle during charging, place decorative surfaces vertically toward the cathode, and prevent gas accumulation.
(4) Ice crystal-like adhesions: formed by high aluminum content in the bath or high phosphoric acid content creating aluminum phosphate precipitate. Countermeasure: Reduce the amount of aluminum dissolved in the bath or decrease the phosphoric acid content.
V. Anodizing of aluminum and anodic oxide film
1. Classification of aluminum anodic oxide film:
(1) Barrier type: Also known as shield-type oxide film or blocking layer, it is closely adjacent to the metal surface, dense, poreless, thin, with a thickness determined by the oxidation voltage, no more than 0.1μm, Mainly used for electrolytic capacitors.
(2) Porous type: Composed of two layers of oxide film, the bottom layer is a blocking layer, with a dense and poreless oxide thin layer structure identical to the barrier film, the thickness depends on the voltage; the main part is a porous layer structure, the thickness of which depends on the amount of electricity passing through.
(Blocking layer: Refers to the oxide layer with barrier film properties and formation rules that separate the porous layer from the porous oxide film of aluminum metal.)
2. Thickness, Structure and Composition of Porous Oxide Film
Porous anodic oxide film composition: blocking layer and porous layer; the structure and formation rules of the blocking layer are equivalent to those of the barrier-type oxide film; the generation rules, structure and composition of the porous layer are completely different from the blocking layer.
1) Thickness of the blocking layer: It depends on the externally applied oxidation voltage and is not related to the oxidation time. The film formation rate or film ratio δb/Va; the film formation rate of the barrier oxide film is greater than the film formation rate of the blocking layer of the porous oxide film.
Porous layer thickness: total thickness = porous layer + blocking layer; the total thickness is directly proportional to the product of the current density and the oxidation time (i.e., the amount of electricity that passes).
2) Composition of the blocking layer: dense, poreless amorphous oxide.
Composition of the porous layer: amorphous Al2O3, but not pure.
3) Lock layer structure: double-layer structure. External layer: contains anions in solution; inner layer: mainly composed of pure aluminum oxide.
Porous layer structure: outer layer: contains γ-Al2O3 and α-AlOOH; inner layer: amorphous Al2O3, water infiltration into the oxide film gradually transforms into boehmite α-AlOOH.
SAW. Anodizing Process
1. What are the influences on the sulfuric acid anodizing process?
Impacts of parameters in the aluminum anodizing process with sulfuric acid
(1) The influence of sulfuric acid concentration:
It affects the thickness of the barrier layer of the oxidation film, the conductivity of the electrolyte, the dissolution effect on the oxidation film, the corrosion resistance of the oxidation film and the quality of subsequent pore sealing.
A high concentration has a significant dissolving effect on the oxidation film, resulting in a thin barrier layer and a decrease in the voltage required to maintain a certain current density; the reverse results in a thick, high-stress film.
A high concentration of sulfuric acid requires low voltage to maintain a certain current, but it has a significant impact on the oxidation film. As the concentration and temperature of sulfuric acid increases, the required voltage decreases.
However, a higher concentration of sulfuric acid increases the erosion of the oxidation film by the acid. As the concentration of sulfuric acid increases, the efficiency decreases: that is, more electricity is consumed to obtain an oxidation film of a certain thickness. As the concentration of sulfuric acid increases, the corrosion and wear resistance of the film decreases.
(2) The influence of bath temperature:
1) When the bath temperature increases within a certain range, the type of oxidation film obtained decreases, the film becomes softer but brighter;
2) When the bath temperature is high, the diameter of the pores and the conicity of the outer layer of the oxidation film tend to increase, making sealing difficult, in addition to being subject to sealing “frost”.
3) The oxidation film obtained at higher bath temperatures is easy to dye, but it is difficult to maintain the consistency of color depth, and the oxidation temperature of the general dyed film is 20 ~ 25 ℃;
4) The oxidation film obtained by reducing the bath temperature has high hardness and good wear resistance, but maintaining the same current density during maintenance requires a higher voltage, and the ordinary film uses 18 ~ 22 ℃.
For films thicker than 15μm, when the bath temperature increases, the film quality and metal loss rate decrease significantly, and the hardness of the outer layer of the film becomes lower.
Temperature significantly affects the quality of the oxidation film: all temperatures above 15°C produce soft non-crystalline films. Lower temperatures help produce dense oxidation films. As the temperature increases, the hardness of the film decreases.
To obtain a film with high hardness and good wear resistance, low temperature anodizing must be used. Except for alloy 3004, generally, alloys have the best corrosion resistance at 20℃. Corrosion resistance decreases as the temperature increases and drops to the lowest level of 40℃.
(3) Influence of oxidation voltage:
Voltage determines the size of the pores in the oxidation film: low voltage – small pore size, more pores – large pore size, fewer pores.
(Within a certain range, high voltage leads to the formation of dense and uniform oxidation films. Under constant voltage, the current density decreases as the oxidation time increases.
The higher the voltage required to maintain a given current, the more heat is released during the oxidation process, which does not contribute to the stable performance of the oxide film. When the current is constant, the lower the temperature, the higher the voltage.)
(4) Influence of oxidation current:
The oxidation current directly affects the production efficiency: the production efficiency of high current is high.
(High current requires a large capacitance capacitor, resulting in significant fluctuations in film thickness and easily causing “burns” to the workpiece. Under low current, the oxidation time is long, which reduces corrosion resistance and wear resistance of the film. The ideal current is 1.2 ~ 1.8A/dm2.
The higher the concentration of sulfuric acid, the better the conductivity of the bath solution and the higher the current density under the same voltage. As the aluminum content increases, the resistance of the bath solution increases and its conductivity decreases.)
(5) The influence of stirring the bath solution:
To standardize the temperature and concentration of the anodic oxidation bath solution, especially when using a larger current, a large amount of heat is generated at the film-bath solution interface, and stirring reduces the interface temperature.
(6) The influence of oxidation time:
Under constant current oxidation, the increase in oxidation film thickness is directly proportional to the time within a certain period. (Based on electrolyte concentration, bath solution temperature, current density, oxidation film thickness and performance requirements, etc.)
2. Briefly describe the characteristics of the anodizing process and the differences in the properties of electrolyte oxide film such as sulfuric acid, chromic acid, phosphoric acid, oxalic acid, boric acid and alkali.
1) Sulfuric acid process: Low production cost; high transparency of the film; good resistance to corrosion and wear; easy electrolytic and chemical coloring.
2) Chromic acid process: The thickness of the oxide film is medium, with a rough surface; the film is soft; It has less wear resistance than sulfate film, but has good elasticity.
3) Oxalic acid process: Oxide film has low porosity, better corrosion resistance, wear resistance and electrical insulation than sulfuric acid film, but has a higher cost.
4) Phosphoric acid process: The oxide film is thinner, with larger pores.
3. Comparison between AC sulfuric acid anodizing and DC sulfuric acid anodizing
1) AC: Low current efficiency; low corrosion resistance of oxide film, low hardness.
2) CC: High production cost; high transparency of the film; good resistance to corrosion and wear; easy electrolytic and chemical coloring.
4. The effect of aluminum ions and impurities in sulfuric acid
Mainly affects the wear resistance, corrosion resistance, gloss and electrolytic conductivity of the oxide film
(1) Aluminum ions:
A concentration of 1~10g/L is beneficial, but more than 10g/L will cause an impact. The current decreases as the concentration of aluminum ions increases; coloring becomes more difficult; when the aluminum content is high, insoluble aluminum salts are deposited on the surface of the aluminum part, the tank wall and the heat exchanger, affecting the appearance of the product and the heat exchange efficiency.
(2) Fe, MN, Cu and Ni cations, etc.:
Fe: Harmful impurity, mainly comes from sulfuric acid and aluminum. When the Fe content exceeds 25 ~ 50 μg/g, the oxide film encounters many problems such as decreased gloss and soft film.
Mn: The effect is similar to Fe, but not as significant.
Cu and Ni: Mainly come from aluminum alloy, their effects are similar, when the content exceeds 100μg/g, the corrosion resistance of the oxide film decreases.
(3) Anions such as phosphate, nitrate, chloride, etc.:
Phosphate: Caused by insufficient washing after chemical polishing; the effect is not significant when the content is low (ppm level). The main danger when the content is high is that the phosphate is adsorbed by the oxide film and released during sealing with water, which will harm the sealing quality when it exceeds 5μg/g.
Nitrate: It mainly comes from insufficient washing after the previous process and from commercial sulfuric acid in the bath. When the content exceeds 30μg/g, it is harmful to gloss, and too high will increase the dissolving ability of the bath, which does not lead to film formation.
Chloride: It comes mainly from the water used, the chloride content in tap water is high. When Cl- and F- exceed 50μg/g, the oxide film produces corrosion spots.
VII. Aluminum Hard Anodic Oxidation
1. Briefly describe the similarities and differences in process parameters for the preparation of hard anodic oxide films and ordinary oxide films.
The preparation of a hard anodic oxide film presents no fundamental difference from ordinary anodizing in terms of principles, equipment and processes. The specific technical measures are slightly different. The difference lies in the reduction in the dissolution rate of the oxide film during the oxidation process.
2. Comparison between hard anodic oxide film and regular anodic oxide film
Anodic oxide hard film has greater thickness, greater hardness, better wear resistance, lower porosity, higher dielectric breakdown voltage, but the surface smoothness is a little worse.
(When the applied voltage is high, the concentration is low and the treatment time is long, the film will be thick, hard, wear-resistant, have high dielectric breakdown voltage, low porosity, large pores and low surface smoothness.)
3. Differences and similarities between the hard anodic oxidation process of cast aluminum alloy and the hard anodic oxidation process with sulfuric acid.
(1) Low bath temperature: less than 5°C, the lower the temperature, the harder the film. The bath temperature for common sulfuric acid anodizing is about 20°C.
(2) Low bath concentration: generally less than 15% for sulfuric acid; The bath concentration for common anodizing is around 20%.
(3) Addition of organic acids to the sulfuric acid bath: oxalic acid, tartaric acid, citric acid, etc.
(4) High applied current/voltage: 2~5A/dm2, 25~100V. Common anodizing uses 1.0~1.5 A/dm2, below 18V.
(5) Voltage gradually increasing operation method: step by step pressurization.
(6) Use of pulse power supply or special waveform power supply: for high Cu alloy or high Si die-cast aluminum alloy.
VIII. Electrolytic coloring of anodized aluminum film
1. Briefly describe the process and pros and cons of electrolytic coloring of Sn salt and Ni salt.
1) Sn salt electrolytic coloring process:
This mainly involves single Sn salt and Sn-Ni mixed electrolytic coloring, with SnSO4 being the primary coloring salt. The color is obtained through the reduction of Sn2+ ions in the micropores of the anodized film.
Advantages: Sn salt has good impurity resistance, strong electrolytic coloring solution dispensing ability and simple industrial control. There are no difficulties inherent in coloring Sn salt under alternating current. Disadvantages: Sn2+ has low stability and is difficult to control differences in colors and hues.
2) Ni salt electrolytic coloring process:
Similar to the Sn salt electrolytic coloring process, it involves the deposition of Ni for coloring. Advantages: Ni salt coloring is fast and the bath solution has good stability. Disadvantages: It is sensitive to impurities in the bath solution.
2. Advantages and disadvantages of AC and DC electrolytic coloring.
1) AC staining.
Advantages: Overcomes the risk of peeling of the oxide film in DC electrolytic coloring. Disadvantages: In AC coloring, the anodic voltage affects the speed of the cathodic coloring reaction, causing a decrease in the anode current density and the cathodic current density, thereby decreasing the coloring speed.
2) DC staining.
Advantages: Fast coloring speed, high electrical energy utilization rate. Disadvantages: There is a risk of peeling of the oxide film in DC electrolytic coloring.
IX. Aluminum Anodized Film Dyeing
1. What conditions must an oxide film meet to be subjected to dyeing treatment?
(1) The anodized aluminum film obtained in sulfuric acid solution is colorless and porous;
(2) The oxide film must have a certain thickness, which must be greater than 7um;
(3) The oxide film must have a certain porosity and adsorption;
(4) The oxide layer must be complete and uniform, without defects such as scratches, sand eyes or pinpoint corrosion;
(5) The oxide film itself must be of appropriate color and have no differences in metallographic structure such as irregular grain size or severe segregation.
2. Briefly describe the dyeing mechanisms of organic coatings and inorganic coatings.
(1) Organic dyeing is based on the theory of material adsorption, including physical adsorption and chemical adsorption.
Physical adsorption: Molecules or ions are adsorbed by electrostatic force. The composition of the oxide film is amorphous aluminum oxide, the dense barrier layer close to the aluminum substrate is on the inside, and the porous structure that grows outward in a bell shape is on the top, showing excellent physical adsorption performance. When the dye molecules enter the pores of the film, they are adsorbed on the pore walls.
Chemical adsorption: Adsorption by chemical force. At this time, the organic dye molecules chemically react with the aluminum oxide and exist within the pores of the film due to chemical bonding.
This type of adsorption includes the following: the oxide film forms a covalent bond with the sulfonic group on the dye molecule; the oxide film forms a hydrogen bond with the phenolic group of the dye molecule; the oxide film forms a complex with the dye molecule.
(2) Inorganic dyeing mechanism: During dyeing, the oxidized piece is first immersed in an inorganic salt solution in a certain order, and then successively immersed in another inorganic salt solution, causing these inorganics to undergo a reaction chemical in the pores of the film to form a water-insoluble colored compound. This fills the pores of the oxide film and seals them, thus giving color to the film layer.
3. Process and standards for organic dye coloring technology.
Process flow: Pretreatment – Anodizing – Cleaning – Ammonia neutralization or other processing – Cleaning – Dyeing – Cleaning – Sealing treatment – Drying.
Standards:
1) The concentration to facilitate dyeing: Light colors are generally controlled at 0.1~1g/L, while dark colors require 2~5g/L, and black requires more than 10g/L;
2) Temperature of dye solution: Generally controlled at 50~70℃;
3) PH value of dye solution: The PH range is 5 ~ 6;
4) Dyeing time: Generally between 5~15min.
4. The influence of impurities in the dye solution on organic dyeing and its control.
(1) Effect of sodium sulfate: Sodium sulfate slows down the dyeing rate, this effect increases with the increase of sulfur groups in dye ions, especially in metal complex dyes.
(2) Effect of sodium chloride: The main cause of corrosion (white spots). Corrosion is suppressed by cathodic current.
(3) Effect of surfactants: non-ionic surfactants have no effect on dyeing, but cationic surfactants such as MLW black will delay dyeing, therefore ionic surfactants are not suitable to be added to degreaser as some anions do not conduct to dyeing.
(4) Effect of trivalent aluminum ions: A small amount of Al3+ has no effect on many dye solutions unless it reaches 500~1000ug/g, which may cause color change, such as blue turning red, etc.
(5) Effect of heavy metal ions.
(6) Effect of anions.
(7) Effect of bacterial action on dyeing: Bacteria proliferate in the dye solution, making it moldy. Initially, small bubbles appear on the surface of the dye solution. When the dye solution sits idle, some insoluble colored particles accumulate around the bubbles, causing abnormal dyeing.
If visible to the naked eye, the moldy substance suspended on the surface should be removed and an appropriate bactericide such as dichlorophenol G4 should be added at 0.05~0.10g/L, dissolved in ethanol solution and added to the tank.
Sometimes it is necessary to pour out the coloring solution. At this time, use a bactericidal or hypochlorous acid solution to clean the tank wall and then reconfigure.
(8) Effect of insoluble impurities on dyeing: The dye solution sometimes inevitably carries oil stains, contaminating the workpiece and causing the dyeing to bloom.
At this time, you should use oil absorbent paper to absorb and remove it, or add a small amount of non-ionic surfactant to disperse the oil droplets so that they do not accumulate on the surface of the dye solution.
5. List the commonly used inorganic dyeing processes, steps and parameters (at least five colors).
6. Steps
Generally operated at room temperature, usually in two steps: first soak in the first solution for 5 to 10 minutes, then rinse and soak in the second solution for another 5 to 10 minutes to obtain the desired color.
Common inorganic dyeing process standards.
| Colors | Solution components: | Concentration/(g/L) | Production of Colored Salts |
| Blue | ① (K 4 Fe(CN) 6 .3H 2 Ó)
② (FeCl 3 ) or (Fe 2 (SO 4 ) 2 ) |
30~50 40~50 |
Ferrous Ferricyanide (Prussian Blue) |
| Black | ① (CoAc 2 ) ② (KMnO 4 ) |
50~100 15~25 |
cobalt oxide |
| Yellow | ① (PbAc 2 .3H 2 Ó) ② (K 2 Cr 2 Ó 7 ) |
100~200 50~100 |
Lead Chromate |
| White | ① (PbAc 2 .3H 2 Ó) ② (Na 2 THEN 4 ) |
10~50 10~50 |
Lead Sulfate |
| Brown | ① (K 3 Fe(CN) 6 ) ② (CuSO 4 0.5H 2 Ó) |
10~50 10~100 |
Copper Ferrocyanide |
| Gold | (NH 4 Fe(C 2 Ó 4 ) 2 )(Ph=4.8~5.3, 35~50oC, 2min) | 10 (shallow) 25 (deep) |
7. Common dyeing problems and solutions.
1) Color does not apply.
Solution:
a) Change the pigment
b) Adjust the PH
c) Increase film thickness
d) Dye on the spot
e) Choose the right pigment.
2) Some areas do not acquire color or the color is light.
Solution:
a) Strengthen protection measures
b) Increase pigment concentration
c) Increase film thickness
d) Clamp the workpiece, adjust the position
e) Change dye solution
f) Improve pigment dissolution.
3) The surface appears white and dull after dyeing.
Solution:
a) Remove water vapor
b) Adjust the concentration of the fading solution
c) Reduce fading time.
4) The color blooms after dyeing.
Solution:
a) Adjust the PH and improve cleaning
b) Improve pigment dissolution
c) Lower the temperature of the dye solution.
5) There are stains after dyeing.
Solution:
a) Rinse the surface of the sample with water
b) Filter the dye solution
c) Place the part in a water tank after oxidation
d) Strengthen protection.
6) Color fades easily after dyeing.
Solution:
a) Increase PH
b) Increase the temperature of the dyeing bath, extend the dyeing time, adjust the PH of the sealing bath, extend the sealing time.
7) The dyed surface can be easily removed.
Solution:
a) Reoxidize
b) Increase the temperature of the dye solution
c) Increase the oxidation temperature.
8) The color is very dark after dyeing.
Solution:
a) Dilute the dye solution
b) Lower the temperature
c) Shorten time.
X. Anodized aluminum oxide film seal
1. Sealing
Chemical or physical process carried out on the oxidized film after anodizing aluminum to reduce its porosity and adsorption capacity.
Basic sealing principles include:
(1) hydration reaction; (2) inorganic filling; (3) organic filling.
2. Heat sealing technique
The thermal sealing technique is carried out through the hydration reaction of aluminum oxide, transforming the amorphous aluminum oxide into a hydrated aluminum oxide known as boehmite, or Al2O3•H2O(AlOOH).
The essence of the heat seal mechanism is the hydration reaction, often referred to as “hydration heat seal”.
3. The role of the hydration reaction
It causes a volume expansion of 30%, the increase in volume fills and seals the micropores of the oxidized film, thus increasing its anti-pollution and corrosion resistance, at the same time as it decreases conductivity (increasing impedance) and increases the dielectric constant.
4. The influence of impurities in water
1) Sealing efficiency depends significantly on water quality and PH control;
2) Common impurities include SiO2 and H2SiO3; 3) Countermeasures: ion exchange.
5. Comparison of boiling water sealing parameters and cold sealing parameters
1) The sealing temperature of boiling water: generally above 95 degrees. Cold sealing occurs at room temperature.
2) The PH value of boiling water sealing: the ideal range is 5.5 ~ 6.5. The cold seal range is also 5.5 to 6.5, with an industrial control best at 6.
3) Boiling water sealing time: depends on film thickness, pore size and sealing quality test requirements. Cold sealing generally stipulated as 10~15 minutes.
XI. Inspection and influencing factors
1. Briefly describe the commonly used methods for testing the quality of anodic oxide films and their advantages.
1) Appearance and color difference:
Inspection methods: Visual and instrumental detection.
Pros and cons: Visual inspection is simple, but is easily affected by the shape and size of the sample and the intensity of light. Instrument detection solves the shortcomings of visual inspection and is suitable for measuring the color of reflected light.
2) Oxide film thickness:
Measurement methods:
a) Cross-section thickness microscopic measurement method: Film thickness greater than 5um, vertical.
b) Spectral beam microscope measurement method: Film thickness greater than 5um, oxide film refractive index 1.59 ~ 1.62.
c) Mass loss method: Film thickness less than 5um, dissolution method, surface density, oxide film density (liquid sulfuric acid oxidation) before and after sealing are 2.6 and 2.4 g/cm 3 .
d) Eddy current method: Not suitable for thin films.
3) Sealing quality:
a) Fingerprint test.
b) Quality of stains dyed after acid treatment, not suitable for Cu contents greater than 2% and Si greater than 4%.
c) Experience with phosphochromic acid.
4) Corrosion resistance:
a) Salt spray corrosion test.
b) Corrosion test in a humid SO2 atmosphere.
c) Machu corrosion test.
d) Moist heat corrosion test.
e) Alkali drop corrosion test.
5) Chemical stability:
a) Acid resistance test.
b) Alkaline resistance test.
c) Mortar resistance test.
6) Weather resistance:
a) Natural exposure test.
b) Accelerated artificial weathering test.
7) Hardness:
a) Indentation hardness.
b) Pencil hardness.
c) Microhardness.
8) Abrasion resistance:
a) Abrasion resistance detected by a sandblasting tester.
b) Abrasion resistance detected by a wheel wear tester.
c) Abrasion resistance detected by a sand drop tester.
9) Membership:
a) Grid cutting experiment.
b) Instrumental experiment: Scratch Method.
10) Mechanical properties:
a) Impact resistance.
b) Flexural strength.
c) Fatigue performance.
d) Bond strength.
e) Resistance to fracture due to deformation.
f) Resistance to thermal cracking.
11) Electrical insulation: Breakdown voltage method.
12) Reflective performance.
13) Others:
a) Coating polymerization performance.
b) Resistance to boiling water.
c) Machinability.
2. Factors affecting abrasion resistance
Alloy composition, film thickness, curing conditions of high polymer coatings, anodizing conditions and sealing conditions, etc.
