The natural aluminum oxide on the surface of aluminum products is soft and thin, and has poor corrosion resistance. It cannot be an effective protective layer and is not suitable for coloring. The artificial oxide film is mainly applied chemical oxidation and anodization. Chemical oxidation is a process in which aluminum products react in a weakly alkaline or weakly acidic solution and some of the matrix metal reacts to thicken the natural oxide film on the surface or produce some other passivation film. Commonly used chemical oxide films are chromic acid films. And phosphoric acid films, which are both thin and adsorptive, can be colored and sealed, Table 3 describes the chemical oxidation process of aluminum products. Compared with anodic oxide films, chemical oxide films have a much thinner film, lower corrosion resistance and hardness, and are less likely to be colored. Lightfastness after coloring is poor, so the coloring and coloring of metallic aluminum is only an anodizing treatment.
Table-3 Chemical oxidation process of aluminum products
No.
Solution composition
Dosage g/L
Temperature/degree
Time min
Application range, film color
Note
1
Sodium carbonate
Sodium chromate
Sodium hydroxide
45
14
2
85-100
10-20
Pure aluminum, Al-Mg, Al-Mn alloy, gray
The film is loose
2
Phosphoric acid
Chromic anhydride
Sodium fluoride
Boric acid
55
15
3
1
Room temperature
10-15
Various aluminum alloys, light green
The film layer is better than 1
3
Sodium dichromate
Chromic anhydride
Sodium fluoride
3.5-4
3-3.5
0.8
Room temperature
2-3
Various aluminum alloys, dark yellow or brown
Solution pH = 1.5
The film layer is better than 1
4
Sodium carbonate
Sodium chromate
32
15
90-100
3-5
Pure aluminum and alloys containing Mg, Mn and Si can also be used for alloys containing less Cu, grey
Can do paint floor
5
Sodium carbonate
Sodium chromate
Sodium silicate
47
14
0.06-1
90-100
10-15
Pure aluminum, Al-Mn (light transparent silver), Al-Mg-Si hard, hard Al-Si and Al-Mg alloys, bright metallic colors
Small gap, not very good coloring, not suitable for paint
6
Sodium chromate
Ammonium hydroxide
0.1
29.6
70-80
20-50
Various aluminum alloys, gray spots
Film like enamel
7
Sodium carbonate
Potassium dichromate
20.4
5
90-100
10-18
Various aluminum alloys, gray
Can whiten in acid solution
(A) The general concept of anodizing
1, the general principle of anodized film generation
An aluminum or aluminum alloy product is used as an anode in an electrolyte solution, and the process of forming an aluminum oxide film on the surface by electrolytic action is called anodization of aluminum and aluminum alloys. The cathode of the device is a material with high chemical stability in the electrolytic solution, such as lead, stainless steel, aluminum and the like. The principle of aluminum anodization is essentially the principle of water electrolysis. When the current passes, hydrogen is released on the cathode; on the anode, the oxygen that precipitates is not only molecular oxygen but also atomic oxygen (O) and ionized oxygen, which is usually expressed as molecular oxygen in the reaction. Aluminum, which is the anode, is oxidized by the oxygen deposited on it to form an anhydrous aluminum oxide film. Not all of the generated oxygen reacts with aluminum, and some of it is precipitated in gaseous form.
2. Selection of anodizing electrolytic solution
A prerequisite for the growth of the anodized film is that the electrolyte should have a dissolution effect on the oxide film. However, this does not mean that anodic oxidation can produce an oxide film or the resulting oxide film has the same properties in all electrolytic solutions that have a dissolution effect. The acidic electrolyte suitable for anodizing treatment is shown in Table-4.
Table-4 Oxidized Acidic Electrolyte
Acids
Ionization constant
Forming voltage
Base film color
sulfuric acid
2×10-2 (second ionized H+)
12-20
Transparent, colorless
Chromic acid
30-40
Opaque, white
Sulfosalicylic acid
40-70
Transparent gray
Sulfamic acid
30-40
Gray
Phosphoric acid
1.1×10-2 (first time)
7.5×10-8 (second time)
4.8×10-13 (third time)
30-40
Transparent with white
Pyrophosphoric acid
1.4×10-1 (first time)
1.1×10-2 (second time)
2.9×10-7 (third time)
3.6×10-4 (fourth)
70-100
White
Phosphomolybdic acid
More than 100
Barrier layer
Boric acid
6.4×10-10
0-600
Barrier layer
oxalic acid
6.5×10-2 (first time)
6.1×10-5 (second time)
40-60
With yellow
Malonate
1.61×10-3 (first time)
2.1×10-6 (second time)
80-110
With brown
Succinic acid
6.6×10-5 (first time)
2.8×10-6 (second time)
120 or more
White to yellow
Maleic acid
1.5×10-5 (the first time)
2.6×10-7 (second time)
150-225
Gray yellow
Citric acid
8.4×10-1 (first time)
1.8×10-5 (second time)
4.0×10-6 (third time)
120 or more
tan
tartaric acid
1.1×10-3 (first time)
6.9×10-5 (second time)
120 or more
tan
Benzoic acid
1.26×10-3 (first time)
3.1×10-6 (second time)
More than 100
Barrier layer
Methylene succinic acid
Eclipse, 40
Interference film
Glycolic acid (hydroxyacetic acid)
1.54×10-4
Eclipse
Malic acid (hydroxysuccinic acid)
4×10-4 (first time)
9×10-6 (second time)
Eclipse, 40
Interference film
3, the type of anodizing
Anodizing is divided into current form: direct current anodizing, alternating current anodizing, pulsed current anodizing. Divided by the electrolyte: sulfuric acid, oxalic acid, chromic acid, mixed acid and sulfo-organic acid as the main solution of natural coloring anodized. According to the film layer, there are: anodization such as ordinary film, hard film (thick film), porcelain film, bright modification layer, and barrier layer of semiconductor function. Aluminium and aluminum alloy commonly used anodizing methods and process conditions are shown in Table-5. Among them, the direct current sulfuric acid anodizing method is most commonly used.
Table-5 Common Anodizing Methods for Aluminum and Aluminum Alloys
series
name
Electrolyte composition
Current density
A/dm2
Voltage
V
temperature
/degree
time
Min
colour
Film thickness
Îœm
Note
sulfuric acid
Alumilite (US)
Sulfuric acid, 10%-20%
DC1-2
10-20
20-30
10-30
Transparent
5-30
Easy to color, corrosion resistant
Sulfuric acid exchange method
Sulfuric acid, 12%-15%
AC3-4.5
17-28
13-25
20-40
Transparent
10-25
For paint
Sulfuric acid hard film
Sulfuric acid, 10%-20%
DC2-4.5
23-10
0±2
60 or more
gray
34-150
Wear-resistant insulation
oxalic acid
Anglo-American law
Oxalic acid, 5%-10%
DC1-1.5
50-65
30
10-30
translucent
15
Alumina membrane (Japan)
Oxalic acid, 5%-10%
AC1-2
80-120
20-29
20-60
tan
6-18
Daily necessities decoration, corrosion resistance, wear resistance
DC0.5-1
25-30
translucent
Eloxal Gxh (Germany)
Oxalic acid, 3%-5%
DC1-2
40-60
18-20
, 40-60
yellow
10-20
For pure aluminum wear
Eloxal Gxh (Germany)
DC1-2
30-45
35
20-30
Almost colorless
6-10
Thin film, soft, easy to color
Eloxal Wx (Germany)
AC2-3
40-60
25-35
40-60
Light yellow
10-20
Suitable for aluminum wire
Eloxal WGx (German)
AC2-3
30-60
20-30
15-30
Light yellow
6-20
Al-Mn alloy
DC1-2
40-60
Hard thick film
oxalic acid
AC1-20
80-200
3-5
60 or more
tan
About 20 or more
Thicker than sulfuric acid film about 600μm thick
DC1-20
40-60
4, anodized film structure, properties
The anodized film consists of two layers. The porous, thick outer layer grows on a dense inner layer with dielectric properties, the latter being called the barrier layer (also called the active layer). Observed by electron microscopy, the vertical and horizontal planes of the film layer almost all show tubular holes perpendicular to the metal surface, and they penetrate the outer layer of the film up to the barrier layer between the oxide film and the metal interface. The pores around the main axis are dense aluminas that form a honeycomb hexagonal body called a unit cell. The entire membrane layer is composed of numerous such cells. The barrier layer is composed of anhydrous alumina, thin and dense, with high hardness and resistance to the passage of current. The barrier layer has a thickness of about 0.03-0.05 μm, which is 0.5%-2.0% after the total film. The porous outer layer of the oxide film is mainly composed of amorphous alumina and a small amount of hydrated alumina, and also contains cations of the electrolyte. When the electrolyte is sulfuric acid, the sulfate content in the film is normally 13%-17%. Most of the excellent properties of the oxide film are determined by the thickness and porosity of the porous outer layer, and they are all closely related to the anodizing conditions.
(B) Direct current sulfuric acid anodizing
1. Oxidation film growth mechanism
Anodic oxidation in the sulfuric acid electrolyte, as the anode of the aluminum products, in the initial period of anodization, the surface is uniformly oxidized, resulting in a very thin and very dense film, due to the role of sulfuric acid solution, the weakest point of the film ( Such as grain boundaries, dense spots of impurities, lattice defects or structural deformations) localized dissolution occurs, and a large number of pores, ie, primary oxidation centers, allow the matrix metal to contact the electrolyte that enters the pores, and the current is thus continuously conducted. The generated oxygen ions are used to oxidize new metals, and they are centered around the bottom of the hole. Finally, they form a new film between the old film and the metal, making the partially dissolved old film behave like a “repairâ€. . With the extension of the oxidation time, the film is continuously dissolved or repaired, and the oxidation reaction is developed in depth, so that an oxide film composed of a thin and dense inner layer and a thick and porous outer layer is formed on the surface of the product. The thickness of the inner layer (barrier layer, dielectric layer, active layer) is basically the same as the end of oxidation, but the position is constantly shifted to the depth; while the external oxidation time is thicker with time.
2, oxide film thickness calculation
The thickness of the oxide film resulting from the anodization can theoretically be calculated according to the formula deduced by Faraday's second law.
σ = Kit
In the formula, σ is the thickness of the anodic oxidation film (μm), I is the current density (A/dm2), t is the oxidation time (min), and K is the coefficient (K=0.309 when the alumina density γ=kg/m 3 ). The above formula is calculated on the premise that the passed power is used for alumina precipitation, and the density of alumina and the membrane is also regarded as a pure alumina dense value. However, the actual situation is not entirely the same. In order to make the K value more realistic, the current efficiency and the density or porosity of the resulting film under this process condition should be taken into account, ie:
K = 1.57η/γ
In the formula, η is the current efficiency (the ratio of the mass of the substance actually deposited on the electrode to the total mass of the precipitated substance). The actual value of K varies from country to country, with 0.328, 0.285-0.355 in the United States, 0.352, 0.364, 0.25 in Japan, and 0.25 in China and Russia.
3, the factors that affect the growth and quality of oxide film
When the temperature of the electrolyte rises from 20 degrees to 30 degrees, the dissolution rate of the film increases by about 3 times. As the current density increases, the amount of metal that the product is cured and the thickness of the aluminum oxide film that is formed on the surface increases. Sulfuric acid concentration has little effect on the thickness of the oxide film. In order to obtain a medium-thickness porous film that is easy to be colored and closed, and the resistivity is higher, the concentration is preferably 15%-20%; the solution needs deionized water to require chloride ion. <15mg/L, iron ion <1mg/L, sulfate ion <30mg/L, resistivity 5x10 5-6 power Ω·cm; maximum allowable impurity content of solution in solution 20g/L, copper Ion 2g/L, iron ion 5g/L, chloride ion 0.1g/L. With the prolonged anodizing time, the thickness of the oxide film increases. After a certain thickness, due to the increase in the film thickness resistance and the decrease in the electrical conductivity, the growth rate of the film slows down, and even if the alloy extends the oxidation time, the thickness of the film will not Increase again. The anodized film of different aluminum alloys has different colors, the film on pure aluminum is colorless and transparent, and the gloss of the metal is completely maintained; the high purity aluminum adds a small amount of magnesium, and the film color will not change due to the extension of the oxidation time. When the magnesium content exceeds 2%, the film becomes dark and turbid. When the aluminum-silicon alloy is oxidized, the silicon will not be oxidized or dissolved, and part of the film will enter the film to make the film dark gray. When the silicon content is large, the anodic oxidation will be immersed in hydrofluoric acid and the film color will be improved. Generally, alloys containing 5% or more of silicon are not suitable for bright coloring products. Contents of up to 13% are difficult to anodize; Copper alloy, when the content is less, the film is green, with the increase of copper content, the film is thin, dark and dark. The anodized treatment of some deformed aluminum alloys is shown in Table-6. The aluminum alloy is anodized in the sulfuric acid solution, due to the formation, growth and dissolution of the oxide film on the surface, causing a change in the resistance, which causes the current, the tank terminal voltage, and the current density in the process to change accordingly. The actual increase in voltage should not be too fast, otherwise it will make the resulting film non-uniform.
Table-6 Some Anodizing Effects of Aluminum Alloys
Chinese alloy grade
The main component content%
Suitable for protective anodizing
Suitable for anodizing coloring
Suitable for bright anodizing
LG5
L3
L5
LF21
LF2
LF3
LF5
LF7
LD31
LY11
LY12
LD8
LD2
LD5
LT1
99.99Al
99.8Al
99.5Al
99.0Al
1.25Mn
2.25Mg
3.5Mg
5Mg
7Mg
0.5Mg, 0.5Si
1Si, 0.7Mg
1.5Cu, 1Si, 1Mg
2Cu, 1Ni, 0.9Mg, 0.8Si
4.25Cu, 0.625Mn, 0.625Mg
4.25Cu, 0.75Si, 0.75Mn, 0.5Mg
4Cu, 2Ni, 1.5Mg
2.25Cu, 1.5Mg, 1.25Ni
1Mg, 0.625Si, 0.25Cu, 0.25Cr
1Si, 0.625Mg, 0.5Mn
5Si
1
1
1
1
1
1
1
2
2
2
2
3
3
3
4
2
2
3
2
2
3
3
3
4
4
4
4
1
2
3
2
3
4
3
3
4
4
6
5
4
6
5
4
6
5
4
4
5
4
4
5
2
3
4
3
3
4
3
6
5
Note: 1—excellent; 2—good; 3—still good; 4—may;5—unsuitable; 6—only suitable for dark colors.
4ã€Aluminum anodization process of building
Architectural aluminum is currently the main product of anodizing, and 75%-85% of it is treated with conventional sulfuric acid. The Chinese building profile standard specifies that the thickness of the oxide film is greater than 10 μm. The optimum process parameters of anodized aluminum for building aluminum are: electrolyte sulfuric acid 15%±2%, aluminum ion content less than 5g/L, solution temperature 21±10C, current density (1.3±0.05) A/dm2, time (for LD31 Alloy) 30min, then 10μm; 60 minutes, then up to 18μm (voltage 18V), the solution is prepared with pure water.
(III) Other Anodizing
1, oxalic acid anodizing
Most of the factors affecting the anodizing effect of sulfuric acid are also applicable to the anodic oxidation of oxalic acid. The anodic oxidation of oxalic acid can be superposed by direct current, alternating current, or alternating current and direct current. Oxidation by alternating current is less flexible and elastic than that of direct current under the same conditions; pitting is easily caused by direct current oxidation, and oxidation is prevented by alternating current oxidation. As the alternating current component increases, the corrosion resistance of the film increases, but the color deepens. The coloring is worse than the sulfate film. The concentration of free oxalic acid in the electrolyte is 3% -10%, usually 3% -5%, about 0.13-0.14g per AH in the oxidation process, while 0.08-0.09g of aluminum per AH is dissolved The electrolyte produces aluminum oxalate and requires 5 times as much aluminum as oxalic acid. The concentration of aluminum ions in the solution is controlled below 20g/L. When 30g/L of aluminum is contained, the solution fails. The oxalic acid electrolyte is very sensitive to chlorides. When anodic oxidation of pure aluminum or aluminum alloys, the chloride content should not exceed 0.04-0.02g/L, and the solution is best prepared with pure water. The temperature of the electrolyte rises and the film thickness decreases. To obtain a thick film, the pH of the solution should be increased. Direct current anodization uses lead, graphite or stainless steel as the cathode, and its area ratio to the anode is (1:2)-(1:1). Oxalic acid is a weak acid and has a low dissolving power. When aluminum is oxidized, the product and the electrolyte must be cooled. The thickness and color of the oxalic acid film are different depending on the composition of the alloy. The film thickness of pure aluminum is yellowish or silver white, while the alloy is thin in color, such as yellow and brass. After oxidation, the membrane is washed and if it is not dyed, steam can be sealed for 30-60 minutes at a pressure of 3.43×10 4 Pa.
2, chromic acid anodizing
The chromic acid anodizing process is shown in Table-4. Concentration analysis should be performed frequently during the oxidation process and chromic anhydride should be added at a proper time. Electrolytic cathode material can be used lead, iron, stainless steel, the best ratio of positive to negative area (5:1) - (10:1). When the solution contains a large amount of trivalent chromium ions, it can be oxidized into hexavalent chromium ions by electrolysis. The sulphate content in the solution exceeds 0.5%, and the anodic oxidation effect is not good. When the sulphate ion is large, barium hydroxide or cesium carbonate can be added to make barium sulfate precipitate. The chloride content in the solution should not exceed 0.2g/L. When the chromium content in the solution exceeds 70 g/L, the solution should be diluted or replaced. Anodizing of chromic acid has two kinds of anodizing method with voltage cycle change or constant voltage anodizing method (rapid chromic acid method).
3, hard (thick film) anodizing
Hard anodic oxidation is a process that produces a thick and hard oxide film on the surface of aluminum and aluminum alloys. The maximum thickness of the hard film can reach 250μm, the microhardness of the film formed on pure aluminum is 12000-15000MPa, and the alloy is generally 4000-6000MPa, which is almost the same as the hard chrome plating. They have excellent wear resistance in low conformity. The porosity of hard film is about 20%, which is lower than that of conventional sulfate film. Some of the hard anodizing processes are shown in Table-7.
Table-7 Hard Anodizing Process
Numbering
Electrolyte
Temperature/degree
Current density / (A/dm2)
Starting voltage/V
Time/min
Film thickness/μm
Starting voltage
Voltage at the end
1
15% sulfuric acid
+14-+4.4
2-2.1
26
120
90
50
2
15% boric acid, 4% Na2HC6H5O7
+60-+70
0.4-0.6
100
300
240
200
3
10% sulfuric acid
+10
250W/dm2
15-25
80
60
10-130
4
15% sulfuric acid
-1-+4.5
2-2.5
25-30
40-60
60-240
28-150
5
10% sulfuric acid
+8-+10
25
60
60
25-60
6
10%-15% sulfuric acid
0-+4
5
Communication 10-12
60-70
Plug in DC 20-24
120-140
7
6%-8% oxalic acid dihydrate
Conditions vary depending on the alloy
8
6%-7% sulfuric acid +
3%-6% organic additive
+4.5-+18
+4.5-+18
1.3-2
10
150
40
65
9 10%-20% sulfuric acid
-6-+10
30
280
160
115-150
10
10%-15% sulfuric acid
+8
4
20-25
60
60
55-80
11
5.5% formic acid, 8% oxalic acid dihydrate
+15-+25
3-6
45
90
100-250
4, Porcelain Anodizing
Ceramic anodized aluminum and aluminum alloys are anodized in solutions of oxalic acid, citric acid and boric acid in the form of titanium salts, zirconium salts or cerium salts. The hydroxides of the salt metals in the solution enter the pores of the oxide film, thus making the surface of the product appear A process with an opaque and dense enamel or a similar plastic appearance with a special gloss. The anodizing process of porcelain is basically the same as that of conventional anodizing of sulfuric acid. The difference is that the ceramic anodizing is at a high DC voltage (115-125V) and a high solution temperature (50-60 degrees), and the electrolyte is often stirred. , The pH is often adjusted so that it is in the range of 1.6-2.
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