Anodic Oxidation and Dyeing Technology II

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 “repair”. . 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.

4、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 isMPa, 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

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.