Introduction to key process of aluminum alloy anodizing
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Introduction to key process of aluminum alloy anodizingIntroduction to the general process of anodizing aluminum alloyPre-processing:Degreasing (acid degreasing, neutral degreasing, alkaline..........
Introduction to key process of aluminum alloy anodizing


Introduction to the general process of anodizing aluminum alloy


Degreasing (acid degreasing, neutral degreasing, alkaline degreasing)

Alkali etching (sodium hydroxide, sodium nitrate)

Chemical polishing (two acids, triacids)

Neutralizing and removing stains (nitric acid, additives)

Medicine water control method:

Due to the difference in pre-treatment syrup composition provided by different manufacturers, the analysis method provided by the medicinal water manufacturer is regularly analyzed and supplemented.

Post processing:

Anodizing (sulfuric acid)

Coloring (organic dyeing, inorganic salt electrolysis)

Sealing (normal temperature, medium temperature, high temperature)

Key process introduction

First, anodizing

The anodization is usually carried out by using sulfuric acid, oxalic acid, chromic acid, boric acid, an organic acid or the like as an electrolytic solution. Hard aluminum oxidation sometimes requires high-speed electrolysis or a special current waveform, but if a dyeing treatment is required in a subsequent process, a sulfuric acid electrolyte is generally used.

Principle of sulfuric acid electrolysis

As shown in (Fig. 1), anodizing refers to a process in which water is electrolyzed by using aluminum as an anode in a sulfuric acid electrolyte, and oxygen ions generated on the anode are combined with aluminum ions to form alumina.


2. Structure of the oxide film

As shown in (Fig. 2), the oxide film is a porous film (60 billion pieces/cm2), and its structure is divided into upper and lower layers. the following

The thinner layer is dense, non-porous, and electrically conductive, and is referred to as a "barrier." The upper layer is a porous membrane composed of a plurality of vertically grown unit bodies, each of which has a hexagonal columnar hollow structure and is arranged according to a certain rule.


3. Growth of barrier layer When the oxide film begins to grow, its growth rate is proportional to the electrolysis voltage, which is about 10~15Å/V. As the film thickness increases, the surface of the barrier layer is corroded by the electrolyte and slightly dissolved. When the thickness of the oxide film reaches the limit value (about 0.02 μm), the current concentrates on the corroded recess and local dissolution occurs under electrochemical action. This local dissolution promotes the expansion of the oxide film to the interior of the aluminum material. At the same time, the current is concentrated in the depressed portion, which accelerates the dissolution of the portion. A barrier layer is formed during the repetition of growth and dissolution. Further, since the current is concentrated in a certain portion within a certain range, it is determined that the pitch of the dissolved portion has regularity. As the oxide film continues to extend inward, the dissolution reaction of the depressed portion proceeds continuously, and finally an electric conduction hole is formed to constitute a basic structure of the oxide film.

4. The growth barrier layer of the porous film expands to the inner layer and eventually forms the overall structure of the oxide film during the alternate process of growth and dissolution. The upper layer structure of the barrier layer is a regularly arranged porous film having a hexagonal columnar structure. As shown in (Table-1), the porous membrane is composed of a plurality of hexagonal columnar unit bodies having an outer diameter of 0.03 to 0.05 μm, a pore-like structure in the middle, and a pore diameter of 0.01 to 0.02 μm. Since the thickness of the barrier layer is only about 0.01 to 0.02 μm, we can simply consider the anodized film to be the upper porous film.

Photo-1 is a section of the film formed by high-speed oxidation of sulfuric acid, and the unit body and the energized hole can be clearly seen.

(Table-1) Effect of current density on the size of sulfuric acid film


Photograph of grain structure of photo-1 high-speed aluminum oxide film


Second, anodized film dyeing

1. Dyeing mechanism of dye

The sulfonic acid group [D (-SO3Na)-] as a constituent of the dye itself has a negative charge, and the oxide film <Al(H2O) 4(OH) 2+ ·AlOn+ > has a positive charge, and under the electrostatic adsorption, the dye The molecules are attracted into the conductive holes. However, since the electrochemical adsorption force is weak, the thickness of the dye entering the pores is only 1/2 to 1/3 of the thickness of the oxide film. The dye molecules entering the pores are fixed in the pores by ionic bonding and acid precipitation.

As shown in Fig. 3, the oxide film self potential is very important for dyeability. The positive charge carried by the oxide film and the negative charge carried by the dye attract each other under the action of static electricity, and adhere to the oxide film by ion bonding and acid precipitation.

The stronger the acidity inside the oxide film, the more positive charge it has. However, if there is residual acid on the surface of the film, it will not be colored, so it must be washed thoroughly before dyeing. On the other hand, if the washing time is too long, the electric charge of the oxide film is neutralized, and the problem of deterioration in dyeability also occurs.

Figure-3 Schematic diagram of film potential-dye adsorption, etc.

2, aging phenomenon

In addition to the accumulation of impurities, excessive accumulation of dye decomposition by-products and salts is also responsible for the aging of the dye bath.

As the dyeing bath ages, the viscosity of the bath gradually increases, resulting in undesirable dyeing and floating phenomena, so it is necessary to periodically update.

Further, it is to be noted that when the by-products generated by the decomposition of the dye are increased, the ultraviolet resistance and heat resistance of the dyed article are lowered. When the dye bath is aged to a certain extent, the frequent addition of the dye does not restore the dyeing performance, which only causes waste of the dye, and the obtained color tone is also unstable. After the partial update, various aging factors remain in the bath, which will shorten the tank.

Effect of Dissolved Aluminum on the Dyeability of ODM BLACK (BK927)


Effect of sulfate on the dyeability of ODM BLACK (BK927)


Third, sealing treatment

1. Sealing mechanism and type

Sealing, also called hydration treatment, is a method of sealing the conductive holes of the porous cell structure. (Table-2) shows a typical sealing method, in which the mechanism of the methods such as vapor sealing, boiling water sealing, nickel acetate sealing, and chromic sealing is a hydration reaction. The mode from dyeing to sealing is shown in Figure 4.

The high temperature sealing is a process of changing a highly chemically active amorphous oxide film into a chemically passive crystalline oxide film.


The large crystal hydrated alumina is stable and irreversible. In a corrosive environment, boehmite is more stable than bayerite, so the high temperature sealing water temperature must be above 85 °C. The hydrated crystalline oxide film expands in volume, clogging the pores of the membrane.

In addition to the hydration reaction, a high-temperature sealing with a metal salt also has a hydrolysis of a metal salt. For example, nickel is added and the salt is salted. For those who add chromium, there are [A1(OOH)Cr04] and [A1(OH)Cr2O7].


Table-2 representative sealing method

Pattern from dyeing to sealing


2. Nickel acetate sealing hole ODM SEAL EX

Although the nickel acetate sealing is mainly based on the hydration reaction, the filling of the nickel hydroxide in the pores suppresses the elution of the dye, and the sealing film has good corrosion resistance, and thus is widely used. (Fig. 5) shows a conceptual diagram of nickel acetate sealing, and (Fig. 6) shows the adsorption amount of nickel.

Figure-5 Conceptual view of nickel acetate sealing


Figure-6: The amount of nickel adsorbed

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