Factors Affecting The Anodizing Of An Aluminum Metal

Anodizing is a process for producing decorative and protective films on articles made from aluminum and its alloys. It is essentially a process where a thick film of aluminum oxide is built up on the surface of the aluminum through the use of a direct current electrical supply. In the majority of anodizing plants in New Zealand it is carried out in an electrolyte bath containing sulfuric acid with aluminum sheet cathodes and the work to be anodized attached to the anode (Figure 1).

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When the current is flowing in the cell the following sequence of events is believed to occur. Sulfuric acid begins to decompose, the hydrogen ions moving to the cathode where they are reduced to hydrogen gas:

2H+ + 2e- → H2(g) (1)

Simultaneously, negatively charged anions, i.e. hydroxide, sulphate and maybe oxide ions move to the anode. The electrical charge in the circuit causes positively charged aluminum ions (Al3+) to be generated in the anode and in turn move toward the cathode. At the anode surface they react with the oxide/hydroxide ions to form aluminum oxide (in the case of the hydroxide ion, hydrogen ions are released into the solution).

There are two types of ions involved in any Electrolyte. The reactions take place and Anions and Cations are formed and transferred to the opposite ends of an Electrolyte. Anions are positively charged ions and Cations are negatively charged ions in an electrolyte.

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In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible. Ion is a particle which is electrically charged either positive or negative; an atom or molecule or group that has lost or gained one or more electrons.

An ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative charge. An anode is an electrode through which electric current flows into a polarized electrical device. A cathode is an electrode through which electric current flows out of a polarized electrical device.

Anodizing is a method of electrolytic passing to increase the thickness of the natural oxide layer of the surface of various metal parts. This process is called “anodizing” because the part which to be treated forms an anode electrode of the electrical circuit. Anodizing mainly increases corrosion resistance and provides better bond for paint primers and glues than bare metal. Anodizing is even used to prevent galling of threaded components and to make dielectric films for electrolytic capacitors. Anodic films are most commonly applied to protect aluminum alloys, although processes also exist for titanium, zinc, magnesium, niobium, and tantalum. This process is not a useful treatment for iron or carbon steel because these metals exfoliate when oxidized; i.e. the iron oxide, flakes off, constantly exposing the underlying metal to corrosion.

Anodizing changes the texture of surface and also changes the crystal structure of the metal near the surface. Thick coatings are normally porous, so a sealing process is often needed to achieve corrosion resistance. Anodized aluminum surfaces, for example, are harder than aluminum but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances.

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Equations of the anode reactions

Al → Al3+ + 3e- (2)

2Al3+ + 3O2- → Al2O3 (3)

2Al3+ + 3OH- → Al2O3 + 3H+ (4)

For which the overall process is:

2Al + 3H2O → Al2O3 + 6H+ + 6e- (5)

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The sulfate ions also play some part as the oxide coating contains 12 – 15% sulfate ions. It is suggested that the sulfate ions facilitate the movement of hydrogen ions reducing the cell voltages required.

THE DEVELOPMENT OF THE ALUMINIUM OXIDE LAYER

Fresh aluminum reacts readily with oxygen to produce aluminum oxide. Once formed the oxide remains firmly bonded to the surface forming an impenetrable layer. Consequently, further reaction ceases. The film is very thin (0.01m), and despite its tenacity it can be removed by abrasion and chemical corrosion. In such instances the aluminum is subject to wear or the surface will mark or become pitted at the site of corrosion.

Anodizing produces much thicker coatings (12 – 25 m) which, if properly sealed, can extend the life of the surface appreciably. Recent research in New Zealand has shown that pitting of the surface can be reduced by up to 90% with a 12 m coating, and by up to 93% with a 25 m coating.

In the initial stages (i.e. first 60 s) of anodizing the oxide layer formed is dense and of even consistency. It provides the greatest resistance to wear and corrosion and consequently is called the barrier layer. The growth of this layer ceases when the high electrical resistance of the oxide reduces the potential of the applied voltage in the electrolytic cell. The depth of the coating at this stage is about 0.08m. Subsequent growth is very slow and competes with the acid reaction:

Al2O3 + 6H+ _ 2Al3+(aq) + 3H2O

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which releases Al3+ ions into the solution. Note that the H+ can be at high concentration near the oxide layer due to one of the anode reactions above. See equation (5).

At low applied voltages only the barrier layer forms. However, the gradual production of Al3+ ions tends to smooth out the underlying metal surface and give a brightening effect to the article. Objects such as wheel trims and bumper bars are general treated in this way. At higher voltages the growth of the layer continues beyond the barrier layer. Unlike the initial barrier layer this secondary layer, although constitutionally the same, has an open pore-like structure; a consequence of the competing anodizing and acid solution processes. Electron photomicrographs reveal the structure of these anodized surfaces to be as shown in Figure 2.

The conditions required to produce coatings vary according to the concentration and nature of the electrolyte, the voltage – current density applied, the alloy being anodized and the temperature of the bath. In the majority of electrolytic plants articles are anodized at a potential of 15 – 20 V and a current density around 1.6 A d-1 m-2; the electrolyte is 3.5 mol L-1 sulfuric acid maintained at temperatures between 20 and 23oC. Under these conditions the quality of the coating is satisfactory for most applications. At higher electrolyte concentrations and temperatures, and at lower voltages or current densities, the acid solution process occurs earlier in the development producing thin, open oxide coatings. Conversely, hard dense coatings are produced at low temperatures and high current densities. The conditions established in each plant are determined by the type of application.

PRE-TREATMENT

Step 1 – Cleaning

Correct and adequate cleaning of the aluminum object prior to anodizing is essential if the finished work is to have a uniform and attractive appearance. When aluminum arrives from the rolling, casting or extrusion mills it may be soiled following ways: