About Anodizing & Aluminum

About Anodizing

Anodizing is a surface treatment process in which aluminum (anode) is electrolyzed to artificially generate an oxide film (rust).

Aluminum easily reacts with oxygen, creating an extremely thin oxide film when in contact with air.
This naturally produced film forms a protective layer that prevents rust, resulting in aluminum’s characteristically good corrosion resistance.
However, this film is extremely thin and can corrode due to environmentally-induced chemical reactions, necessitating a protective surface treatment. That is where anodizing comes in.

Easily scratched, Corrodes, Wears, So…Anodizing is used to protect the surface

Basic Structure

Anodizing Treatment Process
  • 1

    The jig holding the aluminum is dipped in the electrolysis solution (sulfuric acid or oxalic acid).

  • 2

    Connect electrode to jig and add plus current while simultaneously adding negative current to the cathode.

  • 3

    Electrolysis generates an oxide film (aluminum oxide) on the surface of the aluminum.

Anodizing Treatment Process

Pore Size

The term pore refers to the holes in the oxide film that are 100Å-300Å(Angstrom) in diameter. There are actually an amazing 5-70 billion of them per cubic centimeter. The entire population of the world could easily fit in this small space if shrunk down to the atomic level.

Pore Size

The Basic Structure of Anodizing (anodizing) is a Bunch of Pencils!?

Electrical induction of aluminum melts the surface, simultaneously generating osmotic film and oxide film to form 3-dimensional cells over time. It may be helpful to think of the basic structure as a bunch of hexagonal pencils sprouting from the surface of the aluminum.

The Basic Structure of Anodizing (anodizing)  is a Bunch of Pencils!?

  • 1

    Aluminum is an active metal that naturally reacts in air to form an oxide film of approx. 20Å thick.

  • 2

    Aluminum submersed in the electrolytic bath will oxidize and generate an oxide film.

  • 3

    In order to facilitate better electrolyzation, sulfate ion penetrates depressions on the film’s surface, resulting in a localized elution reaction that creates aluminum sulfate, opening countless pores on the surface.

  • 4

    The oxidative reaction and elution reaction progress simultaneously at the base of the pores, creation a regular pore structure.

  • 5

    Film thickness is proportional to electrolysis time.

The Basic Structure of Anodizing (anodizing)  is a Bunch of Pencils!?

Structure of Color Anodizing

Dye penetrates the pores generated in the anodizing coating (anodizing), sealing the surface to create a colored surface that will remain until the oxide film itself wears away.

  • Color Anodizing

    If each pore in the oxide film is filled with red dye, for example, then red anodizing will be produced. The product of this dyeing process is called color anodizing.

  • Natural Coloring

    The product’s color can be altered by simply changing the dye used on the oxide film. Another method called natural coloring can be employed to alter the color of the actual surface of special aluminum alloys.

Principle of Color Anodizing

Principle of Color Anodizing

Characteristics of Anodizing Film

  • High corrosion resistance
  • High wear resistance
  • High degree of hardness
  • Adds useful surface properties (decreases friction coefficient, boosts water repellency and coating adhesion)
  • Good electric insulation
  • Decoration (color, mirror finish, natural finish)

Standard Anodizing and Hard Anodizing

  Standard Anodizing (white / color) Hard Anodizing
Treatment Overview Most common treatment method using sulfuric acid bath Treatment in low temperature electrolytic bath generates thick, hard film
Color Tone Usually white in color, but dyeing can be used to produce a specified color Naturally has grayish color that will differ with the type of aluminum and film thickness
Hardness Approx. 200HV
Aluminum < Standard Anodizing < Iron
Greater than. 400HV
Iron(non-heat treated) < hard anodizing
Film Thickness Decided by application conditions, generally around 5μ-25μ Generally specified 2.0μ-7.0μ based on wear resistance, electric insulation properties
Dimensions Plus 1/2 film thickness » Click here for details Plus 1/2 film thickness » Click here for details
Main Applications Construction materials, industrial goods, household goods, ornamentsaluminum wheels, window sashes, mobile phones Sliding parts including shafts and rollers, aircraft partsSliding parts including shafts and rollers, aircraft parts

Standard Anodizing and Hard Anodizing

  Standard Anodizing (white / color)
Treatment Overview Most common treatment method using sulfuric acid bath
Color Tone Usually white in color, but dyeing can be used to produce a specified color
Hardness Approx. 200HV
Aluminum < Standard Anodizing < Iron
Film Thickness Decided by application conditions, generally around 5μ-25μ
Dimensions Plus 1/2 film thickness » Click here for details
Main Applications Construction materials, industrial goods, household goods, ornamentsaluminum wheels, window sashes, mobile phones
  Hard Anodizing
Treatment Overview Treatment in low temperature electrolytic bath generates thick, hard film
Color Tone Naturally has grayish color that will differ with the type of aluminum and film thickness
Hardness Greater than. 400HV
Iron(non-heat treated) < hard anodizing
Film Thickness Generally specified 2.0μ-7.0μ based on wear resistance, electric insulation properties
Dimensions Plus 1/2 film thickness » Click here for details
Main Applications Sliding parts including shafts and rollers, aircraft partsSliding parts including shafts and rollers, aircraft parts

Anodizing and Film

Anodizing and Plating are Completely Different Products

While anodizing and plating are often regarded as similar in that they both involve the surface treatment of metal, they are in fact completely different products.

Anodizing generates both a growth film on the surface, and an osmotic film below the surface of aluminum. The two films grow uniformly, meaning that any surface unevenness will result in anodizing with the same imperfections.
This fails to create a smooth surface finish akin to proper coating.

Furthermore, the reapplication of anodizing to a surface already treated with hard anodizing will result in thinning as the osmotic film has to be chemically stripped before reapplication.
Plating, however, is a completely different concept in that it involves the sequential application of metal to the target’s surface.

Plating Layer, Anodizing film, Aluminum, Anodizing film is half osmotic film, half growth film

Growth of Anodizing Film

Growth of Anodizing Film

Desirable Shape of Edge

Desirable Shape of Edge

How Film Adheres to Corners/Edges

How Film Adheres to Corners/Edges

Dimensional Change

If Film is Adhered to Inner Diameter

If Film is Adhered to Inner Diameter

If Film is Adhered to Outer Diameter

If Film is Adhered to Outer Diameter

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