During sulfuric anodizing, no metallics are involved other than aluminum. Unlike electro-deposited metallic plating, where plating builds up by depositing on a base metal, an anodic coating penetrates and grows on the base metal by converting aluminum to aluminum oxide. 

Proper formation of the aluminum oxide coating provides the aluminum surface with an increased resistance to corrosion and abrasion.  Depending on the anodizing type and the aluminum alloy, the thickness of the anodized layer can range from .0002” to .003”.  Dyes can be deposited into the pores of the anodized layer and then sealed.  The result is a highly desirable, rich, and intense color finish

Aluminum Anodizing - Gold | Surface Techiques

The Anodizing Process

Anodizing entails the immersion of an aluminum part in an electrolyte where and electric current is passed though the part.  The surface is converted to aluminum oxide.  Proper formation of an aluminum oxide coating provides the aluminum surface with an increased resistance to corrosion and abrasion.  In addition, it serves a base for color absorption to great decorative finishes.

Each alloy anodizes differently because each alloy contain differing amounts of the elements that make up the alloy.  For instance, 2024 has a higher content of copper than 6061.  This makes it very difficult to anodize alloy 2024.  Most aluminum can be anodized, but some alloys are easier than others.  Castings tend to give the anodizer the most problems because of the porosity/roughness of the casting and the alloying materials used in the casting process.

Part Fabrication

While most surfaces of a part can be anodized, the designer needs to consider the realistic properties of the anodizing process.  Parts that are assembled, welded, joined, and have recesses can pose problems for the anodizer.  Additional steps may need to be taken to overcome these issues.  Contact Surface Techniques and we will discuss with you how to get around some of these issues.

Adding Color

Color can be added to most types of anodizing.  However, it is also affected by type and the aluminum alloy used.  Each alloy can produce a different “natural” color after anodizing.  This can affect the shade or tint of the desired final color finish.  Type of anodizing has a similar effect.  Generally Type II will give the “cleanest” color.  Sometimes the dyes we use can be combined to match a desired color.  Please contact Surface Techniques to see if this might be possible for your application.

Sealing

Sealing is the last step for most of the anodizing processes.  This closes the pores in the aluminum oxide layer and adds corrosion resistance.  Color anodizing requires a seal regardless of the type.  Hard clear anodizing (Type III Class I) may be processed without sealing.  Sealing Type III Class I adds a small increase in corrosion resistance, but also give a small decrease in abrasive resistance.  Surface Techniques utilizes a deionized water/nickel acetate seal for our Type II and Type III color work.  For our non-color work, we utilize a nickel acetate seal designed for this purpose.

Sealing hard coat yields:

  • Better Lubricity
  • Better Weather Fastness

Anodizing improves the performance of your parts for:

  • Wear Resistance
  • Elective Resistive Properties
  • Corrosion Resistance
  • Aesthetics, ideal for producing intense colors

 

Conventional Anodizing Type II

Each alloy anodizes differently because alloy metals change the conductivity of the part and cause it to form aluminum oxide, either faster or slower. All aluminum can be anodized, but some alloys work better than others. Castings tend to give the anodizer the most problems because of the porosity in the castings and alloying materials used.

Types of Aluminum Anodizing | Surface Techiques

Left – Type II Conventional Anodizing
Right – Type III Hard Anodizing

Surface Techniques Can Anodizing In the Following Styles:

  • MIL-A-8625F Type II Class I*
  • MIL-A-8625F Type II Class II* (specify color)

*The class portion of this callout is used to note clear or color work. “Clear” is not a color option. It is another way to say that Class I is intended.

  • Class I – No dye added
  • Class II – Dye added. (specify color)

Surface Techniques may be able to anodize per individual needs. Please send us your specification and we will look through it and let you know.

Conventional Type II anodizing improves the performance of your parts for:

  • Wear Resistance
  • Corrosion Resistance
  • Elective Resistive Properties
  • Aesthetics, ideal for producing intense colors

If your application calls for Type I, we can offer Type IIB in its place.  Type I uses a solution of chromic acid instead of sulfuric acid to produce the anodized layer.  MIL Spec 8625 allows for the use of a sulfuric acid bath under Section 3.4.2.1.  An engineering deviation may be required.  Please contact Surface Techniques for more information.

Hard Anodizing (Type III Anodizing – also referred to as Hardcoat Anodizing)

Usually referred to as hard anodizing, it is also called hard coat anodizing.  This process produces a thicker layer than conventional anodizing.  Hard anodizing also produces a higher corrosion resistance and abrasive resistance than conventional anodizing.  A Type III hard coat anodized layer can range in thickness from .0005” to upwards of .002”.  Color can be added, but as the anodized layers thickness increases, the ability to add color decreases.  The natural coloring of aluminum turns from bronze to a gray or dark gray depending on the alloy that was anodized.  This can change the shade or tint of a color.

Types of Aluminum Anodizing | Surface Techiques

Left – Type II Conventional Anodizing
Right – Type III Hard Anodizing

Surface Techniques Can Anodize In the Following Styles

  • MIL-A-8625F Type III Class I*
  • MIL-A-8625F Type III Class II* (specify color)

*The class portion of this call out is used to note clear or color work.  “Clear” is not a color option.  It is another way to say that Class I is intended.

  • Class I – No dye added.
  • Class II – Dye added. (specify color)

Surface Techniques may be able to anodize per individual needs. Please send us your specification and we will look through it and let you know.

Taber abrasion testing shows that hardcoat exceeds wear characteristics of other hard coatings such as electroless nickel, hardchrome, and ceramic.

Some industrial applications of hardcoat anodizing include highly intricate parts such as pistons, anti-lock brake assemblies, pump components, valves, sliding parts, insulation plates and transmission parts. Some retail applications for hardcoat anodizing and Sulfuric anodizing include cookware, bike rims, locks and levels.

Hard Coat Type III processes improve the performance of your parts for:

  • Increased Corrosion Resistance
  • Increased Wear Resistance
  • Increase Electrical Resistive Properties

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