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Aluminum Surface Anodic Finish

Numerous types of anodic finishes

The anodic finish is the result of the carefully controlled electrochemical oxidation of the aluminum surface. There are numerous types of anodic finishes. Each finish is specified to obtain certain appearance and/or performance characteristics desired according to the end use of the product. Some of the characteristics of anodized finishes are:
  • decorative
  • abrasion resistant
  • reflective
  • dielectric
  • corrosion resistant
  • color
  • absorption/emissivity of heat.
Coating thickness is an important attribute of every type of anodized finish. It may be specified as either coating thickness or coating weight, or both. Common units of measurement are:
  • inches,
  • mils,
  • microns,
where 1.0 mil equals 0.001 inch (one mil = one one-thousandth of an inch); 25 microns equals 1.0 mil; or 1 micron equals 0.00004 inches.

Coating weight, or mass per unit area, may be stated as mg/cm2 (milligrams per square centimeter), mg/dm2 (milligrams per square decimeter) or mg/ft2 (milligrams per square foot). There are both processing and testing specifications that call out coating weight in these terms.

The most commonly used anodizing process is sulfuric acid anodizing. It produces a colorless, transparent anodic coating on most aluminum alloys. It may be specified in different coating thicknesses to satisfy different functions.

Protective and Decorative (< 0.4 mil thick). These coatings are used where somewhat limited protection from corrosion and abrasion is suitable. They are also used in bright finish applications, giving protection, but allowing the brightness to show through. These coatings are not recommended for architectural use, but are very satisfactory where appearance is more important than durability.

Examples of products using this type of anodized finish are:
  • Lighting reflectors 0.03 to 0.3 mil
  • Clear anodized automotive trim 0.3 mil
  • Lithographic plates 0.03 mil
  • Appliance trim 0.1 to 0.2 mil
The following designation / nomenclature / specification systems apply to this group of coatings.
  • Aluminum Association (AA): A2 Series of designations, which include
    A21 Clear anodize
    A22 Integral color
    A23 Impregnated color (dyed)
    A24 Electrolytically deposited color
  • Nomenclature originated by Alcoa:
    Alumilite 200 0.10 mil coating thickness
    Alumilite 201 0.15 to 0.20 mil
    Alumilite 202 0.20 to 0.25 mil
    Alumilite 203 0.30 mil (not commonly used)
  • MIL-A-8625:
    Type II Coating thickness called out in purchase document or drawings.
Note: The "mil spec" is generally used only for military or aerospace finishing, but can be used to designate other finish applications if desired.

Architectural Class II (0.4 to 0.7 mil coating). Anodic coatings of this thickness range have greater resistance to corrosion and abrasion than Protective and Decorative coatings. These coatings are recommended for interior architectural use and they may be used outside with regular maintenance of the finish. This thicker class of coatings will give the aluminum a more matte appearance than the thinner coatings. Greater coating thickness makes it possible to produce darker colors when coloring the product.

Examples of anodized products that have a coating thickness in this range are:
  • Interior architectural panels 0.4 to 0.7 mil
  • Black automotive trim 0.6 mil
  • Bleacher seating 0.4 to 0.5 mil
Architectural Class II finishes are designated as follows,
  • Aluminum Association (AA): A3 Series of designations, which include
    A31 Clear coating
    A32 Integral color
    A33 Impregnated color
    A34 Electrolytically deposited color
  • Nomenclature originated by Alcoa:
    Alumilite 204 0.4 mil coating thickness
    Alumilite 214 0.6 mil (not commonly used)
  • MIL-A-8625:
    Type II Coating thickness called out in purchase document or drawings.
Architectural Class I (0.7 mil and thicker anodic coatings). Anodized coatings produced in this class are thicker than Class II coatings and Protective and Decorative coatings. Class I coatings are used primarily for exterior building products and other products that must withstand continuous outdoor exposure. This coating thickness range is not suitable for highly specular (bright) finishes. Most applications are matte finished.

Class I anodic coatings are thick enough to receive lightfast (fade-resistant) coloring processes. Common coloring methods include adsorptive organic and inorganic dying and electrolytically deposited coloring.

Examples of products specified with Class I finishes are:
  • Exterior architectural products such as curtain wall, window, and door frames 0.7 to 1.2 mil
  • Marine products 0.7 to 1.2 mil
Architectural Class I finishes are designated as follows:
  • Aluminum Association (AA): A4 Series of designations, which include
    A41 Clear anodize
    A42 Integral color
    A43 Impregnated color (dyed)
    A44 Electrolytically deposited color
  • Nomenclature originated by Alcoa:
    Alumilite 215 0.7 mil and greater in thickness
  • MIL-A-8625:
    Type II Coating thickness called out on purchase documents or drawings.
Class I and II finishes have wide use in both clear and colored finishes.

Clear Anodize. These transparent coatings show off the silver-gray metallic properties of the aluminum. They are most often anodized in sulfuric acid and the anodic coating is sealed to enhance the protective qualities of the coating.

Integral Coloring. These finishes are produced in a mixed electrolyte of various organic or inorganic acids and sulfuric acid. The resulting colors range from a champagne color to dark bronze, gray, and black. Integral coloring, although an excellent finish, has seen decreasing usage as an architectural and commercial finish over the past several years, primarily due to the relatively higher cost of producing integral color finishes. Thanks to their extremely hard coating, integral color electrolytes may be used in hardcoat anodizing processes. Electrolytic coloring methods, generally less expensive to produce, have largely replaced integral color processes in the marketplace today.

Impregnated Coloring. Products with these finishes are first clear anodized and then immersed in organic or inorganic dyes. The dye is absorbed by the porous anodic oxide. Anodic oxide thickness and the amount of dye absorbed into the coating are largely responsible for the degree of lightfastness of the colors. Some dyes are not lightfast in any case. Common colors made with organic dyes are red, blue, green, brown, and black. Although virtually any color is available, most organic dyes are not fade-resistant. Inorganic colors are somewhat limited in range, golds and bronzes being the most commonly used.

Electrolytically Deposited Coloring. Products using this type of finish are first clear anodized and then color is added by a second step in a bath of metallic salts. The most common metals used for coloring are tin, cobalt, and nickel. The metal is electrolytically deposited into the bottom of the pores of the aluminum oxide (anodic) coating. The color obtained depends on the metal being deposited and the amount of deposit in the anodic pore. These colors range from light bronze to dark bronze and black. Other metals, such as copper, give a reddish color but are not as colorfast as colors produced from tin, cobalt, and nickel.

Other Coloring Processes. There are other processes used to color anodized aluminum. The most notable of these processes is called Interference Coloring. As this process gains in popularity it is becoming more readily available. In this process the base of the anodic pore is modified by electrochemical processing after clear anodizing. Then electrolytic coloring in a standard metallic salt bath deposits a very thin layer of metal at the base of the modified pores. This thin layer of deposited metal is capable of giving visible color interference. Colors produced are shades of blue-gray, green, yellow, and red.

Special Anodic Coatings. This group of widely used industrial anodize finishes is classed "General" in the AA designation system. Some of the most commonly used categories of finishes in this diverse group are specifically called out in the following manner.
  • Aluminum Association (AA): A1 Series of designations, which include
    A11 Preparation for other applied coatings
    A12 Chromic acid anodic coatings
    A13 Hard, wear- and abrasion-resistant coatings
  • Nomenclature originated by Alcoa:
    Alumilite 1000 Series Chromic Acid anodized
    Alumilite 225, 226 Hard anodized, 0.001 / 0.002 mil, respectively (sulfuric-oxalic mixed acid electrolyte)
  • MIL-A-8625:
    Type I Chromic acid anodized
    Type IB Chromic acid anodized (low voltage method)
    Type IC Non-chromic acid anodized (e.g., boric- sulfuric)
    Type III Hard anodized.
Preparation for Other Applied Coatings. These anodic finishes are produced so that other coatings may be applied over the anodized coating. Very thin coatings (0.01 mil to 0.1 mil) that are produced by the standard sulfuric acid method are sometimes used as a base for organic (painted) finishes.

Phosphoric Acid Anodizing and processes employing a mixture of phosphoric and other acids have gained widespread use as a base for other types of coatings. Typical uses of these very thin coatings include lithographic plates prepared for photo-sensitive emulsions, preparation for adhesive bonding applications in the aircraft and aerospace industries, and as a pretreatment for certain types of electroplating on aluminum.

Chromic Acid Anodizing produces very thin films that are more opaque than sulfuric acid anodized coatings. Coating thickness may range from 0.03 mils to 0.10 mils, depending on the alloy and the processing conditions. Chromic acid anodic coatings are usually gray in color and have an attractive enamel-like appearance. These coatings are very resistant to corrosion, resulting in wide use on military hardware and other products requiring excellent corrosion protection.

Hardcoat Anodizing can be carried out under a multitude of different processing conditions including electrolytes, temperatures, current densities, and times. The single objective, however, is to create a coating that is thicker than other coatings (1.0 mil to 5.0 mil or greater), has good corrosion resistance, and excellent abrasion resistance. Hardcoat anodized coatings generally have a Rockwell C hardness ranging from 50 to 70. These coatings have outstanding abrasion resistance in rubbing or sliding applications. They are said to be "file hard" but are not resistant to point pressure, as the softer aluminum substrate will give way, causing the hard anodic coating to collapse. Hard anodized finishes are often left unsealed, as sealing can be detrimental to coating hardness. Hardcoat anodizing is used on applications that include aircraft, machined parts, cookware, and many others.