Specifying Anodized Aluminum
For those who are not familiar with the many aspects of anodized finishes
This Technical Bulletin is published by the Aluminum Anodizers Council as an educational and informational service. The information contained in this Technical Bulletin has been obtained from sources believed to be reliable. It should be noted that relevant practices, regulations and technologies are subject to change and this Technical Bulletin is current only as of the publication date. Further, it should not be assumed that all processes, standards or requirements affecting metal finishing are included. AAC makes no warranty, guarantee or representation whatsoever as to the absolute validity or sufficiency of the information or any reference set forth in this Technical Bulletin. AAC does not “approve” or “endorse” any products, services or methods. AAC assumes no responsibility for use or misuse of this Technical Bulletin.
In this Bulletin
Introduction
Anodized aluminum is used in diverse applications in many different industries. Its wide range of use has resulted in a correspondingly wide variety of anodic oxide designations. The Aluminum Association (AA), in its Publication #45, “Designation System for Aluminum Finishes,” has provided assistance to industry designers and specifiers to designate the correct finish of their choice for the many applications of anodized aluminum. This designation system is particularly helpful to those who are somewhat familiar with anodized finishes and who have some idea of the finish they require.
Many people desiring anodized finishes on aluminum may not be knowledgeable about the types of anodic finishes available and how to specifically call out their requirements to anodizers. This Technical Bulletin of the Aluminum Anodizers Council is intended to help those who are not familiar with the many aspects of anodized finishes. For those who already are familiar with anodic finishes and their various applications, this Bulletin may serve as a quick reference guide to specifying anodized aluminum.
In addition to the AA designation system, three other designation or specification systems are in common use. These are the Alumilite Finishes nomenclature originated by Alcoa, military specification MIL-A-8625 (most current “F” version as of April 2000) and AAC1119 Automotive Standard. Reference will be made in this Bulletin to all of these “systems” to give the reader some familiarity with them. The American Architectural Manufacturers Association (AAMA) has adopted the AA designation system in its “Voluntary Standards for Anodized Architectural Aluminum.”
The system originated by Alcoa divides its nomenclature into three parts:
- Surface preparation (mechanical and chemical)
- Anodic coatings for wrought products
- Anodic coatings for cast products.
The “Mil Spec” (MIL-A-8625) classifies finishes by Types and Classes.
The AAC1119 Automotive Standard designates the finish types by a letter.
Details of these designation systems used for the specification of anodizing aluminum are listed in the Appendices of this Bulletin. In addition to these commonly used designation systems, many proprietary specifications are written for industry-specific or individual company applications.
General Discussion
Some of the factors to be considered are:
- Appearance: Bright, matte, colored, brushed, other
- Environment of use:
- Interior (architectural, commercial, decorative, other use)
- Exterior (architectural, marine)
- Industrial/Automotive(corrosive or noncorrosive application; resistance to wear or abrasion, other)
- Aerospace (corrosion resistance, dielectric, emissivity, fatigue strength, other)
- Alloy and temper of aluminum to be used
- Mechanical finish, if any, required
- Chemical finish in preparation for anodizing
- Anodic oxide type required.
Alloy and Temper
to the process of anodizing; this is particularly evident between alloy series. As a result, each alloy or alloy series yields a different appearance, even if treated to identical anodizing processes. Also, each alloy exhibits its unique
characteristics such as good formability, ease of machining, strength, response to anodizing, etc. Examples of some of these general characteristics follow:
- The 2xxx Series alloys are high in copper, have relatively high strength, are hard, and have good machinability. As copper content increases, anodizing generally becomes more difficult. Anodizing conditions should be closely controlled.
The anodic oxide on 2xxx alloys is usually softer and has lower corrosion resistance than on alloys with lower copper content. Special anodizing techniques may be used to obtain acceptable coatings. - The 3xxx Series have relatively high levels of manganese. These alloys are work-hardened (non-heat-treatable) and exhibit excellent formability characteristics. They anodize clear silver, grayish, or brownish depending on the production
conditions. - The 4xxx Series alloys are high in silicon and generally are not anodized.
- The 5xxx Series alloys are high purity aluminum with magnesium added. Alloys of this series are work-hardened (non-heat-treatable) and have good formability. Alloys 5252, 5457, and 5657 are low in iron and have good luster for chemically
brightened finishes. These alloys are used largely in automotive and appliance trim applications. Alloy 5052, while higher in iron, chrome, and magnesium, is a high-strength alloy and is used in structural applications such as truck
panels. Alloy 5005 is higher in iron and silicon than 5052. It is a good general-purpose architectural sheet and plate alloy. Alloy 5052 anodizes yellowish in thicker coatings, while 5005 anodizes clear silver, gray, or brownish.
- The 6xxx Series offer good, general-purpose, heat-treatable alloys. Alloys of this series have excellent response to anodizing. For example, 6063 and 6463 are popular extrusion alloys having good strength and excellent anodizing characteristics.
Principal alloying elements are magnesium and silicon; 6463 is low in iron and is used for bright finishes. Both have good luster and anodize clear silver. Alloy 6061 is higher in silicon, iron, copper, magnesium, and chrome than 6063.
It is a high-strength structural alloy having excellent machinability, a favorite of machine shops, especially if the parts are to be hard anodized. - The 7xxx Series alloys are high in zinc. They are considered ultra-high-strength alloys and find wide use in the aircraft and aerospace industries. They anodize gray, blue-gray, and brown-black (mottled), depending on the alloy and the
anodizing process.
Casting Alloys. Several aluminum casting alloys also can be anodized. Unfortunately, the characteristics that make good castings are not necessarily the best for anodizing. Alloys with the best casting characteristics are those containing
up to 12-percent silicon. High-silicon alloys do not anodize well because silicon is not readily soluble in aluminum. Only the aluminum on the surface of the part anodizes, leaving areas with higher silicon unanodized and the entire part
with a black or gray silicon powder.
Aluminum casting alloys containing relatively low amounts of silicon and iron and higher amounts of magnesium, chrome, and zinc tend to anodize well. Aluminum-magnesium alloys such as 514 and 535 anodize well. Aluminum-zinc alloys like 712,
713, and 771 also respond well to anodizing. Some higher silicon casting alloys, such as 356, can be anodized using special techniques and processes. It is best to check with an anodizing expert before specifying casting alloys for projects that call for anodized finishes.
Mechanical Finish
Mechanical finishing, if specified, is done before chemical finishing or anodizing. Mechanical finishes give surface texture or remove surface defects and irregularities. Appearance can be varied but usually gives a uniform, matte finish, directional or nondirectional sanded finish, or a specular (mirror-like) finish. The mechanical finish shows through the anodic oxides because uncolored anodic oxides are transparent or translucent and anodic oxides conform to the surface texture. This means that surface defects, as well as applied surface textures, are not covered up by anodizing. Examples of mechanical finishes are buffing, polishing, sanding, sand blasting, and shot peening. Refer to the appendix for
specific designations of mechanical finishes.
Chemical Finish
Chemical treatment of the part significantly affects its matteness or brightness (specularity). Chemical treatment is done prior to anodizing and is a very important factor in the final appearance of the anodized part. The pre-anodize chemical treatment of aluminum is often called the cleaning, clean-up or pretreatment phase of the process.
Cleanliness of the part is critical to quality in anodizing. A typical chemical treatment process might be as follows:
- Treatment in an inhibited acid or alkaline cleaner to remove dirt and oils
- Deoxidize in strong acidic solution to remove natural oxides or heat-treat scale
- Chemical etch or brightening.
Etching is frequently accomplished in a weak solution of caustic soda (sodium hydroxide). Etching removes metal uniformly and roughens the surface of the metal to give a uniform, matte finish. Acid etching also may be done. Treatment in solutions
such as ammonium bifluoride or trisodium phosphate produces a white, matte (satin) finish without as much metal removal as alkaline etching. Different anodizers customize the cleaning and etching process to meet the needs of the products
they process. Some examples of chemical finishes:
- Fine matte etch: (AA) C21; Alcoa R2
- Medium matte etch: (AA) C22; Alcoa R1
Note: MIL-A-8625 does not specify pretreatment processes by number or type designation.
Brightening is the micro-leveling (micro-smoothing) of the aluminum surface by either chemical or electrochemical means. High luster is created because the process removes the micro-peaks but does not affect the micro-valleys. The surface
is smoothed or leveled, which renders high luster to the surface. Both of these methods of achieving a specular finish may be specified by the AA designation C31. The Alcoa designation for this finish is R5. There are a number of other
chemical bright dip solutions and electrobrightening baths, which may be specified by designation or by trade name.
Different mechanical finishes may be combined with satin or bright chemical finishes to produce a variety of appearances. For example, the AA designation M21C21 indicates a “smooth specular buffed” (M21) mechanical finish followed by a fine
matte (C21) chemical finish. The specifier, working in conjunction with the finisher, may select the exact type of “fine matte chemical finish” to achieve the desired look. This is best accomplished by producing sample coupons of different
versions of the finish and choosing the one that comes closest to the desired appearance. The nomenclature originated by Alcoa for the above finish would be A1R1 or A1R2, “polished and buffed with light etch” or “polished and buffed with satin etch.” See the Appendix for a detailed list of finishes, designations, and nomenclature.
Anodic Finish
desired according to the end use of the product. Some of the characteristics of anodized finishes are:
- decorative
- reflective
- corrosion resistant
- absorption/emissivity of heat
- abrasion resistant
- dielectric
- color
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″ (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.
AAC1119
- See Table 3 in appendix 4.
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 - AAC 1119:
See Table 3 in appendix 4.
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
• 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.
• AAC 1119:
See Table 3 in appendix 4.
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.
Appendices
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