This article takes an in-depth look at anodized aluminum.
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Anodized aluminum is aluminum that has undergone an anodizing process to create an exceptionally durable, corrosion-resistant, and highly aesthetic surface.
Anodizing creates a stable aluminum oxide layer fully integrated with the underlying aluminum substrate. This layer is harder and stronger than raw aluminum. It is resistant to chipping, peeling, scratching, and flaking because this layer is intact with the aluminum substrate and not applied on the surface. The structure of this layer may be highly porous, making the anodized part suitable for the application of dyes, paints, lubricants, and adhesives. Lastly, this layer significantly imparts corrosion and weathering resistance to the raw aluminum, allowing them to withstand harsh environments.
Anodizing does not interfere with the recyclability of aluminum. In addition, this process is more environmentally friendly than electroplating and spraying.
Anodized aluminum parts exhibit a smooth, lustrous surface which makes them appealing. Anodizing allows these parts to be dyed or colored brightly. The colored film is highly fade-resistant. Hence, anodized aluminum is widely used in decorative and architectural applications.
Anodizing is an electrolytic passivation process that increases the thickness of the natural oxide film on the part surface. This film or layer, as discussed earlier, is durable and stable and imparts corrosion resistance to the part. Anodizing is a highly controlled oxidation process. This process occurs in an acidic electrolyte bath contained in a tank with flowing direct current (DC) on both the anode and the cathode. The aluminum part of being anodized acts as the anode. A cathode plate or rod typically made of platinum, stainless steel, lead, or carbon is also present inside the anodizing tank.
Anodizing is commonly performed in aluminum, though non-ferrous metals such as magnesium, zinc, and titanium; even conductive plastics can also be anodized. However, Anodizing is not beneficial to ferrous metals such as carbon steel due to ferrous oxide or rust formation instead of a stable and corrosion-resistant film.
An anodizing operation may be batch or continuous. In batch anodizing, the parts are placed in a rack and immersed in a series of baths. The parts are collected at the end of the process. Batch anodizing is employed in cookware, castings, and bent or machined parts. In continuous or coil anodizing, the pre-rolled material is unwound and continuously goes through the process steps. Once the steps are completed, the material is coiled and shipped to the customer. Continuous anodizing is performed in less deformed parts, wires, plates, sheets, and foils.
There are four stages in an aluminum anodizing operation:
The pre-treatment stage is crucial because it greatly affects the anodized part‘s finishing quality and final appearance. It involves cleaning the contaminants such as dirt and grease from the raw aluminum part surface that can hinder the process and removing minor surface imperfections. In addition, machining processes, such as drilling, cutting, and welding, must be performed before this stage.
Chemical or mechanical methods may be performed as a pre-anodizing treatment:
Chemical pre-treatment involves using several chemical solutions to remove dirt and grease (by an acid or an alkali cleaner) and surface oxides and heat-treat scales (a deoxidizing agent). After that, an etching or a brightening step is performed to modify the texture of the surface, which produces a distinct appearance:
An etching step produces a dull or matte finish. This step removes a uniform layer from the aluminum part surface, thus reducing the minor surface defects. Etching is accomplished by immersing the parts in hot sodium hydroxide or trisodium phosphate (alkali etching) or aqueous ammonium bifluoride (acidic etching).
A brightening step produces a shiny or mirror-like finish. The microscopic peaks and imperfections on the aluminum part surface are flattened and smoothened in this step. Thus, the roughness is reduced, which results in a highly reflective surface. Brightening is accomplished by immersing the parts in a phosphoric or nitric acid bath. Additives are mixed with the acid bath to enhance its brightening ability and reduce toxic fumes.
Mechanical pre-treatment involves procedures such as abrasive polishing, sandblasting, and shot peening to expose. Sandblasting and shot peening can improve the aluminum part‘s fatigue resistance, hardness, and coating adhesion. Good coating adhesion is necessary for the stability of the coating.
Electrolysis is the central process of an anodizing operation. In this stage, the aluminum part is submerged into an electrolytic solution bath that contains many positively and negatively charged ions. The aluminum part and the cathode are connected to the positive and negative terminals of the DC power supply, respectively. As DC is applied to close the circuit, the part becomes positively charged as electrons are withdrawn from its surface. These electrons migrate through the electrolyte bath to the cathode interface and react with hydrogen ions to form hydrogen gas. The aluminum cations at the surface react with water, creating the aluminum oxide (Al₂O₃) layer. The chemical reactions are summarized as follows:
There are two types of oxides films produced during the electrolysis process, depending on the chemical composition of the electrolyte bath:
A barrier oxide film is formed on the part surface when the film is grown in a neutral solution (e.g., ammonium borate, phosphate, or tartrate compositions), in which aluminum oxide is insoluble. This film is durable, unreactive with the solution, and protects the underlying aluminum from the environment. The thickness of the barrier oxide film depends on the voltage applied between the cathode and the anode. However, only a maximum voltage can be used before side reactions such as sparking, solute oxidation, and oxygen evolution occur.
A porous oxide film is formed on the surface when grown in a dilute acidic solution (i.e., around 10% acid content). The commonly used acidic electrolyte bath is sulfuric acid, though anodizing in phosphoric acid, oxalic acid, chromic acid, and mixtures of inorganic and organic acids are performed. The acidic solution can retain a high concentration of All₂Ol₃ molecules.
The reaction at the anode side results in forming a barrier aluminum oxide film. As the current flows through the part, it concentrates on the relatively weak and reactive spots on the part surface, producing a highly porous or cellular structure film. The aluminum oxides removed from the pores are dissolved in the acidic solution.
The film thickness is proportional to the electrolysis time and voltage, and the longer electrolysis times and higher voltages produce thicker oxide films. And a deeper column-like hollow structure. Meanwhile, the pore dimensions depend on the bath‘s voltage, temperature, and acid concentration.
There are three main types of aluminum anodizing processes according to MIL-A-8625 specification (anodic coatings for aluminum and aluminum alloys). These types produce a distinct set of properties.
Type I anodizing utilizes chromic acid to create the aluminum oxide film. This type produces the thinnest aluminum oxide film, measuring around 20-100 microinches thick while offering good corrosion resistance when appropriately sealed. In addition, this film is dielectric and non-conductive and is used as a primer for paint and adhesive application. Type I anodizing is suitable for tight tolerance parts; it does not change the part dimensions significantly.
Type I anodized aluminum parts have good forming qualities, and they can be utilized under high stress and bending conditions. These parts are widely used in the aerospace and aircraft industries. These parts appear grayish, even when dyed black. The low film thickness limits the dye absorbed by the film.
However, there are environmental concerns when employing this type since chromic acid is toxic and carcinogenic. Facilities offering Type I anodizing services require a special wastewater treatment facility to treat the chromic acid wastes.
Type II anodizing is the most common type of anodizing process and utilizes sulfuric acid instead of chromic acid. Sulfuric acid as an electrolyte solution produces porous structures efficiently. Hence, Type II anodized parts can easily absorb dyes, paints, and adhesives. In addition, they can be dyed with various colors for decorative purposes. Type II anodizing process produces a thicker oxide film whose measurement ranges from 100-1000 microinches wide. This film is also dielectric and non-conductive.
Type II anodized parts have good abrasion and corrosion resistance and are harder than type I anodized parts. These parts are used in decorative applications, architectural, consumer electronics, consumer goods, military kitchenware, military weapons, optical components, etc.
Type II anodizing is less expensive than Type I because of the cost of the chemicals consumed, energy requirements, and waste treatment requirements.
Type III anodizing also utilizes sulfuric acid as the electrolyte solution. However, this type is performed using a higher current density, higher voltage, and lower temperature than Type II anodizing. Type III anodizing produces a much thicker and highly porous oxide film which measures greater than 1000 microinches thick. This type also has the hardest and most durable coatings. However, it may not suit parts with extremely tight tolerances as it can slightly modify the part dimensions. This type also produces a dark film typically left undyed or dyed black.
Type III anodized parts, commonly known as a hard coat, offer superior abrasion, wear resistance, and good electrical insulation. In addition, the coefficient of friction may be diminished by PTFE impregnation, which is beneficial for parts frequently subjected to frictional forces. They also have excellent corrosion resistance. However, the increased thickness of the oxide film penetrating beneath the aluminum part surface decreases the fatigue resistance of the part.
Type III anodized parts are widely used in the military, aircraft, and aerospace industries. They can be used to manufacture sliding parts, linear guides, pistons, valves, hinges, gears, insulation plates, compressor fittings, automotive parts, and a lot more.
Like Type II, Type III anodizing is safe and environmentally friendly but more expensive due to the required process conditions.
The other types of aluminum anodizing processes are the following:
Boric-sulfuric acid anodizing (BSAA) is an alternative to Type I anodizing due to the environmental and safety concerns brought by Type I anodizing. BSAA offers excellent paint, lubricant, and adhesive adhesion as Type I anodizing. It also imparts good corrosion resistance and is suitable for parts with tight tolerances. BSAA is also widely employed in aircraft and aerospace parts manufacturing.
Phosphoric acid anodizing, also known as PAA or the Boeing Process, is an alternative to Type I anodizing and utilizes phosphoric acid to create oxide films. The rough morphology of PAA-created films has protrusion and whiskers on its surface, giving the oxide film an excellent adhesive property. This film can also withstand high humidity. Therefore, PAA can be used to prepare the aluminum surface for the application of bonding primer. PAA is commonly used in structural adhesive bonding.
Thin-film sulfuric acid anodizing (TFSAA) uses a sulfuric acid-based electrolyte bath with a lower concentration than Type II anodizing. As a result, TFSAA produces a thinner oxide film than those produced by Type II and Type III anodizing. Hence, it can be considered an alternative to Type I anodizing.
TFSAA-anodized parts have higher fatigue strength than Type II and Type III anodized parts because of their relatively thin oxide film. Hence, these parts are suitable under high-stress conditions. They can also be dyed easily. However, these parts have poorer corrosion resistance compared to Type II and Type III anodized parts.
Clear anodizing involves sulfuric acid anodizing followed by sealing the anodized part in a hot water bath. The finishing produced is not transparent; this process is instead used to create a uniform, transparent film on the aluminum part, which enhances its aesthetic value. The anodized part is usually left undyed, and the color produced depends on the thickness of the oxide film. Clear anodizing is typically used to finish automotive trims, window and door frames, railings, sidings, photography plates, and extrusion profiles.
Bright dip anodizing involves a pre-treatment of phosphoric and sulfuric acid mixture, which gives a glossy and highly reflective finish. The pre-treatment is followed by a Type II anodizing. The anodized part is then dipped into a coloring dye before the porous film is sealed. The resulting appearance depends on the grade of aluminum alloy. Nonetheless, bright dip anodizing enhances the overall aestheticity of the part.
Black anodizing involves a standard anodizing step followed by coloring the anodized part with an organic or inorganic black dye. Black dyes are one of the dyes specially formulated in coloring aluminum parts. Inorganic dyes such as ferric ammonium oxalate produce a more lightfast finish than organic dyes. Lightfast is the property of a finish which describes how fade-resistant it is when exposed to light. Electrodeposition of a coloring metal to produce a black anodized finish may also be performed.
Color anodizing involves a standard anodizing step followed by dip coloring using organic dyes. The colors of organic dyes available are more varied than inorganic dye colors. Color anodizing is mainly used for aesthetic applications. However, organic dyes produce a less lightfast finish than black anodized parts.
The dye or pigment fills the porous aluminum oxide film formed by electrolysis in the coloring step. The dyes of an anodized part are intact with the film. The "coating" of anodized aluminum is durable and cannot be scratched off from the surface. A highly porous surface is ideal for the introduction of dyes and pigments.
After rinsing the anodizing solution from the part and subsequent drying, Coloring is done. It can be accomplished by one of the following methods:
In electrolytic two-step anodizing, the anodized part from the electrolysis step is immersed in a bath containing metallic salts. Next, an electrical current is applied to the part. Metallic ions are electrolytically deposited deep in the pores of the oxide film, which gives a distinct color to the anodized part. The common coloring metals are cobalt, tin, copper, and nickel. The produced color and its quality depend on the metal deposited and the amount of metallic deposits in the pores.
In interference coloring, the oxide film pores‘ bases are enlarged to deposit more metallic ions electrolytically. This method produces light-fast colors ranging from blue, green, and yellow to red due to the optical interference of the visible light waves.
In integral coloring, the anodizing and the coloring steps are combined as the oxide film is colored. The anodizing bath is composed of organic acid and sulfuric acid. As a result, the produced "coating" is thicker and more abrasion-resistant. However, this method is expensive, and the colored oxide films are harder to produce. The colors are also limited to pale to dark yellow and bronze, brown, black, and gray.
The anodized part is immersed in a coloring bath that contains the dye in dip coloring. The dye is adsorbed on the surface of the pore opening of the oxide film. The resulting color depends on the type and chemistry of the dye. Dip coloring is an inexpensive method that allows the manufacturer to coat the aluminum parts with various colors. However, the colored film is not resistant to UV, unlike the colored films produced by other methods.
A lubricant or an adhesive replaces the coloring agent in processes involving the application of such substances. The coloring step is skipped for undyed parts.
The final stage of this process is sealing the aluminum anodizing operation. It locks the absorbed dye, lubricant, or adhesive on the porous film. It protects the porous film from corrosion, staining, and absorbing unwanted molecules. It also protects the color from fading. It is accomplished by treating the part with a sealing agent which closes the pores or reduces the pore opening diameters of the film. Sealing must be performed immediately after the coloring step due to the sensitivity of the oxide film.
There are two classes of anodized aluminum based on MIL-A-8625:
Class 1 anodized aluminum refers to the undyed anodized parts. The coloring step is skipped for these parts. The produced color will depend on the type of alloy, anodic thickness, and anodic treatment and sealing parameters. The produced color is usually clear gray or bronze and shall be the final color of the part.
A class I anodic coating has a minimum thickness of 0.7 mils or 18 microns. This coating is considered a "high-performance anodic finish ."Class I-coated parts are typically used as a building or construction material intended for continuous outdoor exposure. These parts include guard rails, curtain wall panels, rain screens, and fences.
Class 2 anodized aluminum refers to dyed or pigmented anodized parts.
The classes of architectural anodic coating based on the Aluminum Association designation are as follows. These classes are different from those specified by MIL-A-8625.
A class II anodic coating has 0.4 mils or 10 microns minimum thickness. Class II-coated parts are used in interior or light-exterior building applications which are not subjected to high fatigue and wear. These products include radiators, wall fins, column covers, trims, and storefronts.
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