Anodic coatings

Anodic oxidation or anodizing is the common commercial term used to designate the electrolytic treatment of metals in which stable oxide films or coatings are formed on surfaces. Aluminum and magnesium are anodized to the greatest extent on a commercial basis. Some other metals such as zinc, beryllium, titanium, zirconium and thorium can also be anodized to form films of varying thicknesses but they are not used to any large extent commercially.

Aluminum

It is a well-known fact that a thin oxide film forms on aluminum when it is exposed to the atmosphere. This thin, tenacious film provides excellent resistance to corrosion. The ability of aluminum to form an adherent oxide film led to the development of electrochemical processes to produce thicker and more effective protective and decorative coatings.

Anodic coatings can be formed on aluminum in a wide variety of electrolytes utilizing either AC or DC current or combinations of both. Electrolytes of sulfuric, oxalic, and chromic acids are considered to be the most important commercially. Other electrolytes such as borates, citrates, chromates, bisulfates, phosphates, and carbonates can also be used for specialized applications.

Anodic coatings produced in a sulfuric acid type of electrolyte are generally translucent, can be produced with a wide range of properties by varying operating techniques, and have a greater number of pores that are smaller in diameter than other anodic coatings.


Coatings obtained by the chromic acid anodic process are much thinner than those produced by the sulfuric acid process based upon the same time of anodic oxidation. Although the coatings are thin, they provide high resistance to corrosion because of the presence of chromium compounds in combination with their relatively thick barrier layer. Because the conventional anodic coatings produced in a chromic acid electrolyte are thin, they have low resistance to abrasion but a high degree of flexibility.

The chromic acid anodic process is critical with respect to alloy composition. In general, the process is not recommended for aluminum alloys containing more than 7% Si or 4.5% Cu. Alloys containing over 7% total alloying elements are also not recommended. It is difficult to form films on these alloys, particularly on casting alloys. Anodic solution (surface attack) will generally occur unless the processing conditions are controlled very carefully.

On most aluminum alloys, anodic coatings of this type do not require sealing. However, sealing in a boiling dichromate or a dilute chromic acid solution produces coatings with the best resistance to salt spray corrosion on AA2014-T6 and AA7075-T6 alloys.

Anodic oxidation in dilute oxalic acid solutions produces coatings that are essentially transparent. Their color varies from a light yellow to bronze. Such coatings are dense and have little absorptive capacity but possess high resistance to abrasion.

Hard Coatings

Procedures have been developed that produce finishes of greater thickness and density than the conventional anodic coatings. They have high resistance to abrasion, erosion, and corrosion. The coatings have thicknesses in the range of 0.1 to 5 mils, depending upon the application.

Hard coatings are popular for applications requiring light weight in combination with high resistance to wear, erosion, and corrosion. These applications include helicopter rotor blade surfaces, pistons, pinions, gears, cams, cylinders, impellers, turbines, and many others. Also, due to their attractive gray color, hard anodic coatings are now being used for architectural applications.

The processing conditions for obtaining hard coatings are such that thick coatings with maximum density can be obtained on most aluminum alloys. The selection of alloy is of utmost importance.

Variations of the anode oxidation process that produce conventional hard anodic coatings also form thick, dense, colored anodic coatings for architectural applications. Attractive bronze, gray, or black coatings are obtained by utilizing certain organic acids as electrolytes.

Other Coatings

Utilizing oxalic acid in combination with titanium, zirconium and thorium salts for the electrolyte produces a dense oxide coating that has an opaque, light-gray appearance.

Anodic oxidation of aluminum in sulfamic acid produces coatings that are denser than those produced by the sulfuric acid process. This anodic process is expensive owing to the high cost of sulfamic acid.

Alternating current may be used to form anodic coatings on aluminum and alloys with all of the electrolytes previously mentioned, but the aluminum surface is anodic only half of the cycle so that an oxide coating is formed only at half the rate of coatings formed with direct current.

Superimposed AC-DC has also been used for anodizing and has produced hard, thick, anodic coatings.

Alloy Selection

The previous discussion on the anodic oxidation characteristics of various electrolytes was based upon the use of relatively pure aluminum. Alloy selection is important. Even aluminum of 99.3% minimum purity, such as the AA1100 alloy, is in a sense an aluminum alloy from the standpoint of anodic oxidation, because the other elements present have an effect on the characteristics of the coating.

The response of the different constituents of aluminum alloys varies considerably and is an important factor. Some constituents will be dissolved by the anodic reaction, whereas others may be unaffected. For alloys where the constituents are dissolved during the anodic treatment, the coatings will have voids that decrease the density of the coating and also lower its resistance to corrosive action and abrasion. Al-Cu alloys are an example of this type.

Silicon in Al-Si alloys is an example of a constituent that is unaffected by conventional anodic oxidation. The silicon particles remain unchanged in the coating in their original position.

Some constituents of aluminum alloys will themselves oxidize under anodic oxidation and the oxidation products will color the coating. For example, constituents such as manganese will produce a brownish opaque appearance due to the presence MnO5. Also, chromium as a constituent will give a yellowish tint to the coating from oxidation products of chromium.

The lower the concentration of constituents present or the purer the aluminum, the more continuous and transparent will be the oxide coating. The so-called superpurity (99.99%) aluminum produces the most transparent oxide coating.

Properties and Applications

The oxide coatings produced by anodizing have many properties that make them commercially important. Anodic coatings are essentially Al2O3, which is a very hard substance. Because the Al2O3 is integral with the surface it will not chip or peel from the surface; this outstanding characteristic is useful for architectural applications. The combination of high wear resistance and attractive satin sheen of the finish makes it a logical choice for aluminum hardware, handrails, moldings, and numerous other architectural components. Because the anodic finish reproduces the texture of the surface from which it is formed, a wide variety of attractive effects are possible by variation in the surface preparatory procedures. Many commercial architectural applications can use a natural aluminum finish, but for applications where it is desirable to preserve initial appearance, the anodic finish will require less maintenance.

Because the oxide coating is brittle compared with the aluminum underneath, it will crack if the coated article is bent. It is possible, however, to produce oxide coatings that are relatively flexible. In general, the thicker coatings formed in sulfuric and oxalic acid electrolytes will crack or craze to a much greater extent than oxide coatings formed in a chromic acid electrolyte. For many applications this cracking may not be objectionable, because it is usually difficult to detect by visual observation. These fine cracks have an adverse effect on the bending properties of the metal, however, and may sometimes cause fracture if the bends are severe. For this reason, it is generally recommended that the finish be applied after forming.

If fatigue is a critical factor, then proper allowance must be made for the reduction in endurance limits produced by relatively thick oxide coatings. Fatigue tests indicate that a coating 0.1 mil thick on smooth surfaces will have little effect on fatigue strength. Thicker coatings in the range of 0.3 to 0.5 mil on smooth surfaces have a slight detrimental effect at high stresses.

Anodic coatings also provide substantial protection against corrosion. There are many factors that must be considered in this connection such as the continuity of the coating and the choice of alloy. Since continuity is dependent upon constituents present in the alloy, anodic coatings on high-purity aluminum are the most resistant to corrosion. On the other hand, anodic coatings formed from Al-Cu alloys have much lower resistance to corrosion.

Sealing of anodic coatings in dichromate solutions results in an appreciable improvement in resistance to corrosion, particularly by chlorides. The results of atmospheric exposure tests indicate that anodic coatings with a thickness of 0.4 mil or greater will provide greatly increased resistance to weathering.

The ability of anodic coatings to absorb coloring substances such as dyestuffs and pigments makes it possible to obtain finishes in a complete range of colors including black. The colors are unique because the luster of the underlying metals gives them a metallic sheen that is particularly attractive for applications that simulate metals such as gold, copper, bronze, and brass. Colorants are available that, when used to color anodic coatings on the proper alloys, will reproduce the natural colors of the metals listed above. Colored finishes can be used in a wide range of applications for nameplates, panels, appliance trim, optical goods, cameras, fishing reels, instruments, gift-ware, and jewelry.

Anodic coatings have good electrical insulating properties. Anodic films produced in boric acid electrolytes are used commercially on aluminum foil for electrical capacitors. The voltage necessary to break down the anodic coatings is generally proportional to the thickness.

Anodic oxidation in oxalic acid and phosphoric acid electrolytes produces coatings that have been successfully used as preparatory treatments for electroplating. Copper, nickel, cadmium, silver and iron have been successfully deposited over oxide coatings. Plating solutions that are highly alkaline should be avoided as they attack the coating and destroy the porous structure necessary for the best adhesion.

A "Krome-Alume" process utilizes an oxalic acid electrolyte to form the oxide coating, and subsequent modification of the coating with hydrofluoric acid produces a structure satisfactory for electroplating. Furthermore, the anodic oxidation process utilizing a phosphoric acid electrolyte produces a structure that requires no further modification to condition it for electroplating. The alloy has an important effect on coating structure and, in general, the phosphoric anodic process is not recommended for preparing high-purity aluminum, Al-Mg wrought alloys, and most die-casting alloys for electroplating.

Production Methods

Anodic coatings are applied to aluminum and its alloys by a variety of methods, including batch, bulk, continuous conveyor, and continuous strip.

The batch method of applying anodic coatings is similar to that used in electroplating except that the parts are anodic instead of cathodic. The continuous conveyor method is also similar to the conventional plating method.

The bulk method for applying anodic coatings to small parts such as rivets, washers, and screws is radically different from bulk electroplating methods. The barrel-finishing method is not suitable for applying anodic coatings because the initial flow of current forms an anodic coating on the parts and even if they contact each other during the rotation of the barrel, no current will flow because the coating is an insulator. The bulk methods employed for anodic coatings utilize special perforated non-metallic cylindrical containers. Pressure, applied to the parts in the container through a threaded center contact post, maintains the initial contact between the surface of the parts.

The continuous strip process has been used commercially to apply anodic coatings to aluminum sheet that is subsequently roll-formed into weather strip. In Europe, this same process is used to apply anodic coatings to aluminum sheet that is formed into food containers such as sardine cans.

Magnesium

Although all of the magnesium alloys in commercial use today have good resistance to corrosion, many parts are provided with maximum resistance to corrosion and abrasion through electrolytic treatments based on anodic treatments. The anodic treatments produce relatively thick and dense coatings with excellent adhesion and high resistance to corrosion and abrasion. As in the case of aluminum, anodic coatings on magnesium alloys are an excellent base for lacquers and enamels.

It is well known that magnesium alloys are attacked by most inorganic and organic acids. However, because magnesium alloys are resistant to alkalies, fluorides, borates, and chromates, the electrolytes for anodizing are generally based upon these chemicals.

The simplest electrolyte for anodizing magnesium alloys is a 5% caustic soda solution. This electrolyte is used in a temperature range of 60 to 70°C. A voltage of 5 to 6 is satisfactory; anodic oxidation time is generally 30 min. All magnesium alloys will respond to this treatment. The coatings produced are approximately 0.3 mil thick and are essentially crystalline magnesium hydroxide. They have relatively high resistance to abrasion and are gray or tan in color in the as-formed condition. The coatings may be colored for decorative applications by immersion in water-soluble dyestuffs in much the same way as the coloring procedures used for anodic coatings on aluminum alloys. If maximum resistance to corrosion is required, immersion (sealing) in a 5% sodium chromate solution at 77 to 82°C is recommended.

Another anodizing treatment for magnesium alloys capable of producing a coating with many desirable properties is the "H.A.E." process. This process is also based upon an alkaline electrolyte. All magnesium alloys, both wrought and cast, respond to this process to give coatings with excellent resistance to corrosion, high dielectric strength, and high hardness.

Other Metals

Thin oxide films may be formed on beryllium by anodic oxidation in an electrolyte composed of 10% nitric acid containing 200 g/l of chromic acid. The anodic oxidation is carried out at approximately 25 A/ft2. Such films retard high temperature oxidation and corrosion.

Thin oxide films can also be formed on zirconium, titanium and thorium by anodizing in an electrolyte composed of 70% glacial acetic and 30% nitric acid. The corrosion resistance of titanium is substantially improved by anodizing for 10 to 15 min in a 15 to 22% (by weight) sulfuric acid solution (room temperature) at approximately 18 V DC. Anodic coatings on titanium are also used as a base for lubricants.

Zinc

Much of the work on anodic coatings for zinc has been conducted in alkaline electrolytes consisting of a two-stage process. The electrolyte for the first oxidation treatment is conducted in an alkali-carbonate solution followed by anodizing in a silicate solution.

Next post:

Previous post: