Metal Anodizing

Anodizing is a process that is used to change the exterior of a metal. The technique can be used to change the surface topography, crystal structure, as well as to increase the corrosion resistance. The method is often used to protect aluminum and titanium from corrosion and abrasion. It also allows to dye the metals in different colors.

Some of the important anodizing processes include -

• Black Anodizing
• Hard Anodizing
• Color Anodizing
• Titanium Anodizing

While anodizing can be done to change the surface topography, crystal structure, as well as to increase the corrosion resistance of various metals, the most important of them is aluminum anodizing.

Aluminum anodizing is an unparalleled electrochemical process, which increases the thickness of a passive, naturally occurring, protective aluminum oxide film. The resulting finish that depends on the process, is the second hardest material known on earth next to diamond, and is sometimes comparable to sapphire. Metal anodizing expands on the natural oxidation procedure by adding chemicals and electricity to form a hard and transparent aluminum surface. Without this procedure, a typical aluminum will corrode and become weak, and anodizing offers protection from these defects. Once anodized, the metal becomes weather resistant and hard. Some other metals, like titanium can be anodized to produce similar results.

Prior to the start of anodizing process, the metal should be cleaned to prevent surface problems. Next, the metal must be pre-treated with bath to create different degrees of shininess. Some of these methods are bright dip anodizing, matte anodizing and satin anodizing. Now the product is prepared for anodization, and in this step it is coated using several methods, including standard sulfuric, hard coat, and color anodizing. After this, the product is covered with a sealant or color dye. Anodizing is comparatively a short process, which takes only a few hours to complete.

Anodized aluminum is known for its superior corrosion resistance and color variation. The anodized finish is sufficiently porous to admit penetration by color dyes. Once dyed, the color is fixed by sealing in hot nickel acetate or similar solution. Being inorganic, anodized aluminum is unaffected by ultraviolet rays. Another advantage of aluminum anodizing is the possibility of repair or re-coloring in future by stripping the anodized layer from aluminum. In addition to this, the anodized surface does not flake and is both static resistant and insulated.

Among the different available methods of aluminum anodizing, hard coat anodizing has found to be useful because of its non-conductive nature to electricity and better wear resistance than other methods. Standard anodizing is another method, which offers great base for bonding to organic coatings and other metals. Various products are made with anodized aluminum, including dryers, mailboxes, grills, pots, pans, refrigerators, duct covers, bicycle frames, wheel covers and electronic products like clocks. The architectural industry is highly dependent on the production of quality anodized aluminum, as it is frequently used for the framing of buildings.

Other industries, which benefit from the use of anodizing are the military, automotive, medical and aerospace.

There are four different processes that are used for the anodizing of aluminum and aluminum alloys of general commercial interest. Each of these are discussed in brief below -

Type 1 - Chromic Acid Anodizing

Anodic coatings that are formed in the chromic acid type process are comparatively thin (0.1-0.3 mil), they, however, exhibit excellent corrosion resistance. A typical cycle for chromic acid anodizing process would involve the following steps -

• Cleaning
• Rinsing
• Etching
• Rinsing
• Desmutting
• Rinsing
• Anodizing
• Rinsing
• Coloring (Optional)
• Rinsing
• Sealing

Chromic acid anodizing processes have found to be useful in the aircraft and defense industries. The earliest industrial use for anodizing involved the chromic acid anodizing of seaplanes in the 1920s. This coating facilitates an excellent base for painting. As chromic acid has been found to be a carcinogen, its replacements have been investigated.

Type 2 - Sulfuric Acid Anodizing

The most widely used of all the anodizing processes, sulfuric acid bath is also used for Type 3 hard coating processes. The coating thickness may range from low to moderate (0.1 to 1.0 mil), which depends on the application. A general cycle for this process would involve the following steps:

• Cleaning
• Rinsing
• Etching
• Rinsing
• Brightening (optional, in place of etching)
• Rinsing
• Desmutting
• Rinsing
• Anodizing
• Rinsing
• Coloring (optional)
• Rinsing
• Sealing

A wide range of colors are possible on anodized aluminum substrates. Coloring is generally achieved with organic dyes. The dye bath is usually in a concentration of less than 1% at a temperature of about 60°C (140°F). Depending on the depth of color desired, the dyeing period may last from 10 - 30 minutes. A two phase process for electrolytic coloring uses the standard anodizing process for the first phase that is followed by AC treatment in a metal containing bath. An important coloring process that is used primarily in the architectural industry, utilizes a proprietary bath based on sulfuric acid, wherein coloring and anodizing are accomplished in one step. With color anodizing, decorative effects can be obtained by using multiple colors for automotive trim, appliances, nameplates and instrument panels.

Type 3 - Hardcoat Anodizing

Type 3 hardcoat anodizing is distinguished from Type 2 sulphuric acid anodizing by the thickness of coating that ranges from 1 - 5 mils for most alloys. The process is similar to that for Type II, but without the optional brightening step. There are two major operating regimes for hard coating. Low temperature hard coat uses a sulfuric acid concentration ranging from 180 to 225 g/L with an aluminum content of 4 to 15 g/L at -2 to 0°C (50-70°F). Much greater current densities are the main feature of hard coat, up to 36 ASF. Special waveforms are sometimes utilized with superimposed AC or pulse current. Additives and/or pulse current are specifically required when handling alloys high in silica and/or copper.

Coloring and sealing are more problematic with hard coat anodizing, due to the dense nature of the coating. It is normal to utilize higher dye concentrations and longer immersion times. In some cases a short dip in 1:1 nitric acid is used to increase the uptake of the dye, but this may have an effect on the hardness or abrasion resistance of the coating film. A 5% solution of potassium or sodium dichromate can be used at 96-100°C (205-212°F) as a seal.

Type 4 - Phosphoric Acid Anodizing

Phosphoric acid anodizing produces a thin (0.1-0.2 mil) coating, which has found to be very useful for the preparation of plates in the lithographic printing industry and for adhesive bonding of aircraft components. Its first usage was as a base for electroplating on to aluminum. However, it has been supplanted in this application by stannate and zincate immersion processes. A typical cycle for phosphoric acid anodizing would involve the following stages -

• Cleaning
• Rinsing
• Etching
• Anodizing

Some of the important advantages of anodizing are -

• Anodized coatings do not peel, chip, flake, or chalk.
• Anodized coatings are translucent, resulting in a deep, rich metallic appearance.
• Anodized coatings are barely affected by sunlight.
• The anodizing technique uses chemicals without VOCs, and aluminum itself is recyclable.
• Anodizing can be less expensive to produce and maintain.
• Anodic coatings are highly durable and abrasion-resistant. It is especially durable in high-traffic regions where the coating is subject to abrasive
cleaners and physical abuse.
• Anodized aluminum can be colored in a variety of shades.
• Anodic coatings have proved to be excellent finishes for areas that are subjected to filiform corrosion, particularly structures in coastal locations.
• Different coloring techniques facilitate weather fastness suitable for architectural applications.

A thicker and denser anodic coating provides the benefits of durability and longer life. After several years, an anodized surface may accumulate dirt and stains, which look similar to chalking paint. This film can be removed with a mild detergent employed with an abrasive cleaning technique. A little amount of the anodic coating can actually be removed, that leaves behind a renewed anodized finish, maintaining the original appearance.

When a paint film fails, the available options are to repaint the surface or to replace the metal. Scrubbing can damage the finish of a painted surface. However, when an anodized coating appears to have failed, cleaning generally results in a renewed appearance. Anodized surfaces, such as building components, must be protected from chemical attack after installation.