Everything you need to know about Aluminum Anodizing
What is Anodizing?
Anodizing is an electrochemical process that converts the metal surface into a decorative, durable, corrosion-resistant, anodic oxide finish. Aluminum is ideally suited to anodizing, although other nonferrous metals, such as magnesium and titanium, also can be anodized.
The anodic oxide structure originates from the aluminum substrate and is composed entirely of aluminum oxide. This aluminum oxide is not applied to the surface like paint or plating, but is fully integrated with the underlying aluminum substrate, so it cannot chip or peel. It has a highly ordered, porous structure that allows for secondary processes such as coloring and sealing.
Anodizing is accomplished by immersing the aluminum into an acid electrolyte bath and passing an electric current through the medium. A cathode is mounted to the inside of the anodizing tank; the aluminum acts as an anode, so that oxygen ions are released from the electrolyte to combine with the aluminum atoms at the surface of the part being anodized. Anodizing is, therefore, a matter of highly controlled oxidation the enhancement of a naturally occurring phenomenon.
What Are the Types of Aluminum Anodizing Processes?
There are three types of aluminum anodizing processes. They are described in more detail below:
1.Type I – Chromic Acid Anodizing
This process uses chromic acid as the electrolyte and produces the thinnest coating of all the methods, 2.5 μ (0.0001 in). Despite the reduced thickness, this process produces comparable corrosion resistance to the other two processes. The coating produced tends to be darker and does not accept color as well due to its reduced thickness and reduced porosity.
2.Type II – Sulfuric Acid Anodizing
This process makes use of dilute sulfuric acid as the electrolyte. It is the most commonly used technique. A coating with a thickness of 5.1 to 15.2 μ (0.0002 – 0.0006 in) can be developed. This coating is harder than the one produced by chromic acid anodizing. Parts anodized using the Type II process can easily be colored with a wide range of dyes. Sulfuric acid is a relatively low-cost electrolyte when compared to chromic acid.
3.Type III – Hardcoat Anodizing
This process makes use of sulfuric acid as the electrolyte but is used to produce much thicker coatings than Type II anodizing — 12.7 to 76.2 μ (0.0005 to 0.0030 in) due to a higher voltage, longer bath immersion time and lower bath temperature. This coating is harder than tool steel and is used where high levels of wear resistance are required.
The typical workflow of the type II aluminum anodizing process is as follows.
Racking – Pickling – Etching – Chemical Polishing – Anodizing – Coloring – Sealing – Drying
Racking: Hang parts on racks. The racks will carry electric current to the parts and will be immersed in acid, alkali, and Metallic salt solutions. They are made from titanium alloy.
Pickling: Immerse the parts in an acid or alkaline solution to remove oil, grease, and other impurities so they would not affect the anodizing effect.
Etching: Immerse the parts in the acid solution to remove a very thin layer on the surface, avoid the original oxide layer on the surface from affecting the subsequent anodizing effect, and form a uniform matt surface at the same time. The time of this process will affect the size of the parts, and the etching time of precision aluminum parts needs to be strictly controlled.
Chemical Polishing： This process is mainly used for parts with bright surface requirements. Generally, the mixed solution of three kinds of anhydride is used for chemical polishing. This process can be omitted for parts that require a matt surface.
Anodizing: The workpieces are immersed in sulfuric acid solution, and the pore structure aluminum oxide film is formed on their surface by electrification.
Coloring: Type II oxidation generally adopts dip coloring. The anodized aluminum parts are immersed in a bath containing dye. The dye is adsorbed on the surface of the pore opening of the anodized film. The color produced depends on the type and chemical properties of the dye. Dip coloring is a cheap method that allows manufacturers to paint various colors on aluminum parts. That is why the type II anodizing process is the most popular process for aluminum anodization.
Sealing： After anodic oxidation and dyeing, the aluminum parts are immersed in ionized water or distilled water to seal the porous structure oxide layer. There are three commonly used sealing methods, hot water sealing. Add metal salts such as nickel, magnesium, or cobalt to medium-temperature water to seal. Sealing of room temperature water plus nickel fluoride and other metal salts. The oxide layer after sealing is stronger, smoother, and more durable.
Anodizing Metal Parts With Protosoon
Anodizing is a widely-used finishing process for metal parts. It can increase a metal component’s lifespan by improving corrosion resistance, wear resistance, and more. Anodization also simplifies long-term maintenance, creates beautiful and identifiable colored components, and even promotes environmentally friendly manufacturing.
While anodizing your metal parts is incredibly important, knowing how to go about anodizing can be a challenge. Protosoon, a seasoned manufacturing partner, can help you achieve the best anodized finish for your components. Contact us today to get started.