Aluminum Seals: An Overview of the Three Different Types of Anodic Oxide Finish
Aluminum seals can go through a process known as anodization to become anodized. The process involves immersing the aluminum in an acid that contains a work piece anodic metal, which is positively charged, while applying direct current electricity. The direct current electricity will split any water within the solution into positively charged hydrogen atoms and negatively charged oxygen atoms, and the oxygen atoms will attack and coat the surface of the aluminum. There are three different types of acids that are commonly used in these processes, and the type of acid used will affect the type of anodic oxide finish on that you get on the aluminum seals.
Type 1 Finish — Chromic Acid Anodizing
This type of finish is a conventional coating with a thickness of 0.5 to 7.6 microns that resulted from a chromic acid bath. This type of anodization is often performed on aluminum seals that are used for aerospace purposes, and offers excellent corrosion resistance. The main attributing feature of chromic acid anodizing lies in the thickness of the coating, which is able to excellent limit impact on reducing the fatigue strength of all components. A chromic bath is the most common chemical used for a type 1 finish, although the same results can also be achieved with a sulphur bath. Most professionals prefer using the chromic acid bath, as it is relatively less reactive than the sulphur alternative.
Depending on what the aluminum seals are used for, a sulphur bath may be a popular alternative. There have been many concerns raised regarding the environmental impact of chronic acid.
Type 2 Finish — Conventional Sulphuric Acid Anodizing
A type 2 finish can be achieved using a sulphuric acid bath for anodizing purposes at a temperature between 68 to 70 degrees Fahrenheit and a current density of approximately 10 to 15 amps per feet. This type of anodizing typically results in a film thickness on the aluminum seals that fall between 3 and 25 microns. It is difficult to achieve a thicker coating since the dissolution by the acid will prevent additional buildup, and the coating is also non-conductive. The thickness of the coating will be dependent on how pure the aluminum is, and whether there are other metals that are used in the alloy.
The aluminum seals can also be color-coded using electrolytic coloring. The color will be locked into the aluminum seals during the anodizing process, and the degree of saturation of the color will be solely dependent on how thick the coating is.
Type 3 Finish — Hard Coat Anodizing
The type 3 anodizing finish for aluminum is also called the hard coat anodizing, and it is the thickest of all of the options offered. This process involves running the aluminum alloy in a sulphuric acid bath at temperatures of approximately 32 degrees Fahrenheit with a current density between 23 to 37 amps per feet for approximately 20 minutes to two hours depending on the size of the aluminum seals. The amount of time that the aluminum seals spend within the sulphuric acid bath will dictate its thickness. A maximum thickness of approximately 0.002 inches can be achieved with this type of finish.
Hard coat anodizing can offer numerous benefits, and is generally recommended when wear will be of particular concern. Hard coat anodization also increases the aluminum seals' abrasion resistance. Once again, this type of finish can be dyed with electrolytic coloring if color-coding is needed.
Anodizing aluminum seals can be beneficial for many reasons. The anodization ensures that the aluminum is corrosion resistant, is wear resistant, becomes more durable, possesses enhanced adhesive bonding, has an improved appearance, can adhere paint more effectively, and has an enhanced capacitance when used in electronic appliances.