What Is Anodizing?

How this method increases the corrosion resistance of a metal part

Man anodizing aluminum in an industrial setting.
••• James L. Amos / Getty Images

Anodizing is a method of increasing the corrosion resistance of a metal part by forming a layer of oxide on its surface. The part that is being treated forms the anode electrode of an electrical circuit. Anodizing increases resistance to corrosion and wear, and provides better adhesion for paint primers and glues than bare metal does.

Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thick transparent coatings that add interference effects to reflect light. One example of the latter is equipment used by cyclists on gear or clothing so they can be seen at night.

How Anodizing Happens

The process of creating this protective oxide coating is achieved electrolytically.

The metal part to be treated (generally aluminum) is first submerged in an electrolytic solution bath along with a cathode. When a current is passed through the acid solution hydrogen is released from the cathode and oxygen forms on the surface of an anode. This results in a metal oxide film growing on the surface of the part being treated.

Depending on the end-use application and the anodizing process used, the oxide layer can be expanded greatly. The layer that can be grown on an aluminum part can be more than 100 times as thick as an oxide layer that would naturally exist on an aluminum part that is only exposed to oxygen.

Common sense dictates that because the metal part being treated forms the anode in this electrolytic circuit the process is referred to as 'anodizing'.

Anodized aluminum and aluminum alloy parts are more corrosion and wear-resistant than non-treated parts. They also protect against galling. Galling is wear caused by friction when two parts of threaded components are rubbed together.

The end result is that anodized parts have much longer life spans than non-anodized parts.

Anodizing Aluminum

While anodizing aluminum allows the metal to keep its natural appearance, the pores in the protective oxide layer also help to provide a better surface for the adhesion of paints and glues.

While various metals, including titanium, hafnium, zinc, and magnesium, can be protected by applying an anodized layer, by far the process is most commonly applied to aluminum and aluminum alloys.

Different types of anodizing methods are generally characterized by the type of electrolytic solution used. Chromic acid (referred to as Type I) was used in the first commercial anodizing facilities in the 1920s. Today, however, the most common electrolytic solutions for anodizing are produced using sulfuric acid (referred to as Type II or Type III depending on the exact process used).

Broad-based Benefits of Anodizing

Anodized aluminum parts are commonly found in aircraft and architectural components, as well as consumer goods such as appliances (refrigerators, microwaves, and barbecues), sporting goods (baseball bats, golf carts, and fishing equipment) and electronics (televisions, smartphones, and computers).

The broad-based benefits of anodizing include:

  1. It's a very thin coating compared to paints and powders.
  2. Extremely durable, hard, abrasion-resistant and long-lasting. The coating does not peel or chip. Much harder surface than paint and powders.
  3. Lasts indefinitely.
  4. Environmentally-friendly finish. It can easily be recycled.
  5. Inexpensive compared to painting and powder coating.