Aluminum Anodizing

What is Aluminum Anodizing?

Wisconsin Metal Tech offers a full aluminum anodizing service throughout the US.

Anodizing is any type of method which changes the surface chemistry of various materials, in particular metals. The new surface adds a number of new properties to the metal, further protection against corrosion, increase in durability, particularly a new resistance to scratching, as well as of course the aesthetic enhancement. Because aluminum is so widely used in hundreds of industries, it makes sense to have the aluminum anodized to inherit the new properties.

The coating itself can be anything between 0.00393701” to 0.03937012 in thickness. Unlike other finishes, aluminum anodizing preserves the metals’ natural luster, its texture, and the aesthetic of the metal itself.

History of Anodizing

Anodizing of metals was used for the first time on an industrial scale in 1923. It was initially created to be used as a form of protection against corrosion on Duralumin seaplane parts. Obviously, it’s used as parts of a seagoing vehicle that required a tough coating immune to the salty, turbulent sea.

The early process itself was chromic acid-based and was named the Bengough-Stuart process, it was noted as British Defence Specification DEF STAN 03-24/3. This process is still used today, despite the legacy requirements for a complicated voltage cycle, now seen as unnecessary.

By 1927, the process had evolved and a new sulfuric acid anodizing process by patented by Gower and O’Brien. Sulfuric acid remains the most common anodizing electrolyte.

The Japanese used Oxalic acid anodizing from 1923 and it was widely adopted by the Germans, especially in architectural applications. The anodizing of Aluminum extrusions was a popular architectural use throughout the 1960s and ’70s.

The Anodizing Process

Before the above process, the aluminum must follow the following process:

Cleaning

The part to be anodized must first be cleaned to remove all signs of fabrication oils, buffing compounds, and other impurities. This is done by immersion in a water-based solution that contains mild acids or alkalis along with various detergents.

Pretreatment

This step is primarily for aesthetic purposes, it improves the appearance of the surface prior to the anodizing step. The most common pretreatment is etching, this gives the surface a satin finish or a bright dip giving a bright, shiny finish.

Anodizing step

Anodizing aluminum is an electrochemical process. Simply put, it involves taking the aluminum alloy and submerging it in a large tank filled with an electrolytic solution, this is an acid and water-based solution. The exact type of acid used depends on the application. An electrical current is passed through the aluminum part, in this case, the aluminum acts as the anode.

A cathode made from either aluminum or lead is also placed in the tank. The water is broken down, liberating the oxygen on the surface of the aluminum, this then combines to form the coating, a thin, transparent layer of aluminum oxide. The thickness of this coating is determined by the level of electrical current as well as the amount of time the current is applied.

How is the color applied?

When you think of aluminum anodizing the first thing is a bright-colored surface. the color can be applied 2 ways:

Integral Color

This process of coloring the aluminum gives the desired color when the anodizing is carried out in the bath. This process gives the aluminum a more abrasion-resistant coating, but the downside is the cost, it simply requires far more electrical power and so making it a more expensive option.

Electrolytic Coloring

It gives color to the aluminum part because the process of anodizing it creates stable, and consistent pores on the surface of the aluminum, the dye simply fills these pores.

The metal is immersed in a bath that contains an inorganic metal salt. Current is applied and deposits the metal salt in the base of the pores.

Sealing

The pores of the now colored metal need to be closed before the anodized part can be used. If left unsealed the surface of the parts would be susceptible to damage. Parts that do not need to be colored still have this step in order to increase corrosion and abrasion resistance whilst keeping the natural color of the metal.

What is aluminum anodizing used for?

The benefits of anodized aluminum such as anti-corrosion and abrasion resistance combined with the amazing looks from a huge range of colors open up a huge variety of applications. The possibilities of aluminum anodizing for the commercial, industrial, and consumer industries are endless.

Environmental Impact

Aluminum anodizing is one of the most environmentally friendly metal finishing processes. The process only uses very small amounts of heavy metals, halogens, or organic compounds. The coloring process does not produce VOCs or heavy metals. The recyclability of the coated and colored aluminum is equivalent to standard aluminum in terms of environmental impact. The waste products from the anodizing process are easily recycled for the manufacturing of alum, baking powder, newsprint, and cosmetics. The waste can also be used for fertilizer and in the industrial wastewater treatment industries.