Surface Finishes: Anodizing

“Anodizing is an electrochemical process that converts the metal surface into a decorative, durable, corrosion-resistant, anodic oxide finish.” -Aluminum Anodizers Council

Anodizing is one of the most environmentally friendly processes in the coating industry. Since it is not a metal plating process, it produces no hazardous waste. Since aluminum’s game changing introduction in the 1920’s, surface treatments were needed to protect this durable element. Anodizing was the first of these surface treatments. MISUMI offers a wide range of anodized aluminum extrusion available in clear and black!

The Process

The process uses the aluminum object as the anode (+ pole) and a carbon electrode as the opposing pole and performs an electrolysis (applies a DC current) submerged in electrolyte baths. The surface aluminum of the subject dissolves and binds with oxygen to create an aluminum oxide surface layer. This layer is called anodic oxide layer or the anodized layer.

The Layer Structure

When an acid solution is electrolysis processed, the anode normally emits oxygen but not in a case of aluminum anodes. The oxygen from the aluminum anode immediately binds before gasifying with dissolved surface layer metal and forms an aluminum oxide. The layer forming process is shown in [Fig.2] below.

A barrier layer forms during the initial phase of the electrolysis advancing to a formation of the porous layer. As the process progresses, this porous layer grows to a certain thickness. As can be seen in the diagram, an anodized layer is made up of an electrically conductive but non-growing barrier layer and isolating but growing porous layer. The oxygen produced at the anode produces a porous layer with good electrical isolation characteristics, corrosion resistance, and wear resistant properties. The layer porosity is utilized for coloring as well as various functions added, and used for products such as reflector plates, household pots and pans, construction materials, automotive components, machinery/optical/communications, and computer components.

The factors that determine the surface layers depend on the base metal. The mechanism of anodized layer forming differs from electroplating where anodized layers are created by the base metal being electrolytically dissolved to form oxidized layers. The thicker the layer becomes, the more base metal is dissolved. Additionally, the base metal properties dictate how easily the oxidized layers can be formed. This is how anodizing is non-hazardous, there are no solvents or VOCs (Volatile Organic Compounds.

Types of Anodizing Processes

The anodizing process uses various electrolytes such as sulfuric acid, oxalic acid, chromic acid, or organic acid, as well as different temperatures and power source electrical waveforms will produce surface layers of different properties. There are many types of aluminum alloys with various compositions depending on their purposes, and they can be divided into heat treated alloys and non-heat treated alloys. From a machining perspective, there are plates, bars, rods that are rolled, and cast, and die-cast materials.

Various levels of anodizing difficulties based on different alloys exist and several schemes have been developed for the difficult alloys. Generally practiced anodizing processes are shown in [Table 1] below.

There are virtually an infinite number of industrial anodizing process combinations as the chart suggests, based on bath types, power supply waveforms, process speeds, and application purposes.
Currently, the most common electrolyte type is the sulfuric acid bath. The reason for the popularity is that the formed layer is colorless, and the power supply voltage and power consumption required for the electrolysis is low and economical for colored parts production. For specialized colors and wear resistance requirements, oxalic acid bath layers and oxalic acid added sulfuric acid bath is used.

Of the various electrolytes, the chromic acid bath is used almost only in the aerospace industry. As for the classification based on power supply waveforms, AC, AC superimposed, and pulse methods are used when the orthodox continuous DC does not yield satisfactory results. Process speed classification, in turn, is a classification based on electrical current density. Large applied currents will form hard oxide layers by preventing the formed layer from dissolving into the electrolytes.

In addition to aluminum, anodizing is used on magnesium, titanium, and tantalum, but are not for the same purpose as the processes on aluminum but for electrical components such as capacitors utilizing the nature of the surface oxidation layers. Colored anodizing offers more durability than paint and has an aesthetic appeal. Anodizing has many benefits from aesthetics to durability. It is a safe coating process both for the environment and factory workers.

Cover Photo by Seidel

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