Introduction to Anodizing

Manufacturers use Anodizing to increase the thickness of the natural oxide layers on the surface of metal parts, most commonly applied to aluminum alloy substrates, it can also be applied to titanium, zinc, magnesium, niobium, zirconium, hafnium, and tantalum. The anodizing process employs electrolytic passivation to achieve the desired properties such as corrosion and wear resistance, better adhesion for paint and glue, or aesthetic effects. For aesthetic effects, thick porous coatings enable a dye to be absorbed, while thin transparent coatings can add interference effects to reflected light.

The process is called anodizing because the aluminum substrate forms the anode electrode of an electrical circuit. Anodizing is also adopted to prevent the galling or wear of threaded components and make dielectric films for electrolytic capacitors. The microscopic texture of a workpiece and the metal's crystal structure near the surface change with anodizing. Thick coatings usually require a sealing process to prevent corrosion due to their porous nature. Anodic films are beneficial in that they exhibit greater strength and adhesive properties than most types of metal plating; however, they are also more brittle, making them more subject to cracking due to thermal stress.

The main benefits of anodizing include significantly thinner coat than paint or powder, extremely durable, hard, abrasion-resistant, and long-lasting, no peeling or chipping, much harder surface than paint, color fade resistance, excellent corrosion protection, environmentally friendly, good electrical insulator, and affordable.

Historically, anodizing first occurred in an industrial context in 1923 to protect the duralumin parts used in seaplanes from corrosion. Initially, the use of chromic acid was adopted and called the Bengough-Stuart process. Eventually, this led to the adoption of sulfuric acid, which soon became and remains the most common anodizing electrolyte today. In 1923, oxalic acid anodizing was patented in Japan and subsequently widely adopted in Germany, especially in architectural applications. By the 1960s and 1970s, anodized aluminum extrusion was a popular architectural material but was replaced with cheaper plastics and powder coating. Most recently, phosphoric acid has been applied to the anodizing field, exclusively used as pretreatments for adhesives and organic paints for now.

The most widely used anodizing specification in the United States is a U.S. military spec. It defines three types of aluminum anodizing: chromic acid anodizing (Type I) and sulfuric acid anodizing (Types II and III). Chromic acid is the oldest anodizing coating process, while sulfuric acid is the most widely used solution in the anodized coating. It is also possible to anodize with an organic acid, phosphoric acid, borate and tartrate baths, and plasma electrolytic oxidation.


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