Anodized aluminum is created by thickening the naturally occurring oxide layer on the metal’s surface, resulting in a durable finish resistant to environmental damage. Aluminum does not rust, as rust specifically refers to the oxidation of iron. Instead, aluminum forms aluminum oxide, and anodization is a controlled enhancement of this natural protection. While anodized aluminum cannot rust, it remains susceptible to specific forms of chemical degradation and corrosion.
Creating the Protective Layer
The anodization process is an electrochemical procedure that intentionally grows a protective layer of aluminum oxide from the aluminum base metal itself. The aluminum part is submerged in an acidic electrolyte bath and serves as the anode, while an electrical current is passed through the solution. Oxygen ions released in the bath react with the aluminum surface, which results in the formation of a hard, porous aluminum oxide layer. This newly formed layer is fully integrated with the underlying metal, which prevents it from chipping, flaking, or peeling like a paint or plated coating.
The controlled oxidation creates a layer that can be up to three times harder than the original aluminum. This anodic film is much thicker and more uniform than the thin, passive oxide layer that forms naturally on untreated aluminum when exposed to air. A separate, final sealing step is performed after the oxide layer is grown and any desired color is added. Sealing closes the microscopic pores in the oxide structure, transforming the porous layer into a dense, non-reactive barrier that is highly resistant to corrosion.
Specific Types of Anodized Aluminum Degradation
Although the anodized finish is highly protective, it can be compromised by chemical and physical attacks. Pitting corrosion is a common issue, primarily caused by exposure to chloride ions found in salt water or de-icing salts. These ions break down the passive film, creating small, deep holes that penetrate the anodic layer and expose the underlying aluminum metal.
Chemical etching occurs when the aluminum oxide finish is dissolved by substances outside the neutral pH range. Highly acidic or alkaline materials quickly attack and dissolve the aluminum oxide layer. For example, common household cleaners, fresh cement, or mortar runoff often have a high alkaline pH (around 12.5 to 13.5) and can cause severe etching and pitting. The resulting damage appears as dull, white, or discolored spots where the protective layer has been stripped away.
The anodized layer can also be damaged by abrasion, a form of mechanical wear. Although the hard oxide layer provides excellent scratch resistance, physical scraping removes the film. Once the protective coating is scraped off, the bare aluminum is exposed, resulting in a loss of corrosion resistance and creating a weak point for other forms of corrosion.
Practical Steps for Finish Longevity
To ensure the long-term integrity of the anodized finish, regular and gentle maintenance is necessary. Routine cleaning should be performed using only mild, pH-neutral soaps or detergents mixed with water. This process helps remove surface grime and contaminants before they can accumulate and lead to chemical reactions. It is important to rinse the surface thoroughly with clean water immediately after washing to remove all soap residue.
Surfaces exposed to harsh environments, particularly marine air or areas where de-icing salts are used, require more frequent attention. After exposure to saltwater spray or chemicals, promptly rinsing the surface with fresh water minimizes the time chloride ions have to initiate pitting corrosion. When cleaning, users must avoid all abrasive tools, such as scouring pads, steel wool, or harsh bristle brushes, as these can mechanically damage the oxide layer.
For cleaning more stubborn spots, solvents no stronger than mineral spirits or denatured alcohol may be used with caution. Cleaners containing strong acids or alkalis, such as those with trisodium phosphate or hydrochloric acid, must be avoided to prevent chemical etching. The finish’s durability is also influenced by the initial anodization type, with Type III (Hardcoat) offering a thicker, more abrasion-resistant layer than the thinner, decorative Type II finish.