Anodized aluminum does not experience tarnishing, but it can degrade or fade over time, leading to a diminished appearance. This material is a specially treated form of aluminum prized for its durability, corrosion resistance, and aesthetic finish, making it widely used in architectural components and consumer goods. While highly resistant to common environmental factors, the protective layer can be compromised by specific chemical and physical stressors. Understanding the science behind anodization and its potential vulnerabilities is key to maintaining the finish’s longevity.
Defining Tarnish and Aluminum’s Reaction
Tarnish is a thin layer of corrosion that typically appears as discoloration on the surface of certain metals, such as silver, copper, and brass. This phenomenon is usually the result of a chemical reaction with sulfur compounds in the air, creating a superficial film of metal sulfides that darken the surface. Unlike deep corrosion, tarnish is a self-limiting process that often protects the underlying metal, but it remains visually undesirable.
Aluminum, however, reacts differently due to its unique chemistry. When bare aluminum is exposed to air, it instantly forms a thin, stable layer of aluminum oxide (Al₂O₃) on its surface, a process known as passivation. This natural oxide layer is chemically inert and tightly adheres to the metal, acting as a highly effective barrier. This self-protecting quality prevents the rapid, deep corrosion or the formation of unsightly sulfides characteristic of tarnish on other metals.
How the Anodization Layer Works
Anodization is an electrochemical process designed to enhance aluminum’s natural protective layer, transforming the surface into a much thicker, harder, and more durable aluminum oxide coating. The aluminum part is submerged in an acidic electrolyte bath, such as sulfuric acid, and an electrical current is applied, making the aluminum the anode. This controlled oxidation causes oxygen ions from the electrolyte to react with the aluminum surface, growing a uniform oxide layer integral to the base metal.
This newly formed layer is highly porous, resembling a honeycomb structure. These microscopic pores are then utilized to trap color dyes for aesthetic finishes, or they are left clear. The final and most crucial step is sealing, where the porous oxide layer is immersed in hot water or a chemical solution. This sealing process closes the pores, making the aluminum oxide layer non-absorbent and creating a hard, ceramic-like barrier that is far more resistant to corrosion and abrasion.
What Causes Damage to Anodized Surfaces
Since anodized aluminum does not tarnish, its degradation is primarily caused by chemical attack, fading, and physical damage that compromises the protective oxide barrier. A significant threat is chemical etching, which occurs when the surface is exposed to substances with high or very low pH levels. Alkaline or caustic materials like strong cleaners, cement, or mortar can dissolve the aluminum oxide layer, resulting in irreversible white spots or surface dulling.
For colored anodized finishes, fading is a common concern caused by prolonged exposure to ultraviolet (UV) radiation from sunlight. The organic dyes used to color the porous oxide layer can break down when subjected to UV light, causing the color to lighten or change over time. Certain colors, like bright reds and blues, are more susceptible to this photochemical breakdown than darker colors or natural bronze tones.
Physical abrasion is another key cause of damage, as the oxide layer, while hard, is relatively thin, typically ranging from 5 to 25 micrometers. Scratching or rubbing with abrasive materials like steel wool or overly aggressive scouring pads can physically break through the brittle oxide film. Once the softer, base aluminum metal is exposed, it is left vulnerable to environmental factors and localized corrosion.
Cleaning and Preserving the Finish
The most effective way to preserve the anodized finish is to employ consistent, gentle cleaning practices. Routine maintenance should involve flushing the surface with clean water to remove loose dirt and debris, which prevents abrasive particles from scratching the finish. When cleaning is necessary, use a mild, pH-neutral cleaner or a gentle, non-abrasive detergent mixed with warm water. Cleaners should ideally have a pH range between six and eight, as anything too acidic or alkaline will attack the oxide layer.
Apply the cleaning solution with a soft cloth, sponge, or soft-bristle brush, using uniform pressure to avoid damaging the surface. Always avoid harsh cleaning agents containing bleach, ammonia, strong acids, or high-alkaline degreasers. After cleaning, the surface must be thoroughly rinsed with clean water to remove all detergent residue, which is especially important if the cleaner is allowed to dry.
For enhanced protection, particularly in environments exposed to moisture or harsh conditions, a protective wax or oil can be applied periodically. This added layer helps to seal the surface further, repelling water and protecting against minor chemical splashes and atmospheric contaminants. By avoiding abrasive tools and harsh chemicals, the durable, integral nature of the anodized oxide layer can be maintained for a long lifespan.