Anodized aluminum (AA) is aluminum metal that undergoes an electrochemical process to create a protective surface layer. This treatment vastly improves the metal’s durability compared to untreated aluminum, but it does not render the surface completely immune to scratching. The anodized finish is significantly harder than the soft, underlying bare aluminum, but it can still be scratched by materials that exceed its hardness threshold.
How Anodization Changes the Surface
The anodizing process is not a coating applied onto the aluminum, such as paint or powder coat, but rather a conversion of the metal’s surface into a durable oxide layer. While aluminum naturally forms a thin oxide layer when exposed to air, anodizing uses an electrolytic bath and an electric current to thicken this layer dramatically. This reaction creates a robust and integrated layer of aluminum oxide.
The finished surface is composed of aluminum oxide, a ceramic material grown out of the base metal. This ceramic layer is microscopically porous immediately after formation, allowing for the absorption of dyes for coloring. The pores are then sealed through a final treatment, which locks in color and maximizes corrosion and wear resistance. Since the resulting layer is fully integrated with the original metal, it will not peel or chip away like applied paint.
Understanding the Hardness and Scratch Threshold
The scratch resistance of anodized aluminum stems directly from the ceramic nature of aluminum oxide, which is an exceptionally hard compound. On the Mohs scale of mineral hardness, aluminum oxide registers a value of 9, placing it second only to diamond at 10. This makes the finished surface considerably harder than common materials like steel (Mohs 4.5) or glass (Mohs 5.5 to 7).
The surface resists scratches from soft items like plastic, fingernails, or common metal tools such as keys. However, any material with a Mohs hardness of 7 or higher poses a threat to the finish. Common silica-based sand and quartz, prevalent in dirt and dust, are Mohs 7, meaning they are hard enough to abrade or penetrate the layer under pressure. Repeated exposure to grit or specialized items like ceramic tools or diamond-tipped objects will cause a visible scratch.
Why Different Anodized Products Vary in Durability
The scratch durability of an anodized product is determined by the thickness of the oxide layer, not solely the material’s hardness. Anodizing is categorized into different types, with Type II and Type III being the most common, varying significantly in layer thickness and density. Type II anodizing is used for decorative and cosmetic applications, such as consumer electronics, creating a relatively thin layer that typically ranges from 0.0001 to 0.0005 inches thick.
Type III, often called hardcoat anodizing, is engineered for superior wear resistance and durability in industrial or demanding applications. This process uses lower temperatures and higher current densities to produce a denser and substantially thicker layer, generally ranging from 0.001 to 0.004 inches. This thicker hardcoat provides a greater buffer against abrasion and is more resilient to deep scratches than the cosmetic Type II finish. The presence of dyes in Type II anodizing can also result in a slightly softer finish compared to an undyed Type III hardcoat.
Protecting the Finish from Damage
Preventing scratches on anodized aluminum involves environmental awareness and proper cleaning protocols. Since the surface is susceptible to abrasive materials like grit and dirt, regular cleaning is important to remove these contaminants before they cause damage. The cleaning process must be gentle, using only a mild detergent mixed with water and applied with a soft cloth or sponge.
Avoid using highly abrasive materials, such as steel wool, scouring pads, or rough cloths, as these can physically abrade the finish. Protecting the sealed oxide layer from chemical attack is also important, as harsh substances can strip the seal and compromise the finish. Aggressive alkaline and acidic cleaners should never be used, as they can chemically dissolve the aluminum oxide layer, leading to discoloration, etching, and loss of scratch resistance.