What Anodizing Does to Aluminum Profiles
Anodizing is an electrochemical process that converts the surface of an aluminum profile into a hard, porous aluminum oxide layer. The profile is made the anode in an acid electrolyte, and oxygen released at the surface reacts with the metal to grow an Al2O3 film that is an integral part of the profile rather than a coating applied on top of it. Because the oxide layer forms from the aluminum itself, it cannot peel or flake, and it protects the extrusion from abrasion and corrosion while the underlying metal keeps its original mechanical properties. For architectural and industrial profiles, anodizing is one of the most durable finishes available.
Process Sequence: Pretreatment, Anodizing, Sealing
A consistent anodic film starts with correct surface preparation. Profiles are degreased and etched to remove rolling lines, die marks and natural oxide, then desmutted so the surface is clean and uniform. The prepared profile is anodized under controlled current, voltage, temperature and time, which together determine the film thickness, and the porous film is then sealed by immersion in hot water or sealing chemicals. Sealing closes the microscopic pores of the oxide layer and is essential for stain resistance, corrosion protection and long-term durability, because unsealed films absorb dirt and moisture. Each stage must be controlled carefully, since defects in pretreatment appear as streaks or uneven film on the finished profile.
Coloring the Oxide Film
Before sealing, the porous anodic film has a strong adsorption capacity, and this property is used to add color. Metal salts can be deposited inside the pores to produce shades such as black, bronze, champagne, golden yellow and stainless-steel gray, in addition to the natural silver-white of the clear film. Electrolytic coloring is the most common route for architectural profiles because it gives weather-resistant, uniform colors, while organic dyeing offers a wider palette for interior and decorative applications. After coloring, the film is sealed to lock the color in place and restore corrosion resistance.
Film Thickness and Performance Classes
The protection an anodized profile offers depends mainly on film thickness, which is specified in micrometers. Interior profiles may need only about 5 to 10 micrometers, while exterior architectural profiles are normally anodized to thicker films, commonly 15 to 25 micrometers, to withstand decades of weather exposure. International specifications such as ISO 7599 set out general requirements for anodic oxidation coatings on aluminum, and sealing quality can be verified by standard test methods that measure mass loss or film admittance. For coastal or polluted environments, a thicker film and correct sealing are the difference between a finish that lasts and one that fails early.
Alloy and Process Factors That Affect Quality
Not every aluminum alloy anodizes equally. The 6xxx series, including 6063 and 6060, is the standard choice for extruded architectural profiles because its low copper content produces clear, consistent films with good gloss. High-copper alloys such as the 2xxx series produce dull, uneven films and are rarely anodized for appearance. Within the same alloy, extrusion quality, grain structure and surface finish all influence the result, so profiles destined for anodizing should be extruded and handled with care to avoid scratches and contamination. Working with a supplier who controls the whole chain, from billet to anodizing line, gives the most repeatable finish.
FAQ
How long does an anodized finish last on exterior profiles?
An exterior-grade anodized film, typically 15 to 25 micrometers and properly sealed, can remain serviceable for decades in most climates when cleaned periodically.
Can anodized aluminum be colored?
Yes. Before sealing, the porous oxide film absorbs metal salts or dyes, producing colors such as black, bronze, champagne, gold and stainless-steel gray.
Does anodizing make aluminum harder?
The anodic oxide film is much harder than the aluminum beneath it, which improves scratch and wear resistance of the surface while the core keeps its original mechanical properties.
Why does sealing matter after anodizing?
Sealing closes the microscopic pores of the oxide film. Without it the film absorbs dirt and moisture, losing stain resistance and long-term corrosion protection.
Which aluminum alloys anodize best?
6xxx extrusion alloys such as 6063 and 6060 give bright, consistent anodic films, while alloys with high copper content produce dull and uneven results and are usually avoided.
Q: How long does an anodized finish last on exterior profiles?
A 20 to 25 micron anodic film lasts 20 to 40 years outdoors, depending on climate and maintenance. Coastal and industrial environments reduce life, which is why 25 microns is specified for severe exposure.
Q: Can anodized aluminum be colored?
Yes. Colors come from electrolytic coloring, bronze, black and champagne, with metal salts in the pores, or organic dye for bright colors. The color is inside the oxide, so it does not peel like paint.
Q: Does anodizing make aluminum harder?
Yes, Type II anodizing reaches 200 to 400 HV and Type III hard coat reaches 300 to 500 HV plus, far harder than the base alloy. This gives wear resistance but does not increase bulk strength.
Q: Why does sealing matter after anodizing?
Sealing closes the pores of the oxide, preventing dye bleed, stain absorption and corrosion. Unsealed anodizing absorbs dirt and fails quickly outdoors; hot water or nickel sealing is standard.
Q: Which aluminum alloys anodize best?
5xxx and 6xxx alloys with low copper anodize to clear, bright finishes; 6063-T5 or T6 is the classic architectural choice. High-copper alloys such as 2024 and 7075 anodize dark and unevenly.