Corrosion protection
The oxide film isolates the metal from moisture, salts and acids.
Technology
What aluminium anodizing is
Anodizing, or anodic oxidation, is an electrochemical process that builds a protective oxide film on aluminium and its alloys. The part is made the anode in an electrolytic tank, and under applied current a layer of aluminium oxide (Al₂O₃) forms on the surface, integrally bonded to the base metal. Where the naturally occurring oxide film is 2–10 nm thick, an anodic coating runs from a few microns to several hundred, set by the process regime.
Properties of anodic coatings
Wear resistance
Hard anodizing raises surface hardness to 500–600 HV.
Decorative finish
The porous structure of the film allows it to be coloured across a wide range.
Electrical insulation
The oxide layer is a capable dielectric, with breakdown voltage up to 1,000 V.
Adhesion
An excellent base for painting, bonding and subsequent coatings.
Types of anodic oxidation
The classification follows the electrolyte, the process regime and what the coating has to do.
By electrolyte
Sulphuric acid
The most widely used route: a 5–25 µm porous film that takes colour well.
Oxalic acid
Golden and bronze tones, and a harder film than the sulphuric route gives.
Chromic acid
A thin 1–5 µm film, specified for parts of complex geometry.
Phosphoric acid
Used as a surface preparation ahead of adhesive bonding.
Mixed electrolytes
Combined coating properties tuned to a specific requirement.
By purpose and layer thickness
Thin film
Special applications: bonding pre-treatment and chromic acid anodizing.
Protective-decorative
A clear or coloured film for visible surfaces.
Hard
A low-temperature process delivering high hardness and wear resistance.
By colouring method
Clear (natural)
Keeps the metal's natural silver-matt appearance.
Adsorptive (organic)
The pores of the finished film are filled with organic dyes.
Electrolytic
After anodizing the part is treated in a bath of metal salts - tin, nickel or cobalt.
Interference
A variant of the electrolytic method: optical blues, greens and reds with no pigment at all.
By finishing (sealing)
Sealed
Mandatory for corrosion protection: the pores are closed with hot water, steam or salt solutions.
Unsealed
Deliberately left porous - for deep dyeing, or for impregnation with lubricants to reduce friction.
Why specify an anodic coating
Durability
Longer service life for anodized structures
Environmental
No heavy metals; the coating is chemically inert
Hardness
Hard anodizing raises surface hardness to 500–600 HV
Dimensional control
Precise control over the finished part geometry
Electrical insulation
Breakdown voltage up to 1,000 V
UV stability
Does not fade in sunlight once the dye is sealed in
Cannot peel
The anodic film is an oxide of the metal itself, so delamination is not possible
Colourable
Pores are filled with pigment before sealing
Where anodizing is used
Architecture & Construction
External cladding, door and window profiles, roofing and fixings. Protection from the environment together with the finish the design calls for.
Automotive, Marine & Machinery
Body components, engine parts and cooling radiators. In marine work the coating is what protects aluminium alloys against seawater.
Aerospace
Lightweight structural elements of the airframe, landing-gear assemblies and internal units - anywhere strength at minimum weight decides the design.
Electronics & Instrumentation
Phone and laptop housings, heat sinks and reflectors. The oxide film doubles as a reliable dielectric.
Consumer Products
Cookware, sports equipment, furniture hardware and interior fittings.
Questions and answers
Anodizing is an electrochemical process that grows an oxide layer on aluminium. The part is made the anode in an electrolyte tank, and under DC current a layer of Al₂O₃ forms on the surface, integrally bonded to the base metal. The coating is harder and far more corrosion-resistant than the metal beneath it, and can be coloured before sealing.
Need a line for your process?
Take a look at the anodizing lines or send us your specification.