The Anodizing Process...

Anodizing is a process for finishing aluminum alloys that employs electrolytic oxidation of the aluminum surface to produce a protective oxide coating.

The typical process includes cleaning, pretreating, anodizing, coloring (optional), and sealing.

  • Cleaning - Generally accomplished by soaking the aluminum for several minutes in a water-based solution containing mild acids or alkalies along with dispersants and detergents to remove oils or compounds.
  • Pretreatment - The most common pretreatment is etching the aluminum by which the material is dipped in a caustic soda (sodium hydroxide).  The etch imparts a satin or matte finish by dissolving aluminum from the surface in a microscopically irregular pattern.  After etching, the material is dipped in an acidic desmut/deoxidizer solution to remove any residue of alloying agents or oxides.
  • Anodizing - This part of the process actually produces the anodic coating.  It is accomplished in an electrolytic cell using sulfuric acid as the electrolyte.  The aluminum is made the positive electrode and when a direct current is passed through it, water is decomposed, liberating oxygen at the surface of the metal.  The oxygen combines with the aluminum to form the coating (a transparent and microscopically porous layer of aluminum oxide).  The thickness of the coating is determined by the amount of electrical current and the length of time the aluminum is charged.
  • Coloring (optional) - There are two possible processes - integral or two-step.  Integral coloring is accomplished by imparting color to the coating as it is being formed in the anodizing bath.  Two-step coloring is accomplished by imparting coloring to the porous coating after it is formed (absorbed).
  • Sealing - The micro pores of the anodized coating must be sealed to prevent unwanted stains later on.  Sealing is accomplished by dipping the aluminum in a hot water solution of metal salts.
Classification of anodic coatings:

Class I - High performance anodic finishes used in exterior applications receiving periodic maintenance such as curtain walls, store fronts, and commercial windows.  Minimum coating thickness of 0.7 mil (18 microns).

Class II - Commercial anodic coatings used in interior applications or exterior applications receiving regularly scheduled cleaning and maintenance.  Minimum coating thickness of 0.4 mil (10 microns).