Hobbies And Interests
Cleaning &Rinsing
Before being treated, the aluminum is cleaned in a heated detergent mixture. This removes oils and other contaminants such as dust, dirt and lubricants. After cleaning and also after each succeeding step in the process, the metal is rinsed repeatedly, often with de-ionized water. This rinsing stops the progress of the previous reaction and removes any contaminants left from it.
Etching, Deoxidizing &Desmutting
The aluminum is then etched with caustic soda (sodium hydroxide, NaOH) or other strong base. This dissolves the surface of the metal to produce a uniform, matte finish. Following a rinsing cycle, the metal is treated in an acidic solution to remove smut. Smut is a term used for the combination of other metals, metal alloys or metal oxides that adhere to the surface.
Anodizing
After another rinsing, the aluminum is immersed in an acid solution. A negative electrical terminal (cathode) is placed in the solution, and the aluminum is connected to a positive electrical terminal (anode). The electrical current flowing through the electrolyte produces negative oxygen ions, which flow to the positively charged aluminum and produce the oxide film.
The most common acids used in the electrolyte are sulfuric and chromic. Chromic acid is less corrosive than sulfuric and is therefore easier to rinse from complex parts. It also has less negative impact on the aluminum. A sulfuric acid electrolyte produces a thicker, more durable coating that also produces deep, rich colors from dying. A sulfuric electrolyte with a lower temperature, called hard-coat anodizing, will produce an even more durable coating.
Coloring
Following the rinsing cycle, color is applied to the metal, either by absorptive dying or by electrolytic coloring; the film produced is ideal for either process. Electrolytic coloring is achieved by electrolysis of another metal like cobalt or nickel with the aluminum. The aluminum oxide has depressions or pores, and the second metal is deposited into those pores, producing the color. The pores in the oxide are also ideal for the deposition of dyes through absorptive dying.
Sealing
The porous nature of the aluminum oxide that is so beneficial for coloring also makes it susceptible to stains and corrosion. The final step in the anodizing process is sealing, in which the pores in the oxide are sealed by treatment in chemical baths or capped by the precipitation of metal salts.