Info is from www.tricktank.com
Annodizing is an electro-chemical process that converts the surface of raw aluminum to an artificial oxide coating. The process uses an electrolyte, or acid, which conducts electricity, similar to an automobile battery. The acid, along with the electrical charge, causes and sustains the annodize process on the surface of the aluminum.
Annodizing isn't a plating process because, unlike plating, anodizing doesn't deposit any material onto the metal surface. Annodizing is actually a complex series of steps involving pre-cleaning, etching, deoxidizing, anodizing, dying and sealing with careful rinsing after each immersion. Decades of experience have established the exactly correct standards of immersion times, solution temperatures, chemical concentrations and mixtures and electrical conditions.
These standards can achieve a wide range of surface finishes from very bright to dull matte in one or more of a variety of brilliant, subtle or rich colors. Anodizing is also performed to either enhance abrasion resistance, corrosion resistance or to reduce surface electrical conductivity.
Immersion in organic dye solutions can achieve color finishes in annodized but unsealed parts. This is possible because the annodize coating has microscopic pores that, depending on variables already described, allow aluminum to absorb dyes through immersion. Typically, the darker the color desired, the longer the annodize time is required.
After dye immersion parts must be sealed. Sealing with nickel acetate seal literally closes the pores of the annodize coating, preventing the dye from leaching and the anodic film from deteriorating