Sulfuric
The sulfuric acid process is the most frequently used anodize process.
The sulfuric acid process produces films of greater thickness (0.1 to 1.0 mil) and is particularly suited for applications where hardness and some resistance to abrasion is required. However, where parts are subjected to considerable stress, such as aircraft parts, the possible presence of the corrosive acid residue is undesirable. The overall thickness of the coating formed is 67 percent penetration in the substrate and 33 percent growth over the original dimension of the part. Surfaces treated with sulfuric acid anodize have greater durability than surfaces treated with chromic acid anodize.
The porous nature of sulfuric acid films prior to sealing is used to particular advantage in the production of colored surface finishes on aluminum and its alloys. The porous aluminum oxide absorbs dyes well, and subsequent sealing helps to prevent color loss in service. Although dyed anodized films are reasonably color-fast, they have a tendency to bleach under prolonged direct sunlight.
