Sulphuric anodising, is formed by using an electrolytic solution of sulphuric acid at room temperature and a current density of 15 to 22 Amps per square foot.The process will run for 30 to 60 minutes depending on the alloy used. This will produce a generally clear coating, depending on sealing, a minimum of 8um thick. One third of the coating thickness will build up per surface and 2/3 will penetration.
Sulphuric anodising provides for several desirable qualities such as.
Sulphiric anodising coatings are often sealed to enhance corrosion resistance, lock in dyes, or both. Hot water seals produce the clearest sulphuric anodise while sodium dichromate yields a yellow-green appearance but generally a better seal.
There are hundreds of governmental, industrial, and commerical anodising specifications in use, each with their own method of calling out coatings, seals, dyes, etc.
Sulphuric anodising is rather tolerant to aluminum alloys with the exception of high-silicon die-cast alloys. The less alloying elements there are the higher the clarity and depth of the colour of the anodise coating.