Sulfuric Anodizing

Sulfuric anodize, commonly referred to as Type II anodizing, is formed by using an electrolytic solution of sulfuric 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 8µm thick. One third of the coating thickness will build up per surface and 2/3 will be penetration.

Properties

Type II sulfuric anodizing provides for several desirable qualities such as:

• Corrosion Resistance (336+ hours salt spray resistance per ASTM B117)
• Moderate Durability
• Excellent Dyability (yielding deep, rich colors)
• Electrical Insulation

Sulfuric anodize coatings are often sealed to enhance corrosion resistance, lock in dyes, or both. Hot water seals produce the clearest sulfuric anodize while sodium dichromate yields a yellow-green appearance but is generally a better seal.

Specifications

There are hundreds of governmental, industrial, and commercial anodizing specifications in use, each with their own method of calling out coatings, seals, dyes, etc. Any anodizer should recognize the Type II designation to indicate a sulfuric anodize, Class 1 to specify natural color or no dye, and Class 2 to indicate a dye.

Materials

Sulfuric anodizing is rather tolerant of aluminum alloys for anodizing with the exception of high-silicon die-cast alloys such as 380. The less alloying elements there are the higher the clarity and depth of color of the anodize coating.