I am looking for specific information regarding Type IIB anodizing per MIL-A-8625. I’ve read that it is a low coating weight, non-chrome replacement for Type I (chromic acid) anodizing. What is the recommended bath chemistry, current density and temperature? Is this process obtainable from a standard sulfuric acid bath? R.A.
Type IIB anodic coatings are produced in the standard Type II sulfuric acid bath and under virtually the same processing conditions as Type II. It is simply a thinner anodic coating that provides better corrosion resistance than Chem Film (Chromate Conversion Coatings), is a non-chrome finish and provides a very adequate base for organic coatings (painted finishes). When used as a paint base all rinsing after anodizing, as well as the sealing of the parts, must be done in surfactant-free solutions. Surfactants are wetting agents and if they are contained in the sealing chemistry will not provide as good a paint base as that provided by sealing in a surfactant-free sealing bath. Deionized water rinsing should be used before and after sealing. Sometimes Type IIB anodizing is specified where fatigue strength is important. The thinner coatings specified by this process give the base aluminum greater fatigue resistance than thicker anodic coatings on the same substrate.
I would suggest anodizing conditions of 70°F, approximately 12 asf. Bath concentration would typically be in the range of 165 g/liter to 275 g/liter (15% to 25% H2SO4 by weight). Coating thickness could range from 1 to 10 microns (0.00004 to 0.00025 inch). Boric-sulfuric falls in the category of Type IC.