Chromic
Chromic acid, Type I anodize coatings are frequently preferred if there is a possibility of solution entrapment in joints, recesses, and weldments because the chromic acid leaves no corrosive residue.
. The chromic acid is inhibitive to aluminum and, thus, is especially good for coating aluminum castings. It is often used on parts with complex geometries and exceptionally tight tolerancees. Chromic acid anodize is compatible with most alloys and is widely used in aerospace and defense.
Chromic acid-produced films are dense, very thin (0.02 to 0.05 mil), and light grey in color, and have excellent corrosion resistance. The overall thickness of the coating formed is 50 percent penetration in the substrate and 50 percent growth over the original dimension of the part. Chromic acid films are typically harder than sulfuric acid films of the same thickness.
