Chromic Anodizing

Chromic anodize, commonly referred to as Type 1 anodizing, is formed by using an electrolytic solution of chromic acid that is about 100° F and a density of 1.5 to 4.5 A/ft2. The process will run for 40 to 60 minutes. This will produce a clear to gray coating, depending on sealing and alloy used, that is about 2 µm. One third of the coating thickness will build up per surface and 2/3 will be penetration.

Advantages

Chromic anodize, like the Type I and Type II coatings, offers a minimum of 336 hours 5% salt spray resistance per ASTM B117. It is not as durable as Type II or Type II, and does not accept dyes as well as Type II.

Due to the low thickness, it can be an advantage of parts with tight tolerances. Also, since chromic acid is less aggressive towards aluminum than the sulfuric acid used in Type II and Type III coatings, it should be used in parts that are difficult to rinse such as welded and riveted assemblies.

Specifications

There are many 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 I designation to specify a chromic anodize, and Class I to indicate natural color or Class 2 to indicate a dye.

Materials

Most alloys are suitable for chromic anodizing. Exceptions would be high-silicone die-cast alloys and high-copper alloys. Basically, an alloy with more than 5% Cu, 7% Si, or 7.5% of alloying elements should not be used.