Does anodizing cause dimensional growth?

Anodizing is the process of electrochemically converting the surface of an aluminum part to aluminum oxide. Aluminum oxide occupies about two times the volume as that of raw aluminum. Therefore, anodizing will cause parts to grow dimensionally. This factor should be taken into consideration when designing parts that will be anodized.

Typical standard clear and color anodizing creates an aluminum oxide film in the range of .0002 to .0008 inches, (.005 to .020 mm), on each surface. Hard anodizing is typically in the range of .0005 to .003 inches, (.013 to .076 mm), the most common being .002 inches. (.051 mm).

The process of hard anodizing a part to .002 in. film thickness will therefore grow .001 in. on each surface or .002 in. in overall dimension.

What alloys are preferred in anodizing?

Higher purity alloys are always preferred for anodizing. Alloying elements such as copper and silicon do not anodize and leave microscopic voids in the aluminum oxide film. Since the anodizing process converts only the aluminum to aluminum oxide to form the anodized finish, higher purity aluminum will yield a denser and harder layer of aluminum oxide. High concentrations of some alloying elements will also affect the surface finish and color of the anodized finish and will reduce the effectiveness of the sealing process causing reduced corrosion and wear resistance and decreasing fade resistance in dyed parts.

The most popular alloys used for anodizing are 5000, 6000 and 7000 series alloys. These alloys will provide consistently excellent quality finishes for hardcoat and conventional anodizing. High purity alloys like 1100 and 3000 series will also form very good finishes. Although 2000 series alloys are popular alloys because of their strength and machining characteristics ithey are not the best choice for good anodized finishes because of their high copper content.  It has all of the disadvantages noted in the above paragraph. Alpha Metal Finishing Co. regularly provides good anodized finishes for parts made from 2000 series alloys, however a superior anodized finish will be obtained with higher purity alloys.