Anodizing is an electrochemical process that converts the surface of a metal, most commonly aluminum, into a protective, durable oxide layer. Unlike a painted or plated finish, this layer is integrated with the underlying metal substrate. Understanding the thickness this conversion adds to a part’s dimensions is important for manufacturing, especially for components with tight tolerances. This dimensional change is not uniform across all anodizing types and depends on the specific chemical process used.
The Chemical Process and Dimensional Buildup
Anodizing is a conversion of existing aluminum into aluminum oxide, not a deposited coating. The process involves submerging the aluminum part into an acid electrolyte bath and passing an electrical current through it, with the aluminum acting as the anode. Oxygen ions react with the aluminum surface to form a hard, porous aluminum oxide layer.
The final oxide layer is thicker than the aluminum metal it replaces because aluminum oxide occupies a greater volume than the aluminum it originates from. Understanding dimensional change relies on the ratio of penetration versus buildup.
In standard Type II sulfuric acid anodizing, the oxide layer typically penetrates into the original surface and builds up outward in approximately equal proportions, often cited as a 50/50 split. A total coating thickness of 20 microns results in about 10 microns of the original metal being consumed and 10 microns building up on the surface. For a rod, this outward buildup occurs on all exposed surfaces, meaning the total diameter increases by twice the single-surface buildup.
Some processes, such as Type I chromic acid anodizing, or certain Type II applications, can have different ratios. For Type II, some processes result in approximately one-third of the total thickness building up on the surface, with the remaining two-thirds penetrating into the substrate. This ratio difference is a consideration for parts requiring high precision, as dimensional growth is determined only by the outward portion of the coating.
Typical Thickness Specifications by Anodizing Class
The thickness added depends on the class of anodizing performed, which is defined by the military specification MIL-A-8625. These classes dictate the electrolyte used and the intended application, ranging from decorative to high-wear resistance. Thickness is commonly measured in microns (µm) or in mils (0.001 inches).
The thinnest layer is Chromic Acid Anodizing, designated as Type I. This process uses a chromic acid electrolyte and is preferred for parts subject to fatigue, such as those in aerospace, because it creates a thin, dense film that minimally affects the part’s strength. Type I coatings usually range from 0.5 to 5 microns (0.00002 to 0.0002 inches) in total thickness.
The most common process is Standard Architectural or Decorative Anodizing, known as Type II, which utilizes a sulfuric acid bath. Type II provides good corrosion resistance and is highly receptive to dyes, making it popular for consumer and architectural products. Type II coating thicknesses are moderate, typically falling in the range of 5 to 25 microns (0.0002 to 0.001 inches).
Type III, often called Hardcoat Anodizing, is designed for demanding industrial applications requiring superior wear resistance and hardness. This process uses a chilled sulfuric acid bath and higher voltages to produce a significantly thicker, denser oxide layer. Hardcoat thicknesses generally start at 25 microns and range up to 100 microns (0.001 to 0.004 inches) or more for specialized applications.
Measurement and Quality Control of Coating Depth
Verifying the coating thickness is necessary for quality control to ensure the part meets specifications. Since anodizing is non-conductive, the most common non-destructive method uses an eddy current probe. This instrument generates an alternating magnetic field that induces circulating currents, known as eddy currents, in the conductive aluminum substrate beneath the coating.
The probe measures the distance between the sensor and the conductive base metal, which correlates directly to the thickness of the oxide layer. Eddy current gauges are fast and accurate, making them the preferred tool for routine production checks. These devices are calibrated using shims of known thickness on an uncoated sample of the material being tested.
While eddy current is the standard non-destructive test, destructive testing is sometimes used for validation or failure analysis. A destructive method involves cross-sectioning the part, embedding a small sample in epoxy, and polishing it to reveal the layer under a microscope. The thickness is then measured visually against a calibrated scale, which provides a definitive, but time-consuming, measurement.