What Is Anodizing?

Anodizing is an electrochemical process that enhances the surface characteristics of certain metals, particularly aluminum. During the anodizing process, a metal part is submerged in an acid electrolyte bath and subjected to an electric current. This causes oxygen ions from the electrolyte to combine with metal atoms at the surface of the part, forming a layer of oxide.

Unlike paint or plating, anodizing does not add a separate coating on top of the metal. Instead, it transforms the surface itself into a durable, corrosion-resistant, and often porous oxide layer. This layer can then be dyed or sealed for added aesthetic or functional benefits.

Aluminum is the most commonly anodized material due to its naturally occurring oxide layer and widespread use across industries such as aerospace, automotive, architecture, and electronics. However, other metals such as magnesium and titanium can also be anodized under certain conditions.

The primary reasons for anodizing include:

• Enhancing corrosion resistance
• Improving surface hardness and wear resistance
• Enabling decorative finishes and color dyes
• Increasing electrical insulation
• Providing a better base for paint adhesion

Understanding how anodizing affects the dimensions of a component is critical for industries that require precision machining and tight tolerances.

How Anodizing Affects Thickness – Build-Up vs Penetration

One of the most frequently asked questions regarding anodizing is whether it adds thickness to a part. The answer is both yes and no, depending on how you interpret the process.

Anodizing does not apply a separate layer like electroplating or powder coating. Instead, it modifies the outer surface of the metal by converting a portion of it into aluminum oxide. This transformation results in a change in the part’s dimensions.

Approximately one-third of the anodic layer penetrates into the base metal, while two-thirds builds up on the surface. This ratio can vary depending on the anodizing method and alloy, but it provides a useful rule of thumb for estimating dimensional change.

For example, if the total thickness of the anodized layer is 15 microns, then about 10 microns may be added to the part’s outer surface, and 5 microns will replace the original metal beneath. From a design perspective, this means the external dimensions of the part will increase by roughly two-thirds of the total anodized thickness.

To illustrate this concept:

• Total oxide layer thickness: 15 microns
• Build-up: ~10 microns (adds to dimension)
• Penetration: ~5 microns (replaces substrate)

Therefore, anodizing does change the physical size of the part, although it is a surface transformation, not an additive coating. This dimensional change must be accounted for, particularly in applications where parts fit together with tight tolerances.

Typical Thickness Values for Different Anodizing Types

The amount of thickness added by anodizing depends on the type of anodizing used. There are three primary classifications based on U.S. military specification MIL-A-8625: Type I, Type II, and Type III. Each type produces a different oxide thickness and is suitable for different applications.

Type I – Chromic Acid Anodizing:
This method produces the thinnest anodic layers, typically between 0.5 to 2.5 microns in total thickness. It offers good corrosion resistance with minimal impact on dimensions, making it ideal for aerospace components or parts with tight tolerance requirements. The build-up on the surface is negligible.

Type II – Sulfuric Acid Anodizing:
This is the most commonly used type of anodizing. It creates oxide layers between 5 to 25 microns thick. Sulfuric acid anodizing is suitable for decorative finishes, dyed colors, and general corrosion protection. Designers often allow for dimensional growth of about 5 to 15 microns on each surface.

Type III – Hardcoat (Hard Anodizing):
Used in applications requiring maximum wear resistance and durability, Type III anodizing can produce oxide layers up to 150 microns thick. Hard anodizing not only increases surface hardness significantly but also adds measurable thickness to the part. The build-up can be substantial—up to 75 microns per side—requiring careful machining allowances in design and manufacturing.

The following table summarizes the general thickness ranges:

Anodizing Type	Process	Typical Thickness (microns)	Approx. Build-Up (microns)
Type I	Chromic	0.5 – 2.5	<1
Type II	Sulfuric	5 – 25	3 – 17
Type III	Hardcoat	25 – 150	17 – 100

These values can vary based on aluminum alloy, process duration, electrolyte concentration, and temperature control. Therefore, it’s important to consult with your anodizing service provider to get precise data for your specific application.

Dimensional Considerations in Design and Manufacturing

When designing parts that will be anodized, it is essential to factor in the dimensional changes caused by the anodizing layer. This is particularly true for components that must fit tightly with others, such as precision housings, sliding surfaces, or mechanical assemblies.

One common mistake is neglecting to allow for anodizing build-up, which can cause interference fits, difficulty in assembly, or improper function. Conversely, overcompensating by reducing dimensions too much can result in loose fits or insufficient material strength.

Here are a few design guidelines to help manage these changes:

1. Adjust External Dimensions for Build-Up: For surfaces that will be anodized externally, subtract approximately two-thirds of the total expected coating thickness from the machined dimensions to maintain final fit after anodizing.
2. Maintain Critical Dimensions Before Anodizing: Internal features such as threaded holes, bores, or grooves should be undersized during machining if anodizing is expected to increase their diameter. Alternatively, these features can be masked before anodizing to preserve original dimensions.
3. Use Clear Communication in Drawings: Engineering drawings should clearly indicate which surfaces are to be anodized, the type of anodizing, and the required thickness. Noting whether dimensions are before or after anodizing is also crucial.
4. Consider Post-Anodizing Machining: In some high-precision applications, a final machining step may be performed after anodizing to restore tight tolerances. However, this is only feasible when the oxide layer is not too hard or thick, as in Type II anodizing.
5. Verify with Inspection Tools: After anodizing, dimensional verification using tools like micrometers or 3D scanners can help ensure compliance with tolerance requirements. In high-volume production, process control is vital to ensure consistent build-up.

Understanding the interplay between anodizing thickness and part geometry helps avoid costly redesigns, reworks, or assembly issues.

Measuring Anodized Coating Thickness

Accurate measurement of anodized coating thickness is essential for quality control, especially in industries with tight tolerance requirements. Several methods are used to evaluate the thickness of the anodic layer:

1. Eddy Current Testing
   This non-destructive method uses electromagnetic induction to measure the thickness of non-conductive coatings (like anodic oxide) on conductive substrates. It is fast, portable, and commonly used in production environments.
2. Cross-Sectional Microscopy
   This method involves cutting a section of the anodized part, polishing it, and examining it under a microscope to measure the oxide layer directly. While highly accurate, it is destructive and typically used for sample testing in quality labs.
3. Weight Gain Method
   By comparing the weight of a part before and after anodizing, and factoring in surface area and coating density, the thickness can be estimated. However, this is less precise and more appropriate for comparative analysis than precise measurement.
4. Scanning Electron Microscopy (SEM)
   In high-end applications such as aerospace or microelectronics, SEM may be used to examine the oxide structure and confirm uniformity and thickness at a microstructural level.

Regardless of the method used, verifying that the actual anodized thickness meets design and functional requirements helps ensure reliability and long-term performance of the component.

When Does Anodizing Thickness Matter Most?

While some applications tolerate moderate variation in anodizing thickness, others demand strict control due to functional, aesthetic, or safety reasons. Here are a few scenarios where anodizing thickness is critical:

• Aerospace Components
  Aircraft parts, especially structural or fatigue-sensitive components, must maintain exact tolerances. Anodizing adds protection without significantly increasing weight, but build-up must be consistent and controlled.
• Medical Device Enclosures
  Devices like surgical instruments or implantable housings may use anodized aluminum for biocompatibility and corrosion resistance. Any deviation in surface thickness could affect device assembly or safety.
• Electronics and Precision Housings
  Anodized aluminum is commonly used in laptop bodies, camera housings, and sensors. The anodizing process must not compromise the fit of internal electronics or exterior seams.
• Sliding or Rotating Components
  Parts that interface with others through sliding or rotation, such as robotic arms or pneumatic pistons, require smooth anodized layers of uniform thickness to avoid wear or jamming.
• Architectural Assemblies
  For applications like curtain wall systems, handrails, or aluminum cladding, anodized finishes must align visually and dimensionally across multiple pieces, especially when modular components are assembled on-site.

Understanding when anodizing thickness is a key factor allows engineers to allocate design allowances appropriately and avoid complications during assembly or field use.

How CSMFG Ensures Precision in Anodized Parts

At CSMFG, we understand that anodizing is not just a finishing process—it’s an integral part of your product’s functional and dimensional performance. Our team provides end-to-end manufacturing solutions that incorporate surface finishing, machining, and inspection into one integrated workflow.

Here’s how CSMFG supports precision anodized parts:

• Tailored Anodizing Solutions
  We offer Type II and Type III anodizing through trusted finishing partners and can match coating specifications based on your part’s intended application, color, and wear resistance needs.
• Dimensional Control Before and After Finishing
  Our engineers evaluate your CAD models and drawings to anticipate dimensional changes from anodizing. We apply pre-anodizing machining offsets and can perform post-anodizing machining if required.
• Clear Communication and Documentation
  Your anodizing requirements—whether they relate to thickness, build-up tolerance, or masking—are clearly documented in our process sheets. Final part inspections verify that coating thicknesses are within spec.
• End-to-End Manufacturing Support
  From aluminum CNC machining to anodizing, assembly, and packaging, CSMFG handles your project with precision and accountability. Whether you’re developing prototypes or scaling up for mass production, we deliver quality at every step.
• Quality Reporting
  Upon request, we provide thickness reports, material certifications, and compliance with relevant anodizing standards like MIL-A-8625, ISO 7599, or ASTM B580.

If you’re unsure how anodizing will affect your part’s final fit or performance, our technical team is ready to advise and offer design-for-manufacturing support.

Conclusion

To answer the original question: yes, anodizing does change the thickness of a part—but not by adding a separate material layer. Instead, it modifies the surface of the base metal, creating an oxide layer that both builds up on the surface and penetrates below. The actual dimensional change depends on the anodizing type and process parameters.

In many applications, especially those involving precise fits or critical tolerances, accounting for anodizing thickness during the design and machining phases is essential. Understanding the difference between build-up and penetration can help engineers make better design decisions and avoid downstream manufacturing issues.

By working with a full-service manufacturing partner like CSMFG, you can ensure that anodizing is integrated into your part’s lifecycle from design to delivery—without compromising on quality or accuracy.

FAQs

Does anodizing always increase the size of a part?

Anodizing generally increases the external dimensions of a part due to oxide layer build-up. However, part of the anodized layer also penetrates into the surface. The net dimensional increase is usually about two-thirds of the total oxide thickness.

Can anodizing be applied selectively?

Yes. Specific areas of a part can be masked prior to anodizing to prevent coating. This is useful when preserving internal threads, mating surfaces, or electrical contact points.

How much build-up occurs in hardcoat anodizing?

Hardcoat anodizing (Type III) can build up as much as 50 to 75 microns on each surface. The exact build-up depends on the alloy and process parameters. It’s important to adjust machining dimensions accordingly.

Do all aluminum alloys anodize to the same thickness?

Different aluminum alloys respond differently to anodizing. For example, 6061 and 7075 produce high-quality anodized layers, while cast aluminum may result in non-uniform coatings. Coating thickness may also vary based on alloy.

How do I specify anodizing thickness in a drawing?

Engineering drawings should indicate the anodizing type (e.g., Type II or Type III), the desired thickness range, and whether dimensions are before or after anodizing. Clearly specifying tolerance expectations helps avoid misunderstandings during production.