Anodizing Thickness Guide: What's Ideal?

Anodizing is the process of adding an oxide layer over the workpiece to enhance its properties.

But how thick is an Anodizing layer? Does it significantly affect the part dimensions?

Anodizing thickness primarily depends upon the type of anodizing being performed. While chromic anodizing can have a thickness of less than 5μm, type II anodizing can have a thickness of around 10-25 μm. For extreme applications, hard anodizing with a coating thickness of up to 25-100 μm is preferred.

This article discusses anodizing thickness in detail and provides insights on methods to measure the anodizing thickness.

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Anodizing Thickness of Every Type of Anodizing

Type of Anodizing	Thickness Range (μm)
Type I anodizing / Chromic anodizing	under 5 μm
Type II anodizing / Decorative anodizing	10-25 μm
Type III anodizing / Hard anodizing	25-100 μm
Composite anodizing	100-250 μm

When compared to powder coating, anodizing involves fusing the coating with the base metal. As a result, the thickness of the coating is partially inside the base metal surface and partially above it.

There are different types of anodizing, and the standard thickness of the anodized coating can vary depending on the specific type of anodizing being used.

In general, the thickness of an anodized coating is measured in micrometers (μm).

Type I anodizing or chromic anodizing has the least coating thickness of less than 5 μm.

Type II anodizing, also known as decorative anodizing, generally has a coating thickness of 10-25 μm.

This type of anodizing is used to improve the appearance of the material, with thicker coatings tending to be more durable and resistant to wear.

Hard anodizing, also known as type III anodizing, has a coating thickness of around 25-100 μm.

This type of anodizing improves the durability and abrasion resistance of the treated material, with thicker coatings providing better resistance to wear and tear.

Type IV anodizing, also known as "composite anodizing", typically has a thickness of around 100-250 μm.

Being thicker than type III anodizing, this type of anodizing is suitable for applications where extreme wear resistance is desirable.

How to Measure Anodizing Thickness?

Different methods are used to measure the exact thickness of an anodized coating.

The best suitable method depends on the anodized part's specific characteristics and the accuracy required.

1. Caliper Measurements 

This method involves using a caliper to measure the thickness of the anodized coating at various points on the part's surface. 

To calculate the thickness of the anodized coating, you have to record the dimensions of the part before coating and then take similar dimensions after coating.

The difference between the two records will give you the thickness of the anodized coating.

This is a relatively simple and inexpensive method; it is comparatively less accurate and is, therefore, suitable for rough estimates.

2. Ultrasonic Thickness Gauges

This device uses the ultrasonic wave to calculate the exact thickness of the anodized coating.

They are relatively accurate and can measure the thickness of anodized coatings on both flat and curved surfaces.

3. Optical Profilometers

Optical profilometers or optical comparators use optics to calculate the thickness of the anodized coating.

They are highly accurate and can measure the thickness of anodized coatings on flat and curved surfaces.

However, it requires costly equipment, making it ideal for industrial applications where accurate anodizing with tight dimensional tolerance is required.

4. Magnetic thickness gauges

These devices use the principle of magnetic induction to measure the thickness of the anodized coating.

They are relatively accurate and can be used to measure the thickness of anodized coatings on ferromagnetic materials.

This means these devices cannot be used to measure the anodizing thickness over non-magnetic metals like aluminum, gold, brass, etc.

5. X-ray fluorescence (XRF) spectroscopy

X-ray fluorescence spectroscopy involves using an X-ray source to generate X-rays, which are then absorbed by the anodized coating.

The absorbed X-rays cause the atoms in the coating to fluoresce, and the fluorescence intensity is proportional to the thickness of the coating.

This method is highly accurate but relatively expensive and requires specialized equipment.

How Does Anodizing Affect Part Dimensions?

The anodized layer's thickness can affect a part's dimensions in several ways.

Anodizing adds a thin layer of material to the part's surface, which can slightly increase its overall size.

Apart from that, there are several factors that can affect the dimensional change of a part during anodizing.

The Thickness of the Anodized Coating

The thickness of the anodized coating can have a significant impact on the dimensions of the part being anodized, both internally and externally.

Generally, the thicker the coating, the more significant the dimensional change that can be expected.

Externally, the coating will add a layer of material to the surface of the part, which can cause the overall dimensions to increase.

This is especially important for parts with tight tolerances, as the added thickness of the coating could cause them to fall outside of the specified tolerance range.

On the other hand, coating internal features such as holes can result in reducing the diameter of the hole by adding a layer over the base metal.

It is generally advised to factor in the coating thickness when providing the tolerance during the designing and manufacturing of the part.

This allows for some flexibility in the final dimensions of the anodized part and can help ensure that it meets the required tolerances.

Effect of Material Properties on Part Dimension

Different metals and alloys can have different coefficients of thermal expansion, which can affect how much a part expands or contracts during the anodizing process.

For example, aluminum tends to have a higher coefficient of thermal expansion than steel, so it may experience greater dimensional change during anodizing.

Effect of Anodizing Process Parameters on Part Dimension

The specific process parameters used during anodizing, such as the current density, temperature, and duration of the process, can all affect the dimensional change of a part.

The temperature and duration will depend on the specific type of anodizing performed and the material being anodized.

Type of Anodizing	Temperature (°C)	Duration (min)
Type II (sulfuric acid)	20-25	60-120
Type III (hard anodizing)	12-25	60-180
Chromic acid	20-25	30-60
Titanium anodizing	15-25	20-60

Varying the temperature and/or duration beyond the recommended value can alter the thickness of the anodizing coating, resulting in poor dimensional accuracy.

Therefore, it is important to ensure good process control to produce parts within the allocated dimensional tolerance.

Apart from that, it is also advised to perform a test run to identify the effect of anodizing parameters on the material before anodizing the actual part.

Effect of Post-processing on Part Dimension

Post-processing steps, such as sealing or coloring, can also affect the dimensional change of a part during anodizing.

Apart from that, certain applications also require painting the anodized part to achieve the required aesthetics.

Although not significantly thick, these processes add to the overall thickness of anodizing layer making it important to factor in the thickness of the sealant coat to estimate the final dimensions of the part.

Factors that affect Anodizing Thickness

1. Type of Anodizing

The type of anodizing required for the application plays an important role in determining the optimal thickness of the anodizing.

Decorative Anodizing

This type of anodizing is used to improve the appearance of the treated material. The anodized layer is thin (10-25 μm) and has a porous surface that can be dyed to produce a wide range of colors. 

Decorative anodizing is commonly used to improve the appearance of aluminum products such as wheels, architectural components, and consumer goods.

Due to its relatively poor corrosion resistance properties, it is best suited for enhancing the aesthetic value of metals that don't rust easily.

Hard Anodizing

This type of anodizing is used to improve the durability and resistance of the material.

The anodized layer is thicker (25-100 μm) and has a hard, smooth surface resistant to wear and corrosion.

Hard anodizing is commonly used to improve the performance of components in high-wear applications such as aircraft, military equipment, and automotive parts.

Composite Anodizing

This type of anodizing is similar to hard anodizing, but the anodized layer is thicker (100-250 μm) and has a higher level of wear resistance.

Composite anodizing is used in applications with extremely high levels of wear resistance, such as in manufacturing rocket nozzles and other aerospace components.

Chromic Acid Anodizing

This type of anodizing is used to produce a thin (1-2 μm) anodized layer that is highly corrosion-resistant.

Chromic acid anodizing is commonly used in the aerospace and defense industries, as well as in the production of medical implants.

Sulfuric Acid Anodizing

This type of anodizing is similar to decorative anodizing but uses sulfuric acid instead of oxalic acid as the electrolyte. 

Sulfuric acid anodizing produces a porous anodized layer that can be dyed to produce a wide range of colors. It is commonly used to produce appliances and sporting materials.

In general, sulfuric acid anodized coatings are typically 10-15 µm thick, although the actual thickness may vary depending on the application's specific process conditions and requirements.

Sulfuric acid anodizing is often used for its anti-corrosion properties and provides additional durability to the metal surface.

2. Durability Required

A thick anodized coating can provide better durability than a thin coating.

For example, a hard anodized coating, produced using a more stringent process, can provide more excellent durability than a decorative anodizing coating, primarily used for aesthetic purposes.

Hard anodized coatings are typically thicker than decorative coatings, as the increased thickness of the oxide layer contributes to their enhanced durability.

The environmental conditions to which the coated parts will be exposed can also affect the required durability of the anodized coating. 

For example, a thick anodizing coating with excellent corrosion resistance is desirable for a part that is to be subjected to harsh environments such as saltwater or high humidity.

Therefore, when selecting an anodizing process, it is important to consider the durability required for the application and choose a process appropriate for the intended use.

3. Type of Base Metal

Anodizing is typically used to improve the surface properties of nonferrous metals, such as aluminum, copper, bronze, gold, titanium, and magnesium.

The Purity and Composition of the Metal

The purity and composition of the metal can affect the anodizing process and the properties of the anodized layer. 

For example, anodizing impure aluminum may be more challenging as the impurities present in the base metal prevent the formation of the oxide layer.

As a result, a thicker anodizing layer is required to enhance the durability of the anodized surface.

The surface condition of the metal

Surface treatment is an important step in the anodizing process. This is because a rough or uneven surface will produce a rough and uneven anodizing layer with a poor surface finish.

As a result, multiple layers of anodizing will be required to produce a smooth layer, above the rough texture.

This increases the thickness of the anodizing layer, making it less economical and more prone to chipping.

Therefore, it is advised to ensure proper surface treatment before anodizing the workpiece.

4. Anodizing Color Required

The choice of color for an anodized coating affects the thickness of the coating to some extent.

This is because the addition of dyes to the anodizing solution can affect the porosity of the oxide layer, which in turn influences the thickness of the coating.

For example, using a more concentrated dye solution or applying multiple dye baths can result in a thicker oxide layer and a deeper, more intense color.

It is also worth noting that the thickness of the anodized coating can be affected by other factors, such as the type and concentration of the anodizing solution, the voltage and current applied during the anodizing process, and the temperature and pH of the solution.

The type of metal being anodized and its surface condition and geometry can also influence the thickness of the coating.

5. Cost of Anodizing

The cost of anodizing is directly proportional to the thickness of the coating. Thicker coatings require more material and, therefore, can be more expensive.

Additionally, applying a thicker coating may require more time and labor, contributing to the overall cost. It is necessary to limit the thickness of the coating to keep costs lower.

It is worth noting that the thickness of the coating can also affect the performance and durability of the anodized material, so it is essential to consider the trade-offs between cost and the desired properties of the finished product.

Final Thoughts

Anodizing thickness plays an important role in determining the durability and surface finish of the coating but also affects the cost of the process.

Generally, a thin anodizing coating is preferable for applications where abrasion resistance is not required.

Whereas for applications where good abrasion resistance is the priority, it is advised to apply a thick anodizing coating which enhances the durability of the part.

However, the thickness of the coating directly affects the cost, making it important to consider your desired application to limit the thickness of the coating, regulating the cost.

Frequently Asked Questions (FAQ)

Can anodizing thickness be increased after the initial process?

No, it is not possible to increase the anodizing thickness after the initial process. Once the anodizing process is complete, the oxide layer will have reached its final thickness.

What are the benefits of anodizing?

The main benefits of anodizing include improved corrosion resistance, improved wear resistance, improved resistance to high temperatures, improved resistance to UV light, improved aesthetic appearance, and improved bonding ability with adhesives.

What are the limitations of anodizing?

Some of the limitations of anodizing include its high cost, time-consuming process, and limited application to metals that facilitate the formation of an oxide layer.