Everything You Need To Know About Hard Coat Anodizing

Hard coat anodizing is a process used to create an artificially thick oxide layer on the surface of a non-ferrous alloy, most commonly-aluminum based alloys. Hard coat aluminum anodizing is sometimes referred to as Type III anodizing. This type of anodizing increases the abrasion and corrosion resistance of the base metal, reduces surface conductivity, and increases surface hardness. Aluminum extrusions are often hard-coated anodized. An example of a hard coat anodized aluminum extrusion is shown in Figure 1 below:

This article will explain what hard coat anodizing is, how it works, and what the various benefits and limitations of this process are. 

What Is Hard Coat Anodizing?

Hard coat anodizing is a process used to create a relatively thick layer of hard aluminum oxide on the surface of a part to improve the surface performance of the component. Hard coat anodizing is often referred to as Type III anodizing, as described in the US military specification, MIL-PRF-8625F. This standard lays out requirements for the anodizing of aluminum parts. The typical thickness of Type III hard coat anodizing is 2 mils (51 µm). Other non-ferrous materials, such as titanium and magnesium, can also be anodized, but aluminum is more frequently protected using this technique. 

For more information, see our guide on Anodizing.

What Is the Purpose of Hard Coat Anodizing?

The purpose of hard coat anodizing is to improve the properties of an aluminum alloy by converting its surface into a layer of aluminum oxide. The properties which can benefit include: surface hardness, abrasion resistance, dielectric strength, and corrosion resistance. This is especially useful when aluminum parts are exposed to extreme operating conditions. Hard coat anodized oxide layers can also be treated with dyes for aesthetic purposes. However, other forms of anodizing, like Type II sulfuric acid anodizing, may be a better choice of process if aesthetics are important as Type II anodizing can be dyed in more colors.

Where Is Hard Coat Anodizing Used?

Hard coat anodizing is frequently used on materials such as aluminum, titanium, and magnesium, with aluminum being the most common. The process converts the surface of the material into a corrosion-resistant, non-conductive, and in some cases, decorative coating. This is why it is widely used in a variety of fields. Some common examples of where hard coat anodizing is used include: the optics, defense, architectural, and medical industries. Its ability to protect the underlying metal from corrosion and stray currents also make it ideal for the aerospace and food service industries. Examples of specific applications include: gears, piston heads, and medical instruments.

How Does Hard Coat Anodizing Work?

Hard coat anodizing (or Type III anodizing) works by creating a hard, durable, and wear-resistant surface on aluminum and its alloys via an electrochemical process. Hard coat anodizing is a multi-step process that begins with cleaning the part to remove all impurities and contaminants from the surface. Failure to do so will result in a poor-quality coating. Next, the part must be etched with an acid to remove any naturally occurring oxide, which inevitably forms when aluminum is exposed to air. This natural coating is uneven and of poor quality. The acid etch also produces a clean uniform surface for the anodizing process. 

Next, the part is connected to the positive electrode and acts as the anode when immersed in a temperature-regulated acidic electrolyte solution (typically sulfuric acid). A cathode made from an unreactive material is permanently situated in the bath and completes the electrical circuit. For hard coat anodizing, higher voltages are required when compared to Type II anodizing, i.e. 18 V vs 120 V. The anodizing cycle is initiated with an applied voltage of around 25 VDC, increasing to 120 VDC near the end of the process. As the anodic layer thickens, its resistance to current increases, which can hamper the oxide deposition process. The voltage must be increased to compensate for the decreasing conductivity of the workpiece. The thickness is controlled by the electrolyte temperature, voltage, acid concentration, and time.

What Are the Benefits of Hard Coat Anodizing?

Hard coat anodizing is widely used to treat parts for applications that require improved mechanical, corrosion, or dielectric performance. Some of the benefits are listed below:

1. Increased Durability: Hard coat anodizing significantly increases the abrasion resistance and surface hardness of an aluminum part. Surfaces hardnesses of between 400 and 600 HV (Vickers Hardness) can be obtained, close to the hardness of some tool steels.
2. Electrical Resistivity: The oxide layer formed during hard coat anodizing has a high dielectric strength, making it behave as an insulator — the thicker the layer of oxide, the higher the resistivity.

What Are the Limitations of Hard Coat Anodizing?

Despite its ability to improve the surface properties of aluminum parts, hard coat anodizing has some limitations that must be considered. The most important of these are listed below:

1. Reduced Fatigue Strength: Very thick hard coat anodizing layers tend to reduce the fatigue strength of aluminum parts. If the service environment includes exposure to cyclic loading, then Type I or Type IB coatings are recommended.
2. Limited Coloring Options: While hard coat anodized parts can be dyed, the resulting colors tend to be darker than those produced by  Type II aluminum anodizing, for example. In some cases, the as-anodized surface can have a dark gray or bronze gray hue. Dying parts after hard coat anodizing requires an additional sealing step. However, sealing will reduce the abrasion properties of the coating. 

What Is the Material Needed to Perform Hard Coat Anodizing?

Hard coat anodizing is a multi-step process that requires specialized materials, the most important of which are listed below:

1. Electrolyte Tank: The anodizing process must be performed in an acid-resistant tank. 
2. Cathode: The cathode is typically made from an unreactive material like graphite or lead sheeting.
3. Power Supply: Anodizing requires a power supply that is able to supply sufficient current as well as voltages up to 120 V.
4. Air Agitation: Air nozzles in the bottom of the electrolyte tank increase turbulence, which helps improve the quality of the coating as well as help with temperature distribution.
5. Temperature Control System: An anodizing bath must be maintained at a constant temperature. During the process, the bath will heat up, and this excess heat must be removed from the tank through a water or air-cooled chiller.
6. Pre and Post-Anodizing Process Equipment: In addition to the anodizing bath, additional degreaser, etching, dying, and sealing tanks are required for an optimal anodizing setup.

How Long Will Hard Coat Anodizing Last?

Hard coat anodizing may last anywhere from a few years to a few decades. It is often used for applications requiring good abrasion resistance. The severity of the abrasion environment impacts the longevity of the protective oxide layer. 

How Long Does It Take To Perform Hard Coat Anodizing?

The time required to hard coat anodize a part depends on several factors, including: the desired thickness, available voltage, surface current density, electrolyte concentration, and bath temperature. A simple formula can be used to calculate a rough estimate for the time needed:

Anodizing Time (minutes) = (Thickness (mils) x 720 ) / Current Density (A/ft2)

This equation assumes a sulfuric acid concentration of 12 to 20% by weight and a bath temperature of up to 80°F. 720 is the number of amp-minutes required per square foot of load to produce one mil of anodic oxide.

Are Hard Coat Anodized Materials Prone to Rust?

No, hard coat anodized materials are not prone to rust. Rust is used to describe the process that forms an oxide when iron-containing metals are exposed to oxygen. This process is destructive to the base metal. Aluminum does form a natural oxide layer but this layer is not destructive to the base metal.

Is Hard Coat Anodizing Permanent?

No, hard coat anodizing is not permanent. While the coating produced during hard coat anodizing is very durable, it can be damaged or worn away, exposing the softer base material beneath. Harsh environmental conditions can also cause the layer to break down over time. For these reasons, it is not permanent in the strictest sense of the word. However, it is a very long-lasting treatment. It must be noted that any exposed base aluminum will form a natural oxide layer; however, this layer is not as good as the artificially created hard coat layer. 

Are Hard Coat Anodized Materials Conductive?

No, hard coat anodized materials are not conductive. While the base metal is conductive, the ceramic-like oxide layer formed during the anodizing process is not conductive.

Summary

This article presented hard coat anodizing, explained what it is, and discussed when to best use it and its various benefits. To learn more about hard coat anodizing, contact a Xometry representative.

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