Hardcoat Anodizing Basics: What You Need to Know

Hardcoat anodizing, also called hard anodizing or Type III anodizing, is a form of aluminum anodizing. It produces a controlled oxide layer on the aluminum surface via an electrochemical process. Compared to other anodizing types, hard coat anodizing creates a harder, denser, thicker, and more abrasion-resistant outside coating. Since this process offers more durability and a much smoother finish, hard coat anodizing has become extremely popular in various applications. Often, it is used in applications requiring corrosion, wear, and abrasion resistance along with dielectric strength properties. 

This post will explain what is hard coat anodizing, its process, benefits, limitations, and applications. Moreover, we shall further discuss the distinction between hard coat anodizing and regular anodizing.

What is Hardcoat Anodizing?

Hard coat anodizing is an electrochemical process that generates a relatively thick oxide layer on the surface of a non-ferrous alloy like aluminum. Some non–ferrous metals, such as titanium and magnesium, can also be anodized, yet aluminum is the most commonly protected using this method. Hard coat anodizing uses specialized acids and higher voltages to produce an extra-hard, thick oxide coating. These oxide coatings can increase wear resistance, corrosion resistance, ability to retain lubricants, electrical and thermal insulation, in some cases, aesthetics. 

Because the oxide coating created from hard coat anodizing is resistant to wear and corrosion, this process is great for aerospace, aviation, robots, food making, the military, medicine, oil, and petrochemical industries. It works well with firearms, machinery, kitchenware, optical instruments, and electronics. It’s also often used in sporting items. It’s also perfect for medical instruments because it’s easily sanitized and non-contaminating. Some specific applications include pistons, cylinder heads, levers, heating and insulation plates, cam discs, bearing rollers, surgical instruments, nozzles, valves, gears, blast shields, and more.

The Hardcoat Anodizing Process

The Type III anodizing process forms a uniform oxide layer in a sulfuric bath, penetrating the material ½ into the aluminum, and ½ builds up on the surface. Now let’s go deep into the process of hard coat anodizing.

Material Used in Hardcoat Anodizing

First, talk about the typical material needed for hard coat anodizing. The hard coat anodizing process necessitates the use of specialized materials, the most essential of which is as follows:

• Electrolyte Tank: The anodizing process must occur in an acid-resistant electrolyte tank. An acidic electrolyte solution is typically sulfuric acid.
• Cathode: The cathode is generally constructed of an inert substance like graphite or lead sheets.
• Power Supply: Anodizing needs a power supply capable of supplying sufficient current and voltages of up to 120 V.
• Air Agitation: Air nozzles on the bottom of the electrolyte tank boost turbulence, which helps enhance coating quality and temperature distribution.
• Temperature Control System: The temperature of an anodizing bath must be kept constant. The bath will heat up during the operation, and this excess heat must be evacuated from the tank via water or an air-cooled chiller.
• Pre and Post-Anodizing Process Equipment: For an optimum anodizing setup, extra degreaser, etching, dying, and sealing tanks are necessary.

How Does Hardcoat Anodizing Work?

The hard coat anodizing process involves several steps that form a durable and corrosion-resistant surface. Let’s explore each step in detail:

Pre-Treatment

Cleaning the part to eliminate any impurities and pollutants from the surface is the first stage in the hard coat anodizing process. If this is done, the coating will be of better quality. Also, the part must be etched with acid to clear any naturally occurring oxide from when aluminum is exposed to air. This naturally occurring coating is usually uneven and of poor quality. But the acid etch creates a clean and uniform surface for anodizing.  

Hard Anodizing the Metal

When submerged in a temperature-controlled acidic electrolyte solution (usually sulfuric acid), the part is connected to the positive electrode and functions as the anode. A cathode formed of an unreactive substance is placed in the bath. An electric current is passed through the system, causing an oxidation reaction on the metal surface.

In the anodizing process, the air bubbles agitation that circulates the sulfuric acid solution is necessary. Circulation contributes to the cooling of the sulfuric acid to near-freezing temperatures. As a result, the sulfuric acid bath provides oxygen for the hard coat anodizing reaction. The oxygen then reacts with the aluminum to form an oxide layer. 

Hard coat anodizing requires higher voltages of up to 120 V. When the anodic layer thickens, its resistance to current increases. This could hinder the oxide deposition process. The voltage must be raised to compensate for the workpiece’s diminishing conductivity in this case. The thickness of this layer can be controlled by adjusting factors such as the electrolyte temperature, voltage, acid content, and duration of the process.

Post-Treatment

Once the desired oxide layer thickness has been achieved, the part is removed from the electrolyte solution, rinsed, and sealed to enhance its corrosion resistance further. Additional post-treatment steps, such as dyeing or applying a lubricating film, may be performed to achieve specific functional or aesthetic requirements.

Benefits of Hardcoat Anodizing

Hard coat anodizing is commonly used for parts that require superior mechanical, corrosion, or dielectric performance. Some of the advantages are as follows:

Enhanced Hardness: Hard coat anodizing significantly increases the hardness of the aluminum substrate, resulting in a surface that is more resistant to wear, abrasion, and scratches. Hardcoat anodized coatings generally have a Rockwell C hardness ranging from 50 to 70, depending on the specific process and alloy used.

Improved Corrosion Resistance: The anodized oxide layer formed through hard coat anodizing acts as a barrier between the metal substrate and the surrounding environment. This barrier provides excellent corrosion resistance, protecting the underlying aluminum from oxidation, moisture, chemicals, and other corrosive agents.

Increased Durability: Hard coat anodizing greatly enhances the durability and lifespan of aluminum components. The thick anodized layer can withstand harsh conditions and repeated use without deteriorating or compromising the part’s structural integrity. 

Wear Resistance: The thick and hard anodic layer formed during the hard coat anodizing process significantly improves the surface hardness of metals, making them more resistant to scratches, abrasion, and wear. Hard anodized coatings are more than ten times more wear resistant than ordinary anodized aluminum.

Electrical Insulation: Anodized aluminum has excellent electrical insulation properties, making it suitable for applications where electrical conductivity must be minimized or prevented. The anodized layer acts as an insulating barrier, reducing the risk of short circuits or electrical interference.

Thermal Resistance: Hard coat anodizing can improve the thermal resistance of aluminum components. The anodized layer acts as a thermal barrier, providing insulation and protecting the part from high temperatures, heat transfer, and thermal shock. Hard anodized coated parts can withstand short exposures of temperatures up to 2000 ℃ because of the inert nature of the coating.

Aesthetic Appeal: Anodized aluminum can be produced in various colors and finishes, providing an attractive and uniform appearance to the surface. This makes hard coat anodizing a popular choice for applications where aesthetics are important, such as architectural elements, consumer products, and decorative pieces.

Limitations of Hardcoat Anodizing

Despite its capacity to enhance the surface properties of aluminum components, hard coat anodizing has certain limitations that must be considered. Here are some of the key limitations of hard coat anodizing:

Limited Color Options: Hard coat anodizing primarily produces a dark gray or black finish. Unlike conventional anodizing processes, which offer a wide range of color options, hard coat anodizing has limited color variations. If you require a specific color for your application, hard coat anodizing may not be the best choice.

Thickness Limitations: Hard coat anodizing creates a relatively thick anodic coating on the metal surface. While this thickness contributes to improved hardness and durability, it can also present limitations in certain applications. The increased coating thickness may impact the dimensional tolerances of parts, especially when tight tolerances are required.

Surface Roughness: Hard coat anodizing can produce a rougher surface texture than other anodizing processes. The formation of a thick anodic coating can lead to an increased surface roughness, which may not be suitable for applications where a smooth or aesthetically appealing finish is desired. Additional finishing processes may be required to achieve the desired surface texture.

Brittle Coating: The hard coat anodizing process produces a hard and dense coating on the metal surface. While this hardness provides excellent wear resistance, it also results in a relatively brittle coating. The hard anodic coating may be prone to cracking or chipping in applications where the parts are subjected to significant impacts or bending forces.

Limited Substrate Compatibility: Hard coat anodizing is primarily suitable for aluminum and its alloys. It may not be suitable for metals like steel or copper. Alternative surface treatment methods would be more appropriate if you require anodizing for non-aluminum materials.

Increased Cost and Processing Time: Hard coat anodizing is generally more expensive and time-consuming than conventional anodizing processes. The complexity of the process, thicker coating formation, and additional quality control measures contribute to higher costs and longer processing times.

Applications of Hardcoat Anodizing

Hard anodizing applies in various industries due to the durability, ease of maintenance, and aesthetics of hard coat anodize. Here are some specific examples of applications where hard anodizing is commonly used:

Automotive Industry: In the automotive sector, hard anodizing is employed to enhance the performance of various components. For example, pistons, cylinders, engine blocks, and suspension components can benefit from hard anodizing, providing a protective barrier against wear, chemicals, and heat, extending their lifespan.

Aerospace Industry: Hard anodizing finds extensive use in aerospace applications due to its ability to improve aluminum components’ wear resistance and hardness. For instance, critical parts like aircraft engine components, landing gear, and structural parts are often hard anodized to enhance their resistance to friction, abrasion, and corrosion.

Defense and Firearms: Hard anodizing is widely used in the defense industry to improve firearm components’ durability and corrosion resistance. Items like rifle receivers, pistol frames, and other firearm parts undergo hard anodizing to create a tough surface that can withstand harsh environmental conditions and frequent use.

Electronics and Electrical Industry: Many electronic devices and electrical components require a protective coating to safeguard against environmental factors and ensure long-term functionality. Hard anodizing provides a hard and insulating surface on items like heat sinks, connectors, and electronic enclosures, enhancing their resistance to wear, corrosion, and electrical insulation.

Marine and Offshore Applications: Given the corrosive nature of marine environments, hard anodizing is commonly utilized to protect metal components used in marine vessels and offshore structures. Examples include boat fittings, propellers, valves, and underwater equipment, which undergo hard anodizing to improve their resistance to saltwater, chemicals, and abrasion.

Sporting Goods and Outdoor Equipment: Hard anodizing is utilized in producing various sporting goods and outdoor equipment to enhance their durability and resistance to wear. Items such as bicycle frames, fishing reels, climbing gear, and camping equipment can undergo hard anodizing to provide a hard and protective surface, extending their lifespan and performance.

Factors Affecting Hardcoat Anodizing Quality

Hard coat anodizing is a surface treatment process that enhances metal parts’ durability, corrosion resistance, and appearance. Several factors can affect the quality of hard coat anodizing. Here are some important factors to consider:

Alloy Composition: The composition of the metal alloy being anodized can significantly impact the quality of the hard coat anodizing. Different alloys have varying responses to the anodizing process, so selecting an alloy suitable for hard coat anodizing is crucial.

Pre-Anodizing Surface Preparation: Proper surface preparation is essential for achieving high-quality hard coat anodizing. The metal surface should be cleaned to remove contaminants like oils, grease, dirt, and oxides. This can be done through degreasing, alkaline cleaning, or mechanical polishing.

Anodizing Bath Parameters: The parameters of the anodizing bath, such as temperature, concentration of electrolyte, and current density, play a crucial role in determining the quality of the hard coat anodizing. These parameters must be carefully controlled and monitored to ensure consistent and uniform anodized coatings.

Anodizing Time: The duration of the anodizing process also affects the quality of the hard coat anodizing. Too short a time may result in an inadequate coating, while too long can lead to excessive coating thickness, affecting the final appearance and performance.

Voltage and Current Density: The voltage and current density applied during the anodizing process influence the thickness and properties of the anodized coating. These parameters must be optimized based on the desired coating thickness and the properties required for the specific application.

Post-treatment: The aluminum is typically sealed to protect the anodized layer after anodizing. The quality of this sealing process can also affect the final quality of the hard coat anodizing.

Equipment: The equipment used for anodizing must be properly maintained and calibrated to ensure consistent results. Any equipment issues can lead to inconsistencies in the anodizing process and affect the final quality.

Operator Skill: The skill and experience of the operator can also play a significant role in the quality of hard coat anodizing. An experienced operator can properly control the process and ensure a high-quality finish.

What is the Difference between Hardcoat Anodizing and Regular Anodizing?

Hard coat anodizing, also known as Type III anodizing or hard anodizing, is a specialized form of anodizing that results in a significantly thicker and harder oxide layer on the surface of a metal. Regular anodizing, or Type II or decorative anodizing, produces a thinner and less durable oxide layer than hard coat anodizing.

The key differences between hard coat anodizing and regular anodizing are as follows:

Thickness

Aluminum anodizing is commonly classified into three types. Type I is chromic acid anodize that is very thin (0.0001″ thick). Type II, also called regular anodizing, is a traditional sulfuric acid anodize that may be decoratively dyed in almost any color and has a 0.0002 – 0.0006″ thickness. Hard coat anodizing is completed under stricter process conditions, resulting in a harder, denser, thicker, and more abrasion-resistant coating. The thickness of the hard coat can range from 0.0005 to 0.0030″ and beyond, depending on the alloy being anodized.

Wear and Corrosion Resistance

The thicker and harder oxide layer in hard coat anodizing provides superior wear and corrosion resistance properties. It forms a protective barrier that can withstand harsh environments and chemical exposure. Regular anodizing offers some level of protection against corrosion, but it is not as robust as hard coat anodizing.

Sealing

In most standards, the coating is left unsealed when hard coat anodizing is applied for wear or abrasion resistance. However, corrosion resistance or a combination of corrosion and wear resistance is the primary goal; the hard coat anodizing should be sealed. Generally, hard coat anodizing requiring dying must be sealed to guarantee that the dye does not fade or bleach off. Deionized water, sodium dichromate, nickel acetate, PTFE, or a mixture of two or more can be used to seal.

Hardness 

Hard coat anodizing produces a harder, more wear-resistant finish than regular anodizing. Hard coat anodizing takes longer, resulting in a thicker oxide layer.

Aesthetics

The color of aluminum after hard anodizing is determined mostly by the aluminum alloys and the thickness of the coating. As a result, given their appearance, there are two primary classes of hard anodize. They are as follows:

Class 1 (Undyed) – Some hard anodize series seem deep gray or black, while others resemble bronze-gray. Because of its dark tone, this class of undyed coating is not frequently ornamental or uniformly colored.

Class 2 (Dyed) – In most circumstances, black is the dye of choice for hard coat anodizing to provide an attractive and even look.

However, regular anodizing may be dyed practically any color. Consequently, this aluminum anodizing is easily decorated with appropriate looks, making them aesthetically pleasing.

Applications

Designers frequently utilize hard coat anodizing for goods or parts that require impact and wear-resistant surfaces due to their improved hardness and other enhanced surface properties. Hydraulics, culinary utensils, pistons, and various building materials are among them. On the other hand, regular anodized aluminum is commonly used in automotive and architectural parts.

Summary

Type III (hard coat) anodizing is an anodizing process in which an exceptionally hard, abrasion-resistant, porous oxide is formed on aluminum. It is sometimes referred to as an “engineering hard coat” since the oxide has been found suitable for situations where hardness and increasing oxide thicknesses are advantageous. Type III anodizing is widely used in aluminum cookware, military purposes, and other applications.

At LEADRP, we provide top-notch aluminum anodizing and prototyping services. Our advanced equipment and experienced processes guarantee parts of superior quality and precision. Feel free to request a quote today and get our one-stop service.

Reference

Hardcoat Anodize – From Anoplate