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What Does Hard Coat Anodizing Mean?
Anodizing or anodic oxidation is defined as an electrochemical process that deposits a non-conductive aluminium oxide layer on the surface of treated non-ferrous metals like aluminum, magnesium, and titanium. The first anodizing process dates back to the 1920s to 1930s, and it was based on chromic acid (chromic anodizing). Due to the carcinogenicity of chromium salts, chromic anodizing was typical of specific sectors, including aeronautics and military. However, sulfuric acid-based anodizing is the most widely embraced and used anodizing process.
Hard anodizing, also called hard coat anodizing or type II anodizing, is a typical aluminum finish that forms a controlled oxide layer on a metal surface through an electrochemical conversion process. The parts of the underlying metal become entirely synchronized with the anodized coating to create a high-strength bond with a denser surface.
Furthermore, hard anodizing of aluminum enhances specific properties of treated aluminum materials, including aesthetic qualities, surface hardness, gliding properties, aesthetic qualities, abrasion resistance, good corrosion resistance, and thermal and electrical insulation.
Stepwise Guide to the Process of Hard Coat Anodizing of Aluminum
Hard coat anodizing forms a controlled oxide layer on aluminum components through a series of meticulous steps. The procedures involved in the hard anodizing process include:
Pre-Treatment
The initial and most critical step in the aluminum hard coat anodizing process is preparing the metal. It is important to clean and deoxidize the aluminum material to eliminate contaminants such as grease or surface dirt that can adversely impact the hard anodizing process. You can employ chemical treatments like etching or mechanical methods like polishing to eliminate micro-imperfections and achieve a clean, uniform aluminum surface.
Aluminum Hard Anodizing
Specialized acids like sulfuric acid solution, higher voltage, and near-freezing temperatures are integral elements in the hard anodising process. The low temperature (-5°C to +5°C) controls chemical reactions, allowing the formation of thicker and stronger oxide coating.
Immerse the deoxidized aluminum substrate in a tank containing electrolyte solution – sulfuric bath. The aluminum material serves as the anode connected to a positive electrode, which forms a crystalline oxide layer on the metallic parts’ surface. Then, an oxidation reaction occurs as a controlled electric current passes through the electrolytic cell. Oxygen ions form on the surface of the aluminum material and bond with the aluminum atoms to develop thicker coatings of aluminum oxide.
Air bubble agitation occurs through the sulfuric bath, circulating the electrolytic solution, and it allows it to cool to a near-freezing temperature. The oxygen from the sulfuric acid bath reacts with the aluminum surface to develop a measurable oxide film. You can adjust the voltage of the current consistently until you achieve the preferred oxide thickness.
Post Treatment
Extract the aluminum part from the tank once the required thickness of oxide layer has been attained and rinse in a deionized water bath to wash off residual acids. Then, you can apply sealant to increase corrosion resistance. Other post-treatment steps designers often utilize to achieve specific aesthetic and functional requirements include the application of a lubricating film or dyeing.
Inspection and Quality Control
Inspect the anodized aluminum parts thoroughly to verify that you have achieved uniform coating and appearance during the aluminum hard coat anodizing process.
Common Properties of Aluminum Hard Anodizing
Hard anodizing possesses specific, unique properties that make it a vital choice in many industries today. We’ll consider some of these features below:
Heat Resistance
Hard anodized aluminium components are well known for their excellent heat-insulating properties compared to conventional standard anodized aluminum. For this reason, hard anodizing can withstand moderate exposure to higher temperatures around 2000°C because of the inherent nature of the anodized layer.
Electrical Properties
Hard-coated anodized aluminium parts exhibit superior electrical resistivity. Anodic coatings exhibit excellent insulation properties, making them ideal for applications where electrical conductivity is not necessary or must be prevented. Moreover, a typical anodic film has an electrical resistivity of about 1011 to 1015 ohms. Depending on the aluminum alloy, the resulting aluminum oxide layer from hard anodizing is more electrically non-conductive.
Corrosion Resistance
Hard coat anodizing forms dense layers of hard anodized aluminum components, enhancing their corrosion resistance. Besides, the thicknesses of hard anodize vary based on the aluminum alloy. Hence, aluminum surfaces can resist oxidation better depending on the thickness of the hard anodic film.
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Not all aluminum alloys anodize equally. Use 6061 or 6082 for best hardness and color uniformity; avoid casting alloys containing high silicon.
Hardness and Wear Resistance
Generally, the chosen aluminum alloy often determines the hardness of the hard anodic coating. Regular anodized aluminum used for decorative purposes often possesses a softer coating with a hardness range of about 200 to 400 HV. The ISO 10074 is a standard specification that defines the minimum hardness of specific alloys.
Conversely, a typical hard coat anodize possesses a consistent hardness between 400 and 600 HV along the whole section of the aluminum component. As such, hard coats are perfect for technical applications since they exhibit excellent corrosion and abrasion resistance.
Agreeably, you can compare the hard anodized aluminum components’ wear resistance to the hard chromium and high steel. Therefore, it is right to conclude that anodic coatings are ten times durable and resistant to wear than regular anodic films.
Color
Hard-coated anodized aluminium parts can accept varying colors depending on the aluminum part and the hard anodizing thickness. Hence, certain hard anodized aluminum surfaces can appear bronze gray or dark gray in undyed form. Also, it can occur in full black in the undyed form.
Comparing Hard Coat vs Regular Anodizing of Aluminum
There are three standard types of aluminum anodizing processes in general. These include
- Chromic acid anodize (Type I)
- Conventional sulfuric acid anodizing (type II)
- Hard coat anodize (Type III)
However, we will explore the key differences between the regular anodizing and hard coat anodizing of aluminum, including:
Coating Thickness
The thickness of the oxide layer is a significant difference between the Type II and Type III anodizing processes. The special feature of hard coat anodizing is the ability to develop a uniform, calibrated thickness of about 30-50µm. Depending on the particular requirements, you can achieve greater or lesser thicknesses of the hard anodizing layer. Design experts tend to reduce the anodizing thickness and define tight tolerances like 30±5µm.
When compared to regular anodizing, it is a thicker and harder aluminum oxide coating commonly used in precision mechanics for machined parts. It increases the surface hardness, maintaining the aluminum parts’ resistance to wear and corrosion in aggressive environments.
Regular or decorative anodizing produces a thinner anodic coating of aluminum oxide that offers moderate wear and scratch resistance in non-aggressive environments. Designers choose the regular anodizing type for decorative purposes since it can accept a variety of colors and works at warmer temperatures.
Appearance
The aluminium alloys and coating thickness mainly determine the color of the aluminum after hard coat anodizing. Hard anodized are grouped into two main categories according to their appearance. These include:
- Class 1-Undyed: Includes some hard anodize grades that have black or dark gray color, while others have more bronze-gray color. Consequently, this group of undyed coatings is not evenly colored or usually decorative because of its dark color.
- Class 2- Dyed: Even though black is the typical dye commonly specified for hard anodizing to create an even and aesthetically appealing appearance. However, you can dye regular anodize, including type II, into any preferred color. As such, this aluminum anodizing type is readily decorated into desired appearances to make them more appealing.
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Hard anodizing adds 25–75 microns thickness; half penetrates inward, half outward. Always account for this dimensional change during part design and tolerancing.
Seal
Since hard coat anodizing develops a thicker oxide layer on aluminum surfaces, sealing is optional for hardcoat aluminum parts. In most cases, designers prefer not sealing hardcoat anodize in technical applications since leaving the coating unsealed increases surface durability and resistance to wear.
However, regular aluminum anodizing requires sealing to cover the pores created during the conversion to aluminium oxide. Thus, sealing offers a standard anodized, robust protective shield against corrosion. Besides, the regular anodic coats are usually not as durable and resistant to wear as hard coat anodizing.
Applications
Product designers often utilize hard coat anodizing for aluminum parts requiring wear and impact-resistant surfaces since it offers increased hardness and other improved surface properties. Hard anodizing is widely used for cooking wares, hydraulics, cylinder heads, pistons, surgical instruments, firearms, and building materials.
More so, hard anodized aluminum parts are ideal for applications involving moving parts since the oxide coatings help retain lubricants. In contrast, regular anodized coating finds use in the architecture and automotive parts.
Is It Important to Seal Hard Coat Anodize?
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Sharp edges prevent uniform anodic coating and may burn during processing. Add small radii or chamfers to improve film adhesion and consistency.
Sealing of hard anodized aluminum plays a significant role in prolonging service life and increasing the aesthetics of the products since it helps prevent the dye from fading. Even though it is an optional requirement, aluminum parts undergo a sealing process to cover pores that might absorb oil, grease, and dirt, which can make the anodic coating sticky.
Sealants are generally applied to all dyed hardcoat anodized layers to avoid color erosion or bleaching out. Product designers achieve sealing with different methods (cold sealing and hot sealing) using nickel acetate, nickel fluoride, sodium dichromate, deionized water, PTFE, or joining of two or more of these elements.
There are various process parameters to consider when sealing the surface of hard anodized aluminum to achieve desired results. Nevertheless, it is essential to note that the application of the hard anodized aluminum product will determine the importance of the sealing.
For instance, it would be best to leave an anodized aluminum part unsealed when it is intended for applications where resistance to wear or abrasion is non-negotiable. The dense anodic coating maintains its surface durability and resistance to abrasion and wear when left unsealed. However, it would be best to seal the hard anodic coating if your hard anodized parts are intended for environments that require high corrosion resistance or a combination of wear and corrosion resistance.
Conclusion
As a viable and widely embraced metal finishing process, hard anodising aluminium significantly increases insulation properties and prolongs the service life of aluminum parts by improving surface hardness, wear and corrosion protection, and ease of maintenance. Its uniqueness resides in the ability to produce thickened and corrosion-resistant oxide film on the surface of aluminum components, and gives a variety of color options.
AT Machining is the one-stop CNC shop that provides the superior aluminum hard coat anodizing services you need for your project. Our certified engineering team has the practical experience and expertise of advanced surface treatment technology and can provide customized hard coat anodizing solutions to elevate your products to meet industry standards and performance needs. Contact us now!
FAQs
How Does Hard Anodizing Aluminum Impact Dimensional Growth?
Aluminum increases beneath the surface by 50% and above the surface of the aluminum material by 50%. However, the thickness influences the dimensional tolerances of aluminium parts, limiting suitability in certain applications where tight tolerances are paramount.
What Are the Technical Specifications and International Standards for Hard Anodising Aluminium?
Hard anodize process conforms to UNI 7796, MIL-A-8625 Type III, AMS 2469, ASTM B580 (Type A), and the ISO 10074. So, it is one of the go-to options when you’re looking to improve corrosion resistance and surface quality.
Which Aluminium Alloys Are Compatible with the Hard Anodizing Process?
All aluminum alloys work well with anodizing except those containing alloying elements other than aluminum in high amounts. The 5xxx and 6xxx aluminum series, including 5052, 6082, and 6061, are typical aluminum alloys that exhibit excellent response to anodizing. Certain alloys, including the 2xxx series, contain high quantities of copper, resulting in less wear and corrosion-resistant properties.
What Are the Common Limitations of Hard Anodizing for Aluminum?
Even though hardcoat anodizing is widely embraced across industries for its many benefits, it has certain limitations, including limited color options, rougher surface texture, relatively brittle coating, and increased processing time and cost.
