Surface finishing encompasses a wide range of industrial techniques that play a central role in downstream processes. These post-processing processes largely aim to modify the surface of manufactured parts, in order to give them certain properties and make them more attractive.
Today, anodizing is one of the most important surface treatment processes in aluminum CNC machining. Anodizing is ideal for aluminum to improve its corrosion resistance and aesthetic qualities. Hard anodizing further improves the surface properties of aluminum parts.
In this article you will learn everything you need to know about hard anodizing aluminum and the properties of hard anodizing. We will also explain the differences between hard anodizing and normal aluminum anodizing.
What is hard anodizing?
Anodizing is an electrochemical process in which a layer of non-conductive anodic oxide is formed on the surface of a non-ferrous metal, mainly aluminum. This process makes aluminum more durable, decorative and resistant to corrosion, allowing it to be used in various finished parts in different manufacturing industries.
Hard anodizing, also called hard coat anodizing, is a form of aluminum anodizing. More specifically, it involves refining aluminum by forming a dense anodic coating of aluminum oxide on the surface of the metal through electrolysis.
Additionally, hard anodizing completely synchronizes parts of the aluminum base material with the anode coating, creating a high-strength bond with a thicker surface.
Additionally, hard anodized aluminum improves certain properties of treated aluminum materials, which is important in many industries. These include abrasion and corrosion resistance, surface hardness, thermal and electrical insulation, sliding properties and aesthetic qualities.
Aluminum hard anodizing
Hard anodizing creates a controlled oxide layer on anodized parts. This process offers significant advantages, including economic benefits and longer life than hard anodizing.
Additionally, you may want to know how to anodize aluminum. Check out the steps for hard anodizing aluminum below:
Pretreatment
This is the first step in anodizing aluminum. The aluminum alloy component is sufficiently cleaned and deoxidized by applying a glossy or satin surface. In this way, grease and superficial dirt are removed.
The satin finish uses light embossing to create a uniform, attractive matte surface. This process also removes surface defects from the aluminum material with hot sodium hydroxide solution.
The gloss finish involves using a concentrated mixture of phosphoric and nitric acid to create an almost mirror-smooth surface. This softens and cleans the aluminum surface.
Hard anodizing process
In general, hard anodizing occurs through the electrochemical reaction of aluminum with oxygen. Furthermore, the process requires the use of sulfuric acid and low bath temperatures. You will find that sulfuric acid is the most common electrolyte for Type III hard anodizing (3). Below are the procedures for hard anodizing aluminum:
i.First, the aluminum piece is placed on a rack to make the anode and then dipped into a sulfuric acid solution. The sulfuric acid bath contains about 180 to 200 grams of acid per liter and small amounts of dissolved aluminum particles.
ii. Then the air bubbles penetrate the acid. This movement of the air bubble circulates the sulfuric acid solution.
iii. Additionally, circulation helps cool the sulfuric acid to a temperature close to freezing. The oxygen for the hard anodizing reaction therefore comes from the sulfuric acid bath.
4. The oxygen then combines with the aluminum and forms a measurable oxide film. This occurs when voltages of about 100 volts or more are applied at a current density of 24 to 40 amps per square foot.
vCurrent is applied continuously until the desired oxide thickness is formed. Operating temperatures at the freezing point of water remain between -2 and 0 °C.
saw. Note that this coating is chemically bonded to the substrate to form hard anodizing.
Hard anodizing thickness ranges from 0.0005 to 0.006 inches (13 to 150 microns), depending on the aluminum alloy. Furthermore, the formation of thick films requires higher voltage and controlled processing in cooling tanks.
Hardfacing vs. hardfacing Regular aluminum anodizing: What's the difference?
In general, there are three types of aluminum anodizing. These are:
- Type I is anodized with chromic acid
- Type II is conventional sulfuric acid anodizing
- Type III is hard anodizing
So what are the differences between hard anodizing and regular aluminum anodizing?
Check out the differences below:
1. Thickness
Hard anodized aluminum is thicker than regular anodized aluminum. Consequently, this thicker surface gives hard anodized aluminum a more corrosion and wear resistant finish.
You will find that Type III hard anodizing produces thicknesses from 13 to 150 microns. In contrast, Type I anodizing produces a thin layer approximately 2.54 micrometers thick, while Type II anodized aluminum has thicknesses of 5.08 to 15.24 micrometers.
2. Seal
Typically, hard anodized aluminum is not sealed due to its thicker surface. This increases the durability and wear resistance of the hard anodized coating. This reduces the need for sealants in the technical application of hard anodizing.
In contrast, regular aluminum anodizing is sealed to close the pores during conversion to aluminum oxide. This sealing process therefore provides standard anodizing with a solid protective layer against corrosion. Additionally, regular anodizing coatings are less durable and wear-resistant than hard anodizing. Therefore, they are recommended for parts that need to be resistant to oxidation.
3. Appearance
You will find that the color of aluminum after hard anodizing mainly depends on the aluminum alloys and the thickness of the coating. Therefore, there are two main classes of hard anodizing based on appearance. These are:
Class 1 (colorless) – Some hard anodizing series take on a deep gray or black color, while others appear more bronze gray. This means that this type of colorless coating is generally neither decorative nor uniformly colored due to its dark color.
Class 2 (Colored) – In most cases, black color is used for hard anodizing to achieve an attractive and uniform appearance.
However, you can color regular anodizing like Type II in almost any color. So, you will find that this type of aluminum anodizing can be easily decorated with suitable appearances, making it more attractive.
4. Applications
Due to hard anodizing's greater hardness and other enhanced surface properties, it is often used by designers for products or parts that require impact- and wear-resistant surfaces. This includes hydraulic systems, kitchen utensils, pistons and some construction materials. Regular anodized aluminum, on the other hand, is often used in automotive and architectural parts.
Importance of hard anodizing sealing
Anodized aluminum sealing remains essential. Additionally, it helps unclog pores that absorb dirt, stains, oil and grease. The presence of these substances often causes the anodized coating to become sticky. Therefore, the seal provides a stronger protective layer.
Regardless, there are certain factors you need to consider when sealing hard anodizing. Therefore, please note that the application of the hard anodized aluminum component mainly determines the importance of the hard anodized seal.
For example, if you plan to use primarily hard anodized aluminum for parts that need to be wear or abrasion resistant, it is best to leave the hard coating unsealed. Because the hard, thick coating maintains its durability and wear resistance even without sealing. On the other hand, if you are using hard anodized aluminum for parts that need to be corrosion resistant or a combination of corrosion and wear resistance, you will need to seal the hard anodized finish.
Sealing can be done with deionized water, sodium dichromate, polytetrafluoroethylene (PTFE), nickel acetate, or a combination of these chemicals. Generally, sealant is applied to colored hard anodized to prevent color erosion.
Properties and characteristics of hard anodizing
Hard anodizing has specific properties suitable for various applications. Check out some of the properties and characteristics of hard anodizing below:
1. Electrical properties
The hard anodized aluminum parts have excellent electrical resistance. Furthermore, anode coatings have excellent electrical insulation properties due to their breakdown voltages. Furthermore, the electrical resistance of a typical anode film is between 1011 and 1015 ohms. Therefore, depending on the aluminum alloy, the aluminum oxide coating resulting from hard anodizing is less electrically conductive.
2. Heat resistance
Conventional standard anodized aluminum has good thermal insulation properties. However, hard anodized aluminum has much better heat resistance. Therefore, it is suitable for parts where good heat dissipation is crucial. Furthermore, due to the inert nature of the coating, hard anodizing can withstand higher temperatures of up to 2,000°C in the short term.
3. Hardness and wear resistance
The hardness of a hard anodized coating varies depending on the aluminum alloy selected. Regular anodizing with a softer coating has a hardness of around 200 to 400 HV and is therefore suitable for decorative purposes. On the other hand, a typical hard anodized coating has a consistent hardness of about 400 to 600 HV across the entire cross-section of the aluminum part. This makes hard coatings suitable for technical applications.
The wear resistance of hard anodized aluminum components is also comparable to that of hard chrome and high speed steel. This means that hard anodized layers are ten times more resistant to wear than conventional anodized layers.
4. Corrosion resistance
Hard anodizing can create thick layers of hard anodized aluminum parts, increasing their corrosion resistance. Furthermore, the thickness of hard anodizing varies depending on the aluminum material. Therefore, the thicker the hard anodized film, the better its ability to resist oxidation.
5. Color
Hard anodized aluminum components have different colors depending on the aluminum component and the thickness of the hard coating. Some hard anodizing may appear dark gray or bronze gray in its colorless form. Furthermore, they also come in completely black color in colorful form.
Standard Specifications for Hard Anodizing
Below are some of the standard specifications for hard anodized aluminum:
- MIL-A-8625, Type III
- AMS 2469
- ASTM B580, Type A
- ISO 10074
- MIL-A-63576 (PTFE sealed)
- AMS 2482 (PTFE sealed)
Application areas of hard anodizing coating
Hard anodizing is used in various industries due to its durability, ease of maintenance and aesthetics. These industries include medical, food manufacturing, military, automobile manufacturing, housing, electrical engineering, mechanical engineering, petrochemicals, aviation, robotics, etc.
More specifically, hard anodizing is used for components such as pistons, cylinder heads, levers, heating and insulating plates, cam discs, bearing rollers, surgical instruments, nozzles, valves, gears, blast shields, and more.
Concluding
Hard anodizing is still one of the most important aluminum finishing processes in many manufacturing industries. Additionally, hard anodizing imparts enhanced properties to the treated aluminum part while maintaining many of the inherent benefits of aluminum.
Although hard anodizing offers several advantages, the process requires high-quality manufacturing to obtain high-quality products.
Common questions
How long does hard anodized aluminum last?
In short, hard anodized aluminum parts are durable and will last a lifetime. However, the hard anodized aluminum components sealed with PTFE (non-stick coating) last three to five years as the surface wears. Therefore, it is necessary to regularly replace hard anodized cookware such as pots and pans.
What sealants can be used for aluminum hardfacing?
You can use seals such as sodium dichromate, polytetrafluoroethylene (PTFE), nickel acetate, and nickel fluoride. However, nickel acetate hot sealing and nickel fluoride cold sealing are the main treatment methods to improve the corrosion resistance of aluminum hard coatings.
How thick is the coating for aluminum hardfacing?
The coating thickness of hard anodized aluminum varies depending on the aluminum material. In general, the natural oxide layers on aluminum surfaces are 2 to 3 nanometers thick. However, the coating thickness can be increased up to 120 micrometers by hard anodizing aluminum.
