Surface finishings comprise a broad range of industrial techniques pivotal in post-manufacturing processes. To a large extent, these post-processes aim to alter the surface of the manufactured parts to get specific properties and make them more appealing.
Today, anodizing is one of the most crucial surface treatment processes applied in CNC aluminum machining. Anodizing is ideally suited for aluminum to improve corrosion resistance and aesthetic qualities. Yet, hard coat anodizing further reinforces the superficial characteristics of aluminum parts.
This article contains all you need to know about aluminum hard coat anodizing and the characteristics of hard coat anodize. We will also walk you through the difference between the hard anodize and the regular anodize of aluminum.
What is Hard Coat Anodizing?
Anodizing is an electrochemical process that forms a layer of non-conductive anodic oxide on the surface of a non-ferrous metal especially aluminum. This process makes aluminum more durable, decorative, and corrosion resistant, informing its use for various finished parts across different manufacturing industries.
Hard anodizing, also called hard coat anodizing, is a form of aluminum anodizing. Specifically, it involves refining aluminum by forming a dense anodic coating of aluminum oxide on the metal surface through electrolysis.
Further, in the hard anodizing process, parts of the base aluminum material become fully synchronized with the anodic coating to form a high-strength bond with a thicker surface.
Also, aluminum hard coat anodizing improves specific properties of treated aluminum materials, which applies in many industries. They include abrasion and corrosion resistance, surface hardness, thermal and electrical insulation, gliding properties, and aesthetic qualities.
Hard Coat Anodizing of Aluminum Process
Hard anodizing produces a controlled oxide layer on anodized parts. This process offers significant advantages, including the economic benefits and prolonged longevity of the hard coat anodize.
Further, you might want to know how to hard anodized aluminum. Check the steps taken in the hard coat anodizing of aluminum below:
This is the first step in the anodizing of aluminum. Here, the aluminum alloy component gets adequately cleaned and deoxidized through the application bright or satin finish to the aluminum component. As such, this helps remove grease and surface dirt.
Satin finishing comprises light etching to create an even and appealing matte surface finish. More so, this process uses hot sodium hydroxide solution to remove the aluminum material’s surface inadequacies.
Bright finishing produces a near mirror finishing with a concentrated mixture of phosphoric and nitric acids. Thus, this process smoothens and cleans the aluminum surface.
Hard Anodizing Process
Generally, hard coat anodize gets formed by the electrochemical reaction of aluminum with oxygen. Furthermore, the process involves using sulfuric acid and low bath temperatures. You’ll find that sulfuric acid is the most common electrolyte for type III (3) hard coat anodizing. Below are procedures for aluminum hard coat anodizing:
i.First, the aluminum part gets racked to make the anode and then immersed in sulfuric acid solution. The sulfuric acid bath contains about 180 to 200 grams per liter of acid and minute amounts of dissolved aluminum parts.
ii. Then air bubbles through the acid. This air bubble agitation circulates the sulfuric acid solution.
iii. Further, circulation aids in cooling the sulfuric acid to a near-freezing temperature. Thus, the oxygen for the hard coat anodizing reaction comes from the sulfuric acid bath.
iv. The oxygen then combines with aluminum to produce a measurable oxide film. This occurs when voltages of about 100 volts upwards get applied at a current density of 24 to 40 amps per square foot.
v.The current is continually applied until the desired oxide thickness forms. Meanwhile, the operating temperatures remain at the freezing point of water between -2 to 0°C.
vi. Note that this coating is chemically bonded to the substrate, thus forming the hard coat anodize.
The hard anodize thicknesses vary from 0.0005 to 0.006” (13 to 150 microns) thick, depending on the specific aluminum alloy. Moreover, forming thick films requires higher voltage and controlled processing in refrigeration tanks.
Hard Coat vs. Regular Anodize of Aluminum: What's the difference?
Generally, aluminum anodizing comprises three types. They are:
- Type I is chromic acid anodize
- Type II is the conventional sulfuric acid anodize
- Type III is the hard coat anodize
So, what are the differences between the hard coat anodize and the regular anodize of aluminum?
Check the differences below:
Hard coat anodize is thicker than regular anodized aluminum. Consequently, this thicker surface gives hard anodized aluminum more corrosion and wear-resistant surface.
You will find that type III hard coat anodizing produces thicknesses ranging from 13 to 150 microns. In contrast, type I anodize has a thin coat with a thickness of about 2.54 microns, while type II anodized aluminum has thicknesses ranging from 5.08 to 15.24 microns.
Typically, hard anodized aluminum does not get sealed due to its thicker surface. This increases the durability and wears resistance of the hard anodic coating. As a result, the need for a sealant reduces during the technical application of hard coat anodize.
In contrast, regular aluminum anodize undergoes sealing to close the pores during conversion to aluminum oxide. Hence, this sealing process gives standard anodize a firm protective coat against corrosion. Moreover, the regular anodic coatings are less durable and wear-resistant than hard anodize. As such, this makes them suitable for parts that need oxidation resistance.
You’ll discover that the color of aluminum after hard anodizing depends mainly on the aluminum alloys and the thickness of the coating. Hence, there are two major classes of hard anodize based on appearance. They are:
Class 1 (Undyed) – Some hard anodize series take on deep gray or black color, while others appear as more bronze-gray color. By implication, this class of undyed coating is not usually decorative or evenly colored due to its dark color.
Class 2 (Dyed) – in most cases, black is the dye often specified for hard coat anodizing to produce an appealing and even appearance.
Yet, you can dye regular anodize such as type II into almost any color. Thus, you’ll find that this type of aluminum anodize is readily decorated into suitable appearances, making them more appealing.
Due to the increased hardness and other enhanced surface properties of hard coat anodize, designers often use it for products or parts requiring impact and wear-resistant surfaces. They include hydraulics, cooking wares, pistons, and some building materials. On the other hand, regular anodize aluminum is widely applied in automotive and architecture parts.
Importance of Sealing Hard Coat Anodize
Sealing anodized aluminum remains essential. Moreover, it helps remove pores that absorb dirt, stains, oil, and grease. The presence of these substances often leads to the sticky nature of the anodic coating. Hence, sealing provides a firmer protective coat.
Regardless, there are specific factors you must consider when sealing hard anodize. Thus, note that applying the hard anodized aluminum component mainly determines the importance of sealing hard coat anodize.
For instance, if your primary aim is to apply hard anodized aluminum for parts requiring wear or abrasion resistance, then it is best to leave the hard coating unsealed. This is because the thick hard coating maintains durability and wear resistance when left unsealed. Meanwhile, if your hard anodized aluminum applies to parts that need corrosion resistance or the combination of both corrosion and wears resistance, you must seal the hard anodize.
Sealing can be applied using deionized water, sodium dichromate, Polytetrafluoroethylene (PTFE), nickel acetate, or a combination of these chemicals. Generally, sealing applies to dyed hard coat anodize to prevent color erosion.
Characteristics and Features of Hard Anodizing
Hard anodize comprises specific characteristics suitable for diverse applications. Take a look at some of the characteristics and features of hard anodize below:
1. Electrical properties
Hard anodized aluminum parts have excellent electrical resistivity. Moreover, anodic coatings have superior electrical insulation properties due to their breakdown voltages. Besides, the electrical resistivity of a typical anodic film ranges from 1011 to 1015 ohms. Hence, the resulting aluminum oxide coating from hard anodizing is more electrically non-conductive, depending on the aluminum alloy.
2. Heat Resistance
The conventional standard anodized aluminum has good heat insulating properties. But then, the hard coat anodized aluminum has much better heat resistance. Thus, this makes it well-suited for parts where good heat dissipation remains crucial. Besides, hard anodize can withstand short exposure to higher temperatures up to 2000°C due to the inert nature of the coating.
3. Hardness and Wear Resistance
The hardness of hard anodic coating varies depending on the aluminum alloy selected. Regular anodized aluminum with a softer coating has a hardness of about 200 to 400 HV, thus suitable for decorative purposes. Meanwhile, a typical hard coat anodize has a consistent hardness of about 400 to 600 HV along the entire section of the aluminum part. This makes hard coats suitable for technical applications.
Also, the wear resistance of hard anodized aluminum components compares agreeably with hard chromium and high-speed steel. Thus, hard anodic coatings are ten times more wear resistant than regular anodic films.
4. Corrosion Resistance
Hard coat anodizing helps produce thick layers of hard anodized aluminum parts, increasing their corrosion resistance properties. More so, the thicknesses of hard anodize vary depending on the aluminum material. Thus, the thicker the hard anodic film, the better the ability to resist oxidation.
Hard anodized aluminum components have varying colors depending on the aluminum component and thickness of the hard coating. Thus, some hard anodize can appear dark gray or bronze gray in undyed form. Further, it also occurs in full black color in the dyed form.
Standard Specifications of Hard Coat Anodize
Below are some of the standard specifications governing hard coat anodizing of aluminum:
- MIL-A-8625, Type III
- AMS 2469
- ASTM B580, Type A
- ISO 10074
- MIL-A-63576 (PTFE Sealed)
- AMS 2482 (PTFE Sealed)
Applications of Hard Anodizing Coat
Hard anodizing applies in various industries due to the durability, ease of maintenance, and aesthetics of hard coat anodize. These industries include medical, food manufacturing, military, automotive, housing, electrical, machinery, petrochemical, aviation, robotics, etc.
More specifically, hard coat anodized applies to components such as pistons, cylinder heads, levers, heating and insulation plates, cam discs, bearing rollers, surgical instruments, nozzles, valves, gears, blast shields, and lots more.
Hard anodizing remains one of the most vital finishing processes for aluminum in many manufacturing industries today. Besides, the hard anodizing provides the treated aluminum part with improved properties while it retains many advantages intrinsic to aluminum.
Even though hard coat anodizing offers various benefits, the process requires quality manufacturing service to get high-quality products.
At WayKen, we offer professional hard coat anodizing and aluminum anodizing service that will give you the best value. With our high-quality processes and cutting-edge equipment, we assure quality, precise and affordable coated parts. Please don’t hesitate to get a quote today for one-on-one support service.
How Long Will Hard Anodized Aluminum Last?
Concisely, hard anodized aluminum parts are durable and will last a lifetime. However, hard anodized aluminum components sealed with PTFE (non-stick coating) last for three to five years as the surface begins to wear down. Hence, this informs the need for the periodic replacement of hard anodized cookware such as pots and pans.
What Seals Can Be Used for Aluminum Hard Coat?
You can use seals like sodium dichromate, Polytetrafluoroethylene (PTFE), nickel acetate, and nickel fluoride. Even so, nickel acetate hot sealing and nickel fluoride cold sealing are the primary treatment methods for enhancing aluminum hard coat corrosion resistance.
What Is the Coating Thickness for Aluminum Hard Coat?
The coating thickness for hard-anodized aluminum varies depending on the aluminum material. Generally, the natural oxide layers on aluminum surfaces are 2 to 3 nanometers thick. Even so, aluminum hard coat anodizing can increase the coating thickness can up to 120 microns.