11 / 100

Some of my best customers have asked me, “Paul, why is hardcoat anodize a good option for my metal finishing application?” There are actually more than a few good reasons why this is such a popular choice industrywide.

Hard anodizing aluminum allows the aluminum to keep the basic advantages of aluminum while providing greatly improved wear, heat resistance, and corrosion resistance. By applying a dense anodic coating of aluminum oxide, the process forms an extremely hard abrasion-resistant oxide film, plus thickness can be added for additional strength and corrosion resistance.

Three Types of Aluminum Anodizing
● Type I is chromic acid anodize and has a thickness of 0.0001”.

● What is often referred to as sulfuric acid anodize is Type II in America (MIL-A-8625); Type II is moderately thicker at 1.8 μm to 25 μm (0.0003″ to 0.001″). Standards for thin (Soft/Standard) sulfuric anodizing are commonly referred to by MIL-A-8625 Types II and IIB, AMS 2471 (undyed), and AMS 2472 (dyed), BS EN ISO 12373/1 (decorative), BS 3987 (Architectural).

● Type III is what’s known in the industry as a hardcoat anodize. This type of engineered anodizing can be produced in the 13 to 150 μm (0.0005″ to 0.006″) thickness range. This electrochemical process involves sulfuric acid and low bath temperatures.

Standards for thick sulfuric anodizing are:
✔ MIL-A-8625, Type III
✔ AMS 2469
✔ ASTM B580, Type A
✔ ISO 10074
✔ MIL-A-63576
✔ AMS 2482

Hard anodized aluminum coating uses a special electrolytic process that produces a dense hard layer of aluminum oxide not only on the aluminum surface, but also inside the material, penetrating the base metal in an equal ratio to the build-up found on the surface of the finish. The coating is thick by normal anodizing standards, typically ranging from 1 to 3 mils, or more if desired.

Sulfuric acid is the most widely used solution to produce an anodized coating. The solution typically contains 180-200 grams per liter of sulfuric acid with a small amount of dissolved aluminum. The acid is then cooled to near freezing, and an electro current of up to 100 volts is applied. The current is applied until the desired oxide thickness is achieved; it can then be colored or sealed.

Thicker coatings require more process control that requires a refrigerated tank chilled to near the freezing point of water with higher voltages compared to thinner layers.

Characteristics and Features of Hardcoat Anodize
1. Increased wear resistance
2. Increased corrosion resistance
3. Improved aesthetics
4. Enhanced abrasion resistance
5. Non-conductive
6. Sterilized, non-contaminating attributes
7. Can be dyed black

Anodizing thickness increases wear resistance, corrosion resistance, and it gives the aluminum the additional features of better retention of lubricants and PTFE coatings, and it provides improved electrical and thermal insulation. The wear- and corrosion-resistant film created by hard anodizing makes this type of finish ideal for the aerospace, aviation, robotics, food manufacturing, medical, and oil, and petrochemical industries. It’s also an ideal finish for electronics and food processing equipment, as well as the military and firefighting equipment markets. And this option is often used in the manufacture of sporting goods, optical devices, and firearms. It’s also often used in machinery. Finally, since it can be sterilized and has non- contaminating properties, it’s ideal for manufacturing medical instrumentation as well.

There are also hard anodized aluminum applications for hydraulic gears, pistons, cylinders, and other mechanical devices that typically experience a lot of wear resistance.

The Benefits of Hard Anodizing

There are actually many benefits of using hard anodizing, including:

● Tolerances – The coating follows the contour of the part with uniform buildup, providing very close tolerance control. Holes 1/4 -inch or larger in diameter will show coverage. It is also very effective on parts with unusual shapes due to the excellent throwing power obtained in the hard anodizing process.
● Wear Resistance – Hard anodized coatings are more than 10 times more wear resistant than ordinary anodized aluminum. Hard anodized aluminum exhibits only half the amount of wear that cyanide case hardened steel shows after 50,000 cycles of the standard Tabor abrasion tester.
● Hardness – Microhardness tests on hard anodized aluminum typically have values of 500 to 530 VPN. Microhardness is nearly independent of coating thickness up to 1.5 mils.
● Heat Resistance – Hard anodized coated parts can withstand short exposures of temperatures up to 2,000 degrees centigrade due to the inert nature of the coating.
● Thermal Properties – Hard anodized coatings exhibit very low thermal conductivity and expansion but excellent thermal emissivity. At higher thickness, emissivity is comparable to a black body in terms of heat dissipation, offering little advantage to dying it black.
● Corrosion Resistance – Hard anodized coatings typically pass the 1000-hour 5% salt spray test (ASTM B-117).
● Electrical Properties – Hard anodized coatings are excellent electrical insulators, exhibiting electrical resistance on the same order of magnitude as glass and porcelain.

Appearance of Hardcoat Anodize by Class & Other Properties

There are two classes when it comes to hardcoat anodize appearance:

Class 1 – undyed: The color of hard coat anodize bases on the specific alloy and the anodic thickness. Some will take on dark gray or black color while others are more bronze-gray in color.

Class 2 – dyed: The most common dyed coating for hard coat anodizing is black.

Other hard anodizing properties include:
1) Color Varies from colorless to light brown on pure aluminum as coating thickness increases from1 to 5 mils. Alloys vary in color from tan to jet black, depending upon the alloy composition andcoating thickness.

2) Surface Finish roughness increases as the coating thickness increases; typical increases are 10 to 20 micro inches for wrought alloys and 50 to 100 micro inches for castings. Parts can be used “as is” or honed, lapped, or ground whenever fine finishes are required.

Aerospace Metals is proud to announce our
new location at 54 West Industrial Drive in
O’Fallon, Missouri. Our centrally located and
state-of-the-art facilities allow us to fill your
orders faster and more efficiently.
We’re equally proud to offer Hardcoat
Anodizing. If you have questions or want
further info about our processes, call us
today at (800) 398-0790 or email us at