You may wonder what Hard Coat Anodizing is and why is it so well favored in the anodizing industry?

There are 3 types of aluminum anodizing. Hardcoat anodizing otherwise known as type 3 anodizing or hard anodizing had its commercial introduction in the 1950s. This anodic film gives aluminum parts superior abrasion and corrosion resistance. Additionally, the purpose is to develop and increase a surface’s solidity and enhance the thermal and dialectical assets of treated aluminum parts. All while retaining the numerous advantages inherent to aluminum including the ability to retain lubricants and PTFE coatings, as well as 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 is also an ideal finish for electronics and food processing equipment, in addition to the military and firefighting equipment markets. This option is often used in the manufacture of sporting goods, optical devices, machinery and firearms.


The hard anodized coating is aluminum oxide formed by the electrochemical reaction of oxygen and aluminum.

The aluminum part is racked to make it the anode and is then immersed in a sulfuric acid solution. Air is bubbled through the acid to accommodate for the necessary ventilation. Air Bubble agitation is the most common method to circulate the sulfuric acid solution. Pumping is another option for circulation to occur.

Circulation assists in cooling and guiding fresh acid to the surface of the part. The oxygen for the anodizing reaction comes from the sulfuric acid bath. The oxygen merges with the aluminum constructing an oxide growth that is chemically bonded to the underlying substrate that is readily measurable. For example, in the case of a .002” coating, a growth of .001” will be visibly experienced on top of the original surface. The other .001” growth is located below the original surface of the part and is commonly known as the penetration. This coating is thick and can range between 1 and 3 mills – or higher if desired.

The thicker the coating, the more control necessary for the process to operate smoothly and efficiently. A refrigerated tank chilled to the near freezing point of water with higher voltages would also necessary. Once the solution has reached the desired temperature, it is then electrified with currents up to 100 volts. The volts are applied until the selected oxide thickness is achieved and is then colored or sealed.

Finally, since it can be sterilized and has non-contaminating properties, this process is 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.

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

Class 1- undyed. The color of hard coat anodize is determined by the specific alloy and the anodic thickness. Some will appear as dark gray or black color while others are more bronze-gray in color.

Class 2 – dyed. The most common color in which hard coat anodizing is dyed is black.

Color may vary from colorless to light brown on pure aluminum as the coating thickness increases from 1 to 5 mils. Alloys vary in color from tan to jet black, depending upon the alloy composition and coating thickness. The surface finish roughness also 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.

The Benefits of HardCoat Anodizing Broken Down:

Corrosion resistance: Hard anodized coatings typically pass the 1000-hour 5% salt spray test (ASTM B-117).

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.

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.

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.

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.

Electrical properties: Hard anodized coatings are excellent electrical insulators, exhibiting electrical resistance on the same order of magnitude as glass and porcelain.

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.

Aerospace Metals LLC specializes in Hardcoat Anodizing, as well as Bright Dip Anodizing, CNC Machining, CNC Turning and Milling, metal finishing, plating, and sheet metal fabrication for the automotive, aerospace and defense, industrial machinery, medical, robotics, energy, and R&D industries. With 30+ years of experience, Paul has helped clients achieve their metal finishing application goals by using a customized approach and a network of professionals who have the equipment and the capability to produce high-quality metal products to exact specifications. Visit our website at High-Quality Metal Finishing Services – Aerospace Metals ( to get a free, no obligation quote today!