Hard anodizing aluminum makes aluminum retain the basic advantages of aluminum while providing greatly improved wear, heat resistance, and corrosion resistance.
The number of applications for hard anodizing continues to grow in the appliance, aerospace, food processing, military, and fire fighting equipment markets. There are also hard anodized aluminum applications for cylinders, hydraulic gears, pistons and other mechanical devices experiencing considerable wear resistance.
The Process: Hard Anodized Aluminum Coatings
Hard anodized aluminum coating uses a special electrolytic process that produces a dense hard layer of aluminum oxide both on and in the aluminum surface. The coating is thick by normal anodizing standards, ranging typically from 1 to 3 mils, or more if desired. Hard anodizing penetrates the base metal in a ratio equal to the build up on the surface.
Other hard anodizing properties include:
- Color Varies from colorless to light brown on pure aluminum as 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.
- Surface Finish Surface 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 where fine finishes are required.
The Benefits of Hard Anodizing
The benefits of using hard anodizing include:
- Tolerances The coating follows the contour of the part with uniform buildup, providing very close tolerance control. Holes 1/4 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 typical 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 2000 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.
White Paper: Hard Anodizing
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