|Hardcoat Anodizing, also known as "Type III anodizing", is an electrochemical procedure that creates a controlled oxide film on the surface of aluminium. Hardcoat Anodizing utilizes sulfuric acid, high voltage, and low bath temperatures to produce a very high surface hardness (60-70 Rockwell C).
Hardcoat Anodizing is a combination of corrosion, abrasion, and wear resistant coating. There are different shades of coating available varies from light to dark grey, depending upon the alloy makeup of the aluminium substrate, and it can also be zed black. Parts are available in multiple finishes such as Hard Coat/Electroless Nickel Plating or Hard Coat/Chem Film.
Basic Hardcoat Anodizing Properties
The hard coat anodizing process results in a surface that is both smooth and durable. It provides the substrate metal surface with a coating that is:
1. Resistant to corrosion – both marine and atmospheric
2. Wear resilient
3. Abrasion resistant
4. Compatible with many adhesives
5. Excellent lubricity
6. Resistant to UV light spectrum
7. Non-conductive insulating properties
8. Electrically resistant
9. Provides a neutral barrier between two dissimilar metals to prevent galvanic corrosion
The benefits of using hard anodizing include:
1. Tolerances- The coating spread all over the surface uniformly. Holes with 1/4 or larger in diameter cover up automatically. It is also very effective to the parts without any shapes, due to the excellent throwing power include in the hard anodizing process.
2. Heat Resistance- Hard anodized coated parts can hold off short exposures of temperatures up to 2000 degrees centigrade due to the inert nature of the coating.
3. Wear Resistance- Hard anodized coating is 10 times more wear resistant than ordinary anodized aluminium. Hard anodized aluminium displays only half the amount of wear that cyanide case hardened steel shows after 50,000 cycles of the standard Tabor abrasion tester.
4. Hardness- For the hard anodized aluminium, Microhardness tests have values of 500 to 530 VPN. Microhardness is nearly independent of coating thickness up to 1.5 mils.
5. Thermal Properties- Hard anodized coatings provide very low thermal conductivity and also low expansion but excellent thermal emissivity. At higher thickness, emissivity is comparable to a black body in terms of heat resistance, gives little advantage to dying it black.
6. Corrosion Resistance- Hard anodized coatings can pass several tests. On it, this is can also pass through the 1000-hour 5% salt spray test (ASTM B-117).
7. Electrical Properties- Hard anodized coatings are electrical resistant. As an electrical resistance, it works on the same order of magnitude as glass and porcelain.
During hard coat anodizing, an aluminium oxide layer formed uniformly on the surface that penetrates into the material. The hard coat anodizing process needs submerging the part(s) in the sulfuric acid for 32°F. The density of the acid should be 23 to 37 amps, for 20 minutes to two hours. The amount of time spent on the submerged period determines the anodizing coating.
Aluminium naturally forms the oxide on its surface. But the aluminium oxide which is formed additionally on the surface of the metal during the hard coat anodizing process is different. It is more uniform, its density is high and much harder than the natural oxidation.
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