Aluminum has become a popular alternative to steel in manufacturing because of its lighter weight and non-conductive properties. But many applications require a process called anodizing to give aluminum a stronger surface. Essentially, anodizing involves immersing aluminum in a bath of sulfuric acid called an electrolyte and passing a low-voltage electrical current through the acid solution. The result of normal anodizing is a thin layer of aluminum oxide (rust) on the surface of the original aluminum sheet. However, if the acid solution is cooled to the freezing point of water and the amount of electrical current increases substantially, the process is called hard anodizing.
Hard anodizing is more common in industrial or commercial applications than in consumer products. Some aluminum cookware can be hard anodized, but regular anodizing usually results in a durable nonstick finish that consumers love. Hard anodizing produces a thicker coating of aluminum oxide that penetrates the pores and cracks in the surface, resulting in a more uniform appearance than regular anodized aluminum. Hard anodized aluminum may have a dark brown or black finish, but other colors can also be produced.
The advantage of using hard anodized aluminum instead of stainless steel is lower overall cost and lighter weight. It is easier to machine hard anodized aluminum than to penetrate a similar block of stainless steel. Hard anodizing also produces products that are resistant to severe weather, salt spray and abrasive processes. Hard-anodized aluminum is only a few percent harder than diamond.
Both the automotive industry and the commercial cookware industry have long been supporters of hard anodizing. Nonstick coatings such as PTFE must have a reliable application method to produce a strong bond. Hard anodizing can add Teflon or other substances during electrolysis. Some auto parts also benefit from the hard anodizing process, as the finished product can be heat-resistant and non-conductive. The medical field also benefits from hard anodizing technology. The aluminum used in the prosthetic joint is hard anodized for strength and resistance to the corrosive effects of blood.
Hard anodizing shares many features with sulfuric acid anodizing, but the results of the two processes differ significantly. Hard anodizing creates a thicker aluminum oxide surface that bonds more strongly to the original aluminum layer. When shopping for new aluminum cookware, you may want to look for the description "anodized aluminum" or "hard anodized." Cookware marked "hard anodized" may last longer, but maybe a little more expensive.
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