Because of its lighter weight and non-conductivity, anodized aluminum has become a popular alternative to steel in manufacturing circles. But many applications call for 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 running a low-voltage electric current through the acid solution. The result of normal anodizing is a thin coating of aluminum oxide (rust) on the surface of the original aluminum sheet. If the acid solution is cooled to the freezing point of water and the amount of electric current increased substantially, however, the process is called hard anodizing.

Hard anodizing is more commonly found in industrial or commercial applications than in consumer products. Some aluminum cookware may be processed through hard anodizing, but regular anodizing generally produces the durable non-stick coating preferred by consumers. Hard anodizing produces a much thicker coating of aluminum oxide, penetrating holes and fissures in the surface to create a more uniform appearance than regular anodized aluminum. Aluminum sheets processed by hard anodizing may have a dark brown or black surface, but other colors can be created.

The benefits of hard anodizing aluminum instead of using stainless steel are lower overall cost and weight. Machining hard anodized aluminum is easier than penetrating a similar block of stainless steel. Hard anodizing also yields a product resistant to harsh weather, salt sprays and abrasive machining processes. Aluminum processed by hard anodizing can be only a few points away from the hardness of diamonds.

The automobile industry and the commercial cookware industry have both been long-time proponents of hard anodizing. Nonstick coatings such as Teflon® must have a reliable method for application which can create a strong adhesive bond. Hard anodizing can incorporate Teflon® or other substances during the electrolytic process. Some automotive parts also benefit from the hard anodizing process, since the finished product can withstand heat and is non-conductive. The medical field has also benefited from hard anodizing technology. The aluminum used in prosthetic joints is hard anodized for additional strength and resistance to the corrosive quality of blood.

Hard anodizing shares many characteristics with sulfuric anodizing, but the two processes have clearly different results. Hard anodizing creates a much thicker aluminum oxide surface with a stronger bond to the original aluminum layer. When shopping for new aluminum cookware, it may pay to look for the descriptions 'anodized aluminum' or 'hard anodized.' The cookware marked 'hard anodized' may prove to be more durable, though possibly a little more expensive.