Coating and Plating Processes - Hard Anodizing

Hard Anodizing differs from regular anodizing in the surface hardness and wear characteristics imparted.

Essentially, regular anodizing involves immersing aluminum in an electrolyte bath of sulfuric acid, and running a low-voltage electric current through the acid solution to produce a thin coating of aluminium oxide. 

In hard anodizing the acid solution is cooled to the freezing point of water and the amount of electric current increased substantially. Hard anodizing 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. 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.

Because of its lighter weight and conductivity, anodized aluminium 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. 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 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.

Hard anodizing is commonly used in aerospace components to provide an abrasive and wear resistant surface. Companies who carry out hard anodizing for aerospace components must be certified to do so.

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.