Hard anodizing is a variant of anodizing. It produces similar results to standard anodized surfaces, but its thicker and denser hard anodic film limits the coloring results to dark tones.
Unlike the regular process of sulfuric acid anodizing, hard anodizing involves a special electrolysis process that creates a thick layer of aluminum oxide both on and in the surface. This process protects metals and alloys from deterioration, which is why depending on the application it is intended for, this process is preferred.
The sulfuric method of anodizing involves a vat of sulfuric acid that a metal is dipped into while an electrical current is being sent through the vat. The combination of the acid bath and the electrical current is what changes the body chemistry of the metal. Hard anodizing simply cools the acid to the temperature of freezing water and significantly increases the electrical current running through the acid.
This process is usually utilized with aluminum. The process is easier then using regular anodizing treatments on tougher metals, and also costs less and produces a metal that is tough but lightweight. Hard anodizing allows the positive characteristics of aluminum to be utilized. These include low density, exceptional thermal conductivity and electrical resistance, and good pliability.
Hard anodizing is preferred by many industries, for a variety of applications. The automotive industry often uses the process to create internal car parts that will be non-conductive and heat resistant. Cookware used by industrial food processing plants or bakeries are often hard anodized, again because of the thickness and strength that is its trademark.
Also Teflon, which is a non-stick element popular in cookware, adheres to hard anodized metal better then it does to others. Because blood can be so corrosive of metal parts, the medical field commonly uses products put through the hard anodization process for its prosthetic joints and limbs. Another reason this type of metal is preferred is because of its appearance.
Hard anodized aluminum is less porous then aluminum anodized by sulfuric acid. Therefore aesthetically it is a smoother and sleeker end product that is more pleasing to the eye. The colors can not be as bright and vibrant though, because the pore size of the surface will not support them. Therefore, dark brown and black shades are most common, although darker shades of other colors can also be created.