Anodizing and hard coating are two methods used to change the surface of metal for a variety of reasons. Both processes are commonly used on machined parts used for nearly any purpose.
Hard coat is an electrochemical process that places an aluminum part into an electrolytic bath and applying electrical current in the bath and colored dyes can be added to the bath to give the aluminum part a desired vibrant color. This process results in an oxide coating on the aluminum part. The coating is harder than steel and scratch resistant.
Anodizing is an electrochemical process used on metals, such as aluminum, magnesium and titanium. This process is accomplished by placing metal parts and a cathode into an acid electrolyte bath and passing electric current through the liquid. Anodizing alters the surface of the metal by using a highly controlled method of oxidation and the results leave the surface of the metal porous so it can easily accept other processes, such as coloring and sealing.
A couple of major differences exist between hard coating and anodizing. First, anodizing alters the surface of the metal while hard coating is added to the metal, increasing the thickness of the part. When designing and machining parts that will be hard coated, one must also take into account the fact that hard coating will alter the depth and diameter of holes and will have differing effects on blind holes than it does on through holes. Threaded parts will also be affected.