Anodizing

Anodic oxidation refers to the surface treatment, an electrolytic process for the production of oxide layers on metal. The anodic oxidation represents the contrast to the cathodic reduction dar.

Method

Suitable solutions in the anodic oxidation (for example, sulfuric acid or chromic acid ) is treated electrolytically, i.e., decomposed by an electric current. Is formed on the anode surface of an oxide layer.

Looking at this process greatly simplifies the actual current elementary processes, can the present immediately after the ion discharge as atomic oxygen and thus describe chemically very reactive. It is able to oxidize certain metals, which were used as an anode material.

Applications

The layer thus produced is mainly used as a protective coating for metals against corrosion and abrasion. They are also used as electrical insulation ( dielectric) in electrolytic capacitors (tantalum, niobium, aluminum). The small thickness ( less than 1 micron ) of this oxide layer, thereby allowing one of the main advantages of electrolytic capacitors, high capacity.

Previously, the effect of anodic oxidation of some metals was also used for rectification. One type consisted of a platinum electrode and a niobium electrode, which are immersed in dilute sulfuric acid. When the niobium sheet to the anode, the current flow ceases, because non-conductive niobium oxide is formed, which is reduced to niobium metal again upon reversal of the polarization, whereby a flow of current is possible.

Although the anodic oxidation at different metals is possible, but it has acquired greater only for light metals of industrial importance, particularly for aluminum and its compounds. There, the method is also called anodizing process. " Anodized " aluminum is widely used in architecture ( building facades, doors, etc.) and in vehicle construction.

A further advantage of the method is that by the conversion, only the uppermost metal layer (up to 40 microns in thickness) are affected. Some metals (eg titanium ) initially are therefore inclined to be microporous and are easily stainable with organic dyes. You must then be subjected to a densification.

In medicine, the anodic oxidation for the disinfection of microbially contaminated water used. These substances are produced which are mikrozid and disinfect the water.

Variants

A variant is the anodic oxidation under spark discharge (English anodic spark oxidation, ANOF ). This process will not work with direct current, but a non-zero rising sawtooth voltage ramp oxidized until a spark jumps from the electrolyte to the material to be treated. This spark melting the material such as titanium, magnesium and aluminum and represented by the locally high temperatures of the discharge, a hard oxide. In the case of, for example, aluminum can be deposited without thermal treatment of the component alpha -alumina ( corundum). These layers are particularly suitable as highly chemical resistant protective coatings.