Passivation (chemistry)

Under passivation is understood in surface technology, the spontaneous emergence or deliberate creation of a protective layer on a metallic material, which prevents the corrosion of the base material or greatly slowed.

Spontaneous passivation

Is bare metal of the air or other corrosive environment exposed to, it will depend on the chemical nature of the metal, if it comes to corrosion. For example, while gold and platinum only very slowly corroded by its status as a precious metal, the less noble metals such as iron, zinc and aluminum have a tendency to corrosion. Whether and how fast it comes to corrosion, depends also on the possible emergence of a passivation layer. The best example of such a passivation layer is the metal chromium: Although chromium is less noble than iron in the chemical sense, it behaves in the corrosion to air and water almost like a precious metal - you know this effect with chrome-plated bathroom fixtures, which for decades bright and shiny. remain A very thin, invisible oxide film [for stainless steels of the order of 10 nm (about 50 atomic layers in pure chromium 5 layers ) ] separating the metal from the atmosphere, so that further oxidation is possible only through diffusion through the oxide layer. The passivating layer impedes the diffusion, so that a further corrosion of the material is stopped.

Another example of this phenomenon is stainless steel: The chromium contained is from 12 % by weight of a chromium oxide layer, thereby further oxidation is prevented. If this oxide layer is damaged, bare metal comes into contact with the atmosphere and it automatically creates a new passivating layer, that is, the layer is self-healing. Another industrially important materials, which form passive layers, are aluminum, nickel, titanium, lead, zinc and silicon.

Under unfavorable conditions (halogen- containing medium, electrochemical potential comparisons, also Pourbaix diagram ) can materials with passive film pitting are vulnerable.

The best-known example in which there is no spontaneous passivation is usually steel. The corrosion layer - the rust - consists of a fast-growing layer of iron oxide, which does not slow down the further progress of corrosion.

To calculate whether the oxide layer has protective or non-protective nature, the Pilling - Bedworth ratio can be used.

Passivation

For some metals, it is advisable not to leave the formation of a passivation layer to chance, but to create the passivation layer technically a defined process. One such example is aluminum, it is called in this case, but not by passivation but by anodizing.

For magnesium, silver, zinc and cadmium can be explained by the method of chromate produce, which in addition to the improved corrosion protection as a primer for subsequent process steps to prevent tarnishing (silver), as protection against fingerprints, or to change the appearance ( gloss, color shades, a passivation ) can be used.

A great industrial importance, chromating of zinc coatings. The passivation layer thus produced can delay the corrosion of zinc (white rust ) very long. The passivation can be blue, yellow, black, olive or transparent depending on the process have the colors (weak). Depending on the process, the passivation can toxic chromium ( VI) oxide. In recent years, and chromium ( VI)-free conversion coatings have been developed, but which in some cases can not reach the same level of corrosion resistance such as chromium (VI) - containing processes. The new legislation in the EU, the passivation with chromium ( VI) for use in the automotive industry is (passenger cars <3.5 t) and have been banned in household appliances.

Modern replacement method using chromium ( III) or are entirely chrome free. For chromium-free process, for example, treatment solutions may be used which contain complex fluorides of titanium or zirconium. This then creates a passivating layer of titanium or zirconium oxide.

The relatively new method of thick film passivation of zinc coatings combines the advantages of chromium ( VI) - free and good to very good corrosion resistance.

In semiconductor industry, silicon is a widely used material which is readily oxidized and loses some of its desired positive electrical properties. Up to now commonly used silicon to passivate the surface. This process takes place in a vacuum chamber, which is heated to 400 ° C. This relatively high temperature results in that not all the other suitable materials for the passivation process are eligible, and the products are produced relatively cost-intensive. Scientists at MIT are developing a new method to allow the passivation at room temperature. Also in a vacuum chamber is positioned over the Heizdrahtgeflecht silicon, which is heated to about 300 ° C. The injected polymer material in the vacuum chamber in the vicinity of the heating wires is evaporated and condensed on the surface of the silicon. This vapor phase deposition method is to have the advantage that the energy consumption is reduced drastically, different materials can be used for passivation and the production cost can be significantly reduced. Since the silicon itself is not heated above 20 ° C during this process should be realized in the production of solar cells a much better overall efficiency.