What is zinc passivation

Under Passivation In surface technology, one understands the spontaneous emergence or targeted creation of a non-metallic protective layer on a metallic material, which prevents or greatly slows down the corrosion of the base material.

Spontaneous passivation

If bare metal is exposed to air or another corrosive environment, it depends first of all on the chemical composition of the metal whether or not it will corrode. While gold and platinum, for example, are naturally protected from corrosion due to their property as noble metals, the less noble metals such as iron, zinc and aluminum have a tendency to corrode. Whether and how quickly corrosion actually occurs also depends on the possible formation of a passivation layer. The best example of such a passivation layer is the metal chromium: Although chromium is somewhat less noble than iron in the chemical sense, it behaves almost like a noble metal when it comes to corrosion when it comes to air and water - everyone knows this effect, because the chrome-plated bathroom fittings last for decades shiny and shiny. A very thin, invisible oxide layer separates the metal from the atmosphere, so that further oxidation is only possible by diffusion through the oxide layer. The passivating layer hinders diffusion, so that further corrosion of the material is stopped.

Another example of this phenomenon is stainless steel: the chromium it contains forms a chromium oxide layer from a percentage of 12% by mass, which prevents further oxidation. If this oxide layer is damaged, bare metal comes into contact with the atmosphere and a new passivating layer is automatically formed. H. the layer is self-healing. Other technically important materials that form passive layers are aluminum, titanium, lead, zinc and silicon.

Under unfavorable conditions (halogen-containing medium, electrochemical potential, see also Pourbaix diagram), materials with a passive layer can be susceptible to pitting.

The best-known example in which spontaneous passivation does not occur is ordinary steel. The corrosion layer - the rust - consists of a rapidly growing layer of iron oxide, which does not slow down the further progress of corrosion.

Passivation process

With some metals it makes sense not to leave the creation of a passivation layer to chance, but to technically generate the passivation layer using a defined process. One such example is aluminum, but in this case one does not speak of passivation but of anodizing.

In the case of magnesium, silver, zinc and cadmium, the chromating process can be used to create a passivation layer which, in addition to improved corrosion protection, also serves as a primer for subsequent process steps, as tarnish protection (silver), as protection against fingerprints or to change the appearance (gloss, color tone ) can serve.

The chromating of zinc layers has achieved great technical importance. The passivation layer created in this way can delay the corrosion of the zinc (white rust) for a very long time. Depending on the process, the chromate layers can have the colors (weak) blue, yellow, black, olive or transparent. Depending on the process, the chromate layers can contain toxic chromium (VI). Chromium (VI) -free chromating has also been developed in recent years, but some of these do not achieve the same corrosion resistance as the chromium (VI) -containing processes. The new legislation in the EU has banned passivation with chromium (VI) for use in automotive engineering and household appliances [1], [2].

Modern replacement processes use chromium (III) or are completely chromium-free. For example, chromium-free processes use treatment solutions that contain complex zirconium or titanium fluorides. This then creates a passivation layer made of titanium or zirconium oxide.

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

Individual evidence

  1. DIRECTIVE 2000/53 / EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of September 18, 2000 on end-of-life vehicles
  2. DIRECTIVE 2002/95 / EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of January 27, 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment

Category: Materials Science