White rust

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White rust is the name for corrosion products that are formed on zinc surfaces under certain conditions , for example when there is limited access to CO 2 from the air. White rust can appear after galvanizing ; it is undesirable because it reduces the corrosion protection and is unsightly.

The typical composition of the white rust is: 2 ZnCO 3 • 3 Zn (OH) 2  3 H 2 O, zinc hydroxide , little zinc oxide and very little zinc carbonate .

Formation of white rust

In air, zinc is coated with a dense, firmly adhering, dark gray oxide or carbonate layer and is therefore used as protection against corrosion. Carbon dioxide is necessary to form a dense layer , as Zn 5 (OH) 6 (CO 3 ) 2 is then formed. If there is a lack of CO 2 or under conditions containing chloride or sulphate , problems can arise, since loose and voluminous coatings then form. They do not have a defined composition, but consist of different products.

properties

The structure and properties of the protective layer depend heavily on external conditions such as humidity, temperature or the presence of anions. In general, zinc corrosion is a process of initial dissolution with subsequent formation of deposits, with intermediate Zn (I) stages also occurring. Under special circumstances, however, direct oxide formation can also occur. It is typical of zinc corrosion that several parallel and consecutive reactions occur. This leads to the formation of several different and inhomogeneous oxide and hydroxide layers, with the substrate structure playing a particularly important role, since different reactions take place at defect sites than on the zinc layer. The reaction sequence in solutions containing sulfate and chloride can be summarized as follows:

  • Zn ⇔ Zn (I) + ads + e -
  • ZnZn (I) + ads ⇒ Zn 2+ (aq) + Zn (I) + ads + e - (reaction on the surface of the Zn)
  • Zn (I) + ads ⇒ Zn (II) ads + e - ⇒ Zn 2+ (aq) ⇒ ZnO (s) (with the participation of SO 4 2− or Cl - )
  • Zn (II) ads ⇒ ZnO (s) ⇒ ZnSO 4 (s) ⇒ Zn 2+ (aq) (in sulfate)
  • ZnOH ads ⇔ ZnOH + (aq) + e -

The presence of other substances also leads to the development of further compounds. Carbon dioxide in the air causes the formation of various carbonates and hydroxycarbonates. In addition, it slows down corrosion in the presence of chlorides through the formation of Simonkolleit (Zn 5 (OH) 8 Cl 2 · H 2 O). If chloride and sulphate are present at the same time, a number of other complex products can be formed that slow down the dissolution of iron. Similar processes also occur when bases attack zinc.

Toxicity

White rust is particularly dangerous for pets in galvanized steel cages. Rodents and birds in particular can easily ingest too much zinc by licking and gnawing at the bars, which leads to life-threatening heavy metal poisoning . White rust increases this risk because it comes off more easily than a permanently galvanized surface.

For humans, white rust is probably not harmful, at least when inhaled.

The environmental product declaration “hot-dip galvanized structural steels” comes to the conclusion that if hot-dip galvanized steel is used in accordance with its intended purpose, “no effective relationships with regard to the environment and health are known”.

Avoidance

White rust can be avoided during galvanizing by chromating , organic coatings or the addition of other metals .

literature

  • A. El-Mahdy, A. Nishikata, T. Tsuru: Electrochemical corrosion monitoring of galvanized steel under cyclic wet-dry conditions . In: Corrosion Science 42, 2000, p. 183, ISSN  0010-938X
  • L. Sziráki, E. Szöcs, Zs. Pilbáth, K. Papp, E. Kálmán: Study of the initial stage of white rust formation on zinc single crystal by EIS, STM / AFM and SEM / EDS techniques . In: Electrochimica Acta 46, 2001, p. 3743, ISSN  0013-4686
  • T. Falk, J.-E. Svensson, L.-G. Johansson: The Influence of CO2 and NaCl on the Atmospheric Corrosion of Zinc . In: Journal of the Electrochemical Society 145, 1998, p. 2993, ISSN  0013-4651
  • R. Lindström, J.-E. Svensson, L.-G. Johansson: The Atmospheric Corrosion of Zinc in the Presence of NaCl . In: Journal of the Electrochemical Society 147, 2000, p. 1751, ISSN  0013-4651
  • T. Tsuru, T. Hirasaki, A. Nishikata: Corrosion Inhibition of Galvanized Steel by Corrosion Products of Zinc . In: Proceedings 15th International Corrosion Congress Granada , Sept. 2002

Individual evidence

  1. ^ Worksheets for hot-dip galvanizing from the Institute for Hot-Dip Galvanizing
  2. ↑ When the cage makes the bird sick , article on the broadcast of the WDR from August 18, 2009
  3. ^ Sibylle Schindler: Poisonings. Veterinary practice Panschwitz-Kuckau, accessed on October 14, 2015 : "Poisoning by zinc - Poisoning by metallic zinc occurs through gnawing of galvanized cage grids, zinc-containing coatings and other zinc-containing metal objects (watering bowls, food bowls)."
  4. ↑ technical specifications - technical basis Use of hot-dip galvanized components in objects and systems of the VBS .  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. (PDF) Federal Department of Defense, Civil Protection and Sport DDPS@1@ 2Template: Dead Link / www.ar.admin.ch  
  5. ↑ - Environmental product declaration, hot-dip galvanized structural steels . (PDF) Institute for Building and Environment