Wet storage stain

White rust is the name of the corrosion products formed on zinc surfaces under certain conditions, such as limited access of CO2 from the air. White rust often occurs after galvanizing; here he is undesirable since it reduces the corrosion protection, unsightly and is partially harmful.

The typical composition of the white rust as follows: 2 ZnCO3 • 3 Zn (OH) 2 3 H 2 O, zinc hydroxide, zinc oxide, and little very little zinc carbonate.

Formation of white rust

Zinc becomes covered in air with a dense adherent dark gray oxide or carbonate and is therefore used as corrosion protection. To form a dense layer of carbon dioxide is necessary, because if Zn5 (OH ) 6 ( CO3) 2 is formed. For lack of CO2 or chloride or sulfate-containing conditions but problems can occur because then form loose and voluminous coats. You have no defined composition, but consist of different products.

Properties

The structure and properties of the protective layer are strongly dependent on the external conditions, such as humidity, temperature or anions present. In general, the zinc corrosion is a process of initial dissolution followed by precipitation, whereby Zn (I ) intermediates occur. Under special circumstances, but it can also direct oxide formation occur. It is typical for zinc corrosion that several parallel and consecutive reactions. Leading to the formation of several different and inhomogeneous oxide and hydroxide, wherein in particular the substrate structure plays a role because of defects, other reactions take place as in the zinc layer. The reaction sequence in sulfate and chloride- containing solutions can be summarized as follows:

  • Zn ⇔ Zn ( I) ads e -
  • ZnZn (I ) Ads ⇒ Zn 2 (aq) Zn (I ) ads D ( reaction on the surface of Zn)
  • Zn ( I) ads ⇒ Zn ( II) ads e - ⇒ Zn2 ( aq) ⇒ ZnO ( s ) (with the participation of SO42 - or Cl - )
  • Zn ( II) ads ⇒ ZnO ( s ) ⇒ ZnSO4 ( s ) ⇒ Zn2 ( aq) ( in sulfate)
  • ZnOHads ⇔ ZnOH ( aq) e -

The presence of other substances also leads to the development of other compounds. Carbon dioxide in air results in the formation of various carbonates, and hydroxycarbonates. In addition, it slows down the corrosion in the presence of chlorides via formation of Simonkolleit ( Zn5 (OH) 8Cl2 · H2O). In the simultaneous presence of chloride and sulfate a number of other complicated constructed products can be created that slow dissolution of iron. Similar processes occur on zinc also the attack of bases.

Toxicity

White rust is a danger especially for pets in cages made ​​of galvanized steel. By licking and gnawing at the grids can easily take too much zinc, especially rodents and birds, which leads to a life-threatening heavy metal poisoning. White rust increases this risk because it is easier to solve than a fixed galvanized surface.

Human white rust is at least during the inhalation probably not harmful.

The Environmental Product Declaration " Hot dip galvanized structural steel " comes to the conclusion that "no effect relationships relating to the environment and health are known " in - use fair use of galvanized steel.

Avoidance

By chromating, organic coatings or alloying with other metals white rust can be avoided in the galvanizing.

Heavy metal (chemistry)
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