Local element
Local elements are small-area corrosion elements (or contact elements ) that can hardly be seen with the naked eye. They are significantly smaller than 1 mm 2 . Local elements can be crystallites of an alloy , which are connected to one another in an electrically conductive manner both directly and via an electrolyte and form a short-circuited galvanic cell .
Local elements can arise at points of contact between two different metals through the action of moisture , for example condensation, and often cause considerable corrosion there . Soldering points, weld seams, rivets, screw connections, damaged coatings (e.g. scratched tinplate ) and alloys are susceptible to this .
Often a local element is the cause of pitting corrosion .
Local element formation in technical zinc
Metals used in technology often contain more noble metals as impurities. Technical zinc always contains a small amount of copper . When this zinc comes into contact with water containing carbon dioxide , numerous tiny local or contact elements form on the surface between the copper impurities and the zinc. As a result, the less noble zinc dissolves. The electrons flow to the more noble copper, where oxonium ions are reduced to hydrogen molecules .
literature
- Erwin Riedel, Hans-Jürgen Meyer: General and inorganic chemistry. A textbook for students with a minor in chemistry, 11th edition, De Gruyter Verlag, Berlin / New York 2013, ISBN 978-3-11-026919-2 .
- Horst Briehl: Chemistry of Materials. 3rd edition, Springer Fachmedien, Wiesbaden 2014, ISBN 978-3-658-06225-5 .
Web links
- Teaching material , schule-bw.de.
- Chemistry for doctors: Amalgam problem with tooth fillings - Lokalelement , Johann Gasteiger, Axel Schunk, University of Erlangen-Nuremberg.
- Local element of zinc and copper - contact corrosion (accessed September 7, 2018)
- Corrosion (accessed September 7, 2018)
- Electrochemical Corrosion (accessed September 7, 2018)