# Corrosion element

A corrosion element is a structure in a component that behaves like a short-circuited galvanic cell and leads to corrosion of the material.

## variants

Corrosion elements differ in the structure of the components that act as anode , cathode and electrolyte . What they have in common is the electrical arrangement: anode and cathode are connected to one another in an electrically conductive manner, both via the electrolyte and through direct contact. An external voltage source is only involved in the last case:

### Selective corrosion

Selective corrosion occurs on crystallites in an alloy structure, which consist of compounds with different electrochemical potential , e.g. B. copper and zinc crystallites in brass , which react with each other on the surface via a water film. In the case of selective corrosion, the corrosion attack runs preferentially (selectively) along certain structural areas of the material. According to the area of ​​the destroyed structure one differentiates:

• Intergranular corrosion, if the destruction runs along the grain boundaries,
• Transcrystalline corrosion when it runs through or comes into contact with the grains.

Since selective corrosion occurs in the grain size range, it cannot be seen with the naked eye and is therefore particularly dangerous. Signs: Steps on the corrosion layer (bluish visible under a magnifying glass).

Corrosion element ( local element ) made of iron and copper

### Contact corrosion

Contact corrosion occurs when two metals with different potential for dissolution are conductively connected by an electrolyte (water, moist air ...). The less noble metal becomes the anode and the nobler metal becomes the cathode. This additional polarization leads to an accelerated dissolution of the anode.

Examples: screw made of copper in an aluminum sheet, stainless steel sheet screwed to steel sheet.

The extent of contact corrosion is minimized if:

• the potential difference assumes a minimum,
• the metals involved form corrosion-inhibiting cover layers,
• the conductivity of the electrolyte is minimal.

The area ratios of the electrodes influence the current densities and thus also the corrosion rate. With the widespread oxygen corrosion, the cathodic conversion (and thus also the cathodic current density) is limited by diffusion and convection processes. Minimum anodic dissolution current densities can therefore be achieved using the following area ratio:

${\ displaystyle {\ frac {A_ {Anode}} {A_ {Cathode}}} \ gg 1}$

Since contact corrosion usually only has a short range (<5 mm), the area ratios are of secondary importance.

Model experiment on corrosion due to different concentrations

### Concentration element

This consists of a metal surface, wetted by an electrolyte with locally different concentrations, e.g. B. in a gap . The oxidation of the metal takes place in the area with the lower concentration of the electrolyte, the reduction in the area with the higher concentration.

A ventilation element is a concentration element in which the oxygen content in the electrolyte varies.

### Mechanical stress

A homogeneous structure that has been deformed locally or is locally under tension favors corrosion. Examples: stress corrosion cracking , vibration cracking corrosion .

### External stress corrosion

External stress corrosion occurs when there is a potential difference in the material caused by external voltage sources. Example: metal pipes, laid near electrical DC voltage lines (see also sacrificial anode ).

## Chemical reaction

Generally, the anode will oxidize and dissolve. The reactions at the cathode depend, among other things, on the pH value and the oxygen concentration. e - denotes electrons, H + denotes protons, and Me denotes metal atoms or ions.

Anode reaction:

Me ignoble → Me ignoble + + e -

Cathode reaction:

(1) If there are metal ions that are more noble than those of the anode reaction, they are deposited on the cathode:
Me edel + + e - → Me edel
(2) In an acidic environment (pH value <5) hydrogen is formed:
2 H + + 2 e - → H 2
(3) In an acidic environment, water is created when oxygen is present:
O 2 + 4 H + + 4 e - → 2 H 2 O
(4) In an alkaline environment (pH value> 7) water reacts to form hydroxide:
2 H 2 O + 2 e - → H 2 + 2 OH -
(5) As (4), in the presence of oxygen:
O 2 + 2 H 2 O + 4 e - → 4 OH -

## literature

• Chemical Industry Fund: Corrosion / Corrosion Protection. Slide series and text booklet No. 8, Frankfurt am Main 1994, DNB 948212381 .
• Elsbeth Wendler-Kalsch, Hubert Graefen: Corrosion damage theory . Springer Verlag, Berlin / Heidelberg 2012, ISBN 978-3-642-30431-6 .
• Ulrich Bette, Markus Büchler: Pocket book for cathodic corrosion protection. 8th edition, Vulkan-Verlag GmbH, Essen 2010, ISBN 978-3-8027-2556-2