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Corrosion basics

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Corrosion protection methods

In the different industrial areas of application, the subject of corrosion and corrosion protection is dealt with using differing standards and naming conventions. To make things more understandable, the European Norm has provided with DIN EN ISO 8044 an uniform definition, which satisfies the need for a reasonable degree of precision and comprehensibility.

The term corrosion is understood as a process. It is a part physical, part chemical interaction between a metal and its environment that can lead to the metal’s properties changing. This can impair the function of the metal and/or of the technical system in which it is fitted.

A corrosion system typically consists of a metal or multiple metals and those parts of its environment that influence the corrosion, i.e. that promote or prevent it.

A corrosive agent is a material that when touching metal causes corrosion.

The result of corrosion of any part within the corrosion system is called corrosion manifestation. It is not necessarily damage: not every spot of rust impairs, for instance, the stability of a steel girder.

Actual impairment of a technical system is described as corrosion damage.

Corrosion protection changes corrosion systems in such a way that corrosion damage occurs much more slowly.

Experts differentiate between several kinds of corrosion. On a molecular level, corrosion is essentially of a chemical or electrochemical nature.

  • In the case of chemical corrosion, the reaction partners meet directly without the help of any electrolytes and form a bond. Being a corrosive agent, oxygen oxidises the metal and a metal oxide is created. Chemical corrosion at high temperatures is called high-temperature corrosion. Corrosive gases such as oxygen form products of the reaction on the surface of the steel. These products are described as scale.
  • Electrochemical corrosion always takes place in the presence of an electrolyte. Current gets carried here by ions. Water is such an electrolyte. Wafer-thin films of moisture are sufficient to start processes like the rusting of iron. If two different metals are connected with an electrolyte, a galvanic element is present and one of the metals corrodes.
  • As a result of air humidity, water is practically always present. Corrosion processes therefore mainly take place electrochemically. In order to influence these processes, i.e. to protect steel girders from corrosion for example, the individual reactions have to be known. Important factors here are oxidation and reduction processes, plus the different electrochemical potentials of the reaction partners involved.