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The DIN standard DIN EN ISO 8044 (formerly DIN 50900) defines corrosion as the reaction of a metal material with its environment, which has the effect of a measurable alteration in the material and which can lead to an impairment of the function of a metal component or the whole system.
In most cases a corrosive environmental atmosphere (high levels of humidity or also aerosols) and corrosive fluids (salt water, chemical compounds) are the factors which cause corrosion. But in some cases corrosion (rust) also occurs through the formation of a galvanised element (electric current with different metals in the presence of an electrolyte). In addition to these two main types, a wide number of additional causes of corrosion can be identified: starting with phase differences, i.e. phase boundaries or concentration differences in holes, cracks, deposits or particles right up to static and dynamic tensile loads or also their direct contact with different metals, right up to the absorption of hydrogen, phase inclusions and inhomogeneities in the material.
The main requirement for atmospheric corrosion is a film of moisture (rain, dew, condensation or also hygroscopic salts – i.e. electrolytes) on the surface of the metal. The mechanism which then occurs is the same as in electrochemical solutions – however there must always be good access for oxygen. Thin films of fluid therefore create optimum conditions for corrosion. Additional factors which can be mentioned are the relative humidity of 80 % and a temperature of over 0 °C. And last but not least, the speed of corrosion is dependent on the time of wetness and the pH value of the film of moisture.
In addition to increased temperature, acidification and oxidizing agents accelerate corrosion. For this reason a greater level of corrosion and faster corrosion can be expected in connection with reactive air contamination in the atmosphere (SO2, NOx, etc.) and/or hygroscopic salts, even with humidity of much less than 80 %! Furthermore the microclimate (cracks, rear-ventilated facades, tunnels, …) is an additional accelerator of corrosion, as the occurring concentration of pollutants (mainly chloride) causes an increase in corrosiveness.