Corrosion handbook

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The important variables for atmospheric corrosion are:
Temperature
General influence
An increase in temperature leads to an increase in the rate of chemical
reaction and therefore also an increase in the corrosion rate. This is
especially true at constant relative humidity levels.
Additional information
On the other hand, increasing temperature facilitates the drying of wet
surfaces and can slow down corrosion rates. At temperatures below
freezing point corrosion is negligible. The influence of the temperature on
corrosion can therefore go in both directions.
Examples
For similar environments such as maritime regions, which usually
show high humidity levels, an increase in the average temperature also
accelerates the corrosion rate. As a result, coastal and offshore areas in
tropical climates result in much higher corrosivity than similar zones in
colder regions.
Humidity
General influence Atmospheric corrosion only takes place when a moisture film is present
on the metal surface. In the absence of humidity, most contaminants
would have little or no corrosive effect. The period in which a moisture
film is present is also called time of wetness. As a commonly used rule
of thumb according to the ISO 9223:1998 standard, the time of wetness
is defined as the periods in which the relative humidity exceeds 80 %.
This is only a rough estimation since the formation of a moisture film
on the surface also depends on the presence of hygroscopic salts
(e.g. corrosion products or salt deposits). Condensation in maritime
environments with chlorides will therefore take place at lower levels of
relative humidity. Calculation of corrosion rates according to the latest
ISO 9223:2012 standard takes into account only the mean relative
humidity measured in one year (see 5.1).
Additional information Water in the form of rain does not always increase corrosivity. It may
even have a beneficial effect by washing away chlorides and pollutants.
However, in usually dry and mildly corrosive environments, rainwater can
provide the moisture necessary to trigger a corrosion reaction.
Examples In rural/urban atmospheres, sheltered parts usually show less corrosion
than the ones exposed to rain. On the other hand, the lack of the washing
effect of the rain in coastal areas often leads to stronger corrosion.
Chlorides
General influence Atmospheric salinity distinctly increases corrosion rates.
Chlorides have multiple detrimental effects on the corrosion of metals. In
particular these are:
• Decrease in the saturation humidity: the presence of salts facilitates
condensation at lower relative humidity values. This leads to longer
periods with wet metal surfaces
• Formation of soluble corrosion products: the dissolved metal ions
form metal chlorides, which usually do not provide enough protection
from further corrosion.
• Destruction of passive films: chlorides attack the oxide films formed
on passive metals such as stainless steel and aluminum
Additional information The corrosion behavior of a particular metal in atmospheres containing
chloride strongly depends on its ability to form stable and insoluble
corrosion products together with the chlorides present. For example, this
is the case with zinc, which explains the much lower corrosion rate of
zinc compared to steel.
Examples In maritime atmospheres the main source of chlorides is the seawater.
It contains mainly sodium chloride (more than 90 % of the salt),
accompanied by calcium and magnesium chlorides. The main source
of anthropogenic chlorides is the use of de-icing salts on roads during
winter time.