Corrosion handbook

30 06/21
Sulfur dioxide
General influence Of all the atmospheric contaminants originating from industrial processes
such as fuel combustion and metal smelting, sulfur dioxide is the most
important one in terms of concentration and its effect on corrosion
rates. Sulfur dioxide gas in the atmosphere acidifies the electrolyte on
the surface and leads to the formation of soluble corrosion products.
Corrosion rates are thus increased on many metals, e.g. zinc, steel,
aluminum and stainless steel.
Additional information Sulfur dioxide emissions are on the decline in large parts of the
industrialized world and have reached insignificant levels in many urban
and even industrial areas (below 10 µg/m
3
or even lower). However,
heavily polluted hot spots around the world still exist, where an increase
in corrosion due to SO
2 has to be considered when it comes to material
selection.
Examples The ISO 9223:2012 standard classes atmospheres with SO
2
concentrations of over 50 µg/m
3
(yearly average) as strongly
contaminated environments.
4.2 Assessment of corrosivity for zinc and ZM coated
products
The following section describes a way of helping to assess and estimating the
expected corrosion in a given environment for zinc-coated and ZM coated
products. It has to be noted that this only applies to pure atmospheric corrosion
where the item is fully exposed to the weather. Applications where, for example,
the item is in contact with soil, immersed in sea water or positioned in the splash
zone are excluded, as are additional effects resulting from galvanic corrosion,
erosion or exposure to chemical substances.
Estimation of the corrosivity of the application can be carried out in two different
ways according to the ISO 9223:2012 standard (see Fig. 37).
1. Corrosivity estimation using the classification of ISO 9223:1998 or the dose-
response function given in the new version of this standard (ISO 9224:2012).
2. By conducting one-year exposure tests of the material/product to be used and
measuring the corrosion rate.
For most projects it is not feasible to conduct one-year exposure tests and
therefore estimation becomes the most frequently used approach. For the
estimation approach, the main climatic and environmental parameters previously
described are required as input. These are, in particular, the mean temperature,
mean relative humidity, chloride deposition and SO
2 concentration.
Data on atmospheric
parameters
(humidity, SO
2
etc.)
Exposure tests
A
lgorithms
(e.g. ISO 9223)
Data evaluation
Corrosion measurements
Corrosion classification
Atmospheric
environment
1
2
Fig. 37: Methods for assessment of
atmospheric corrosivity.