Sulfur dioxide, a product of the combustion of sulfur containing fossil fuels, plays an important role in atmospheric corrosion in urban and industrial type atmospheres. It is adsorbed on metal surfaces, has a high solubility in water and tends to form sulfuric acid (acid rain) in the presence of moisture films. Sulfate ions are formed in the surface moisture layer by the oxidation of sulfur dioxide and their formation is considered to be the main corrosion accelerating effect from sulfur dioxide. The acidification of the electrolyte could arguably also lead to accelerated corrosion attack but this effect is likely to be of secondary importance due to buffering effects of hydroxide corrosion products.
Sulfur dioxide may be expressed either in terms of a deposition rate or an airborne concentration. Either measure is to be made in accordance with ISO 9225. The units used for the sulfur dioxide categories in the ISO 9223 are as sulfate deposition (SD) rate in mg m-2 day-1:
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SD <= 10 |
P0 |
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11 < SD <= 35 |
P1 |
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36 < SD <= 80 |
P2 |
|
81 < SD <= 200 |
P3 |
