| Run-off |
| Zinc run-off is the soluble part of the patina washed off by rainwater. |
Various atmospheric pollutants that attack the patina cause zinc run-off.
| Stage 1 |
Stage 2 |
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| Attacks on the patina by various atmospheric pollutants form zinc salts with variable solubility in rainwater. For example, attack by sulphur dioxide (SO2) forms zinc sulphate (ZnSO3 + ZnSO4) and attack by chloride (Cl) forms zinc chloride (among others ZnCl2.4Zn(OH)2 + Zn3OCl4). Zinc sulphate is more soluble than zinc chloride. |
When it rains, only the products which are soluble in rainwater (starting with the most soluble) are detached and washed off, releasing part of the zinc into the environment. This is run-off. |
Among the various atmospheric pollutants, SO2 has the greatest impact on the zinc run-off rate, because when SO2 is present in the atmosphere, there is at the same time, an increase in the corrosion rate of zinc, and an increase in the formation of rainwater soluble zinc sulphate due to SO2 attacking the patina.
Sulphur dioxide is produced by some industrial plants, oil-fired central heating installations and road traffic. As a result, the run-off rate is greater in an urban or industrialised area than in a rural environment.
Fortunately, since the 1970's, SO2 pollution of the atmosphere has been considered as a major environmental problem. Therefore, European and other legislators have reinforced legislation against this pollution. This has resulted in a general reduction of the concentration of SO2 in the atmosphere and an even more drastic reduction in particularly polluted sites such as cities or industrial areas.
As a result, a reduction in zinc run-off into surface water has been observed over recent decades.
The average run-off rate in Europe is officially estimated at about 0.3 and 0.4(1) µm/year.
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