The Runoff Process
Metal runoff is the fraction of oxidized metals within the patina that by the action of rainwater can be released from a metal or metal alloy surface.
The runoff process
By the action of impinging precipitation on exposed copper-based surfaces, a small fraction of oxidized metal in the patina will be released from the surface and dispersed into the environment via dewatering and stormwater systems. This process is very slow and has a marginal influence on the patina that strongly adheres to the surface.
The amount ot released metal
The amount of copper that is released per surface area and time from the patina is denoted copper runoff or release rate. The amount of metal released into the runoff water is strongly dependent on several parameters such as the amount, duration and intensity of precipitation, dry and wet deposition of pollutants, wind direction, time of wetness and surface orientation, geometry and inclination from horizon to name a few.
Affected by rain quantity and intensity
Large volumes of precipitation during a given time interval generally result in lower released metal concentrations compared with small precipitation volumes during the same time period. These processes are for instance highly rain intensity specific and dependent on prevailing environmental conditions prior to and in-between different rain events.
- Critical review: Copper runoff from outdoor copper surfaces at atmospheric conditions, Y.S. Hedberg, J.F. Hedberg, G. Herting, S. Goidanich, I. Odnevall Wallinder, Environmental Science and Technology, 48, 1372−1381 (2014)
- Surface-rain interactions: Differences in copper runoff for copper sheet of different inclination, orientation, and atmospheric exposure conditions, Y. S. Hedberg, S.Goidanich, G. Herting, I. Odnevall Wallinder, Environmental Pollution, 196, 363-370, (2015)
Important aspects to consider
The total runoff quantity is not a measure of any risk. Risk analysis requires knowledge of the chemical speciation of released copper and its changes upon environmental entry.
The runoff quantity is different from the corrosion rate, which is a measure of the total amount of metals that is oxidized per surface area and time unit and that forms corrosion products within the patina.
The corrosion products of the patina are poorly soluble and should not be compared with the properties of water soluble copper salts.
Only a small fraction of copper is released from the patina and its effect on the patina properties is minor.
Solid surfaces such as concrete in pavement and storm water pipes and soil act as efficient sinks for released copper and change its chemical form.
Dilution of the released total metal fraction (concentration) will take place during environmental entry.