A COMPARISON BETWEEN CORROSION RATES AND RUNOFF RATES FROM NEW AND AGED COPPER AND ZINC AS ROOFING MATERIAL
Recently there has been an increased environmental concern in several countries in Europe, in particular in Sweden and in the Netherlands regarding the amount of copper and zinc that is released from building materials into society. Due to lack of runoff data, the legislators have so far used corrosion rates measured during the last 20 years to calculate quantities of metal released from buildings assuming that the quantity of metal corrosion equals the quantity of metal runoff. With decreasing levels of environmental pollutants during the last decade in Europe, it is important to determine more recent and hence more representative corrosion and runoff rates to be used in the calculations.
For this reason a field exposure program was implemented during 48 weeks in an urban atmosphere in Sweden determining corrosion and runoff rates for copper and zinc of different age. New copper exposed for 48 weeks in the urban atmosphere shows a corrosion rate of 6.7 gm−2y−1 and an almost constant runoff rate of 1.3 gm−2y−1 during the period. The runoff rate is significantly lower than the corrosion rate and represents only a fraction (≤20%) of the total amount of corroded metal during this period. Zinc shows a gradually decreasing corrosion rate with time being 5.0 gm−2y−1 after 48 weeks of exposure. The runoff rate is relatively stable with an average rate of 3.1 gm−2y−1 during the same period. This value represents ≈60% of the total amount of corroded zinc.
The effect of panel age has been investigated in parallel field and laboratory studies. The results show that naturally aged copper exhibits somewhat higher average runoff rates (2 gm−2y−1) than new copper, probably due to a combined effect of storage and weather conditions. No significant difference in runoff rate can be found between new and naturally aged zinc. The field and laboratory investigations show that precipitation rate and amount influence the magnitude of the runoff rate for both copper and zinc.