Definitions background and experimental approaches

Metals and metal containing products are used in large amounts in buildings and in constructions making up the infrastructure of modern society. Among the frequently used metals in building applications, such as building shells, roofing materials and fencing, are copper (in pure form or as a copper-based alloy), zinc (mostly as a plating on steel), and chromium and nickel (as alloying elements of stainless steel). Their dispersion is caused by atmospheric corrosion and the subsequent precipitation-induced release (the "runoff") process (Odnevall Wallinder and Leygraf, 2002).

Before looking closer at these processes, it may be pertinent to define some of the key concepts in this field:

Corrosion rate is the amount of a metal (g) that corrodes per surface area unit (m2) and time unit (year). The rate is given in g/m2/y. Part of the corroded metal will precipitate and remain on the metal surface as a corrosion product, whereas another part will run off during precipitation events. The runoff rate is the amount of metal (g) that is released from the corrosion product per surface area unit (m2) and time unit (y).

Corrosion and runoff are generally governed by completely different physical, chemical and electrochemical processes. It follows that corrosion and runoff proceed independently of each other and with rates that usually are not equal (Odnevall Wallinder and Leygraf, 1997). Differences between corrosion rate and runoff rate can mainly be attributed to successive accumulation - or reduction - of metal in the corrosion product. The metal released with the runoff occurs in a chemical form (as a chemical species)

with specific reactivity, mobility and bioavailability, depending on the ambient chemical and physical environment. In order to determine the environmental hazard of metals released as a result of corrosion and runoff from metal coatings and metallic materials used on buildings and constructions, it is obviously of great importance to carefully examine the properties (speciation) and the fate of the metal during its successive transport away from the original site of release. Such a holistic approach was chosen by the team of researchers at the Division of Corroson Science, KTH, Stockholm, under the leadership of C. Leygraf and I. Odnevall Wallinder (op. cit., 2002).

Although the rates of corrosion and degradation of different materials have been studied extensively for many years, systematic investigations of runoff from metallic surfaces have been rare. The first scientifically well-planned runoff studies in Sweden were implemented by the KTH team in 1995, and have ever since been continued to cover all relevant materials and various aspects of the problem (Odnevall Wallinder and Leygraf, 2002). As a parallel effort, studies of metal runoff from about a dozen different common roofing and facing materials (including various paints) were conducted in 1997-98 after both short-term and long-term (up to 1 year) outdoor exposures in the Stockholm atmosphere by the Swedish Corroson Institute (Persson and Kucera, 2001). Thus, a considerable amount of data has been collected over the past few years on the corrosion - and particularly on the runoff - process, together with detailed examination of the fate, bioavailability and ecotoxicity of the released metals. In fact, several academic theses have already resulted from this work (He, 2000; 2002; Karlen, 2001).

The experimental approaches to measuring the rate of metal runoff from various materials has been a combination of field experiments and laboratory follow-up studies. During the many individual studies that were conducted as complements with the aim of broadening the basic knowledge obtained during long-term (up to 4-5 years) continuous exposures of materials to well-defined atmospheric conditions, several specific questions have been addressed:

• How does the air quality (urban - rural air) influence metal runoff ?

• How does temperature, rain intensity and length of dry periods affect metal runoff ?

• How does metal runoff change during a rain episode (effect of first flush) ?

• What is the role of pH of precipitation ?

• How does surface inclination and wind exposure, etc. affect metal runoff?

• What is the effect of age of the roofing material (natural or artificial patination) ?

• How large quantities of metals are dispersed via corrosion and runoff from buildings ?

• Can artificial rain in the laboratory be used to simulate natural rain ?

• What chemical species of metals occur in the runoff water leaving the roof ?

• How does the chemical speciation of metals change when the runoff reaches the ground?

• What are the bioavailability and toxicity properties of metals in runoff, immediately and after transport in contact with limestone, concrete and/or soil ?

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