Environmental and health aspects of corrosion– importance of chemical speciation

Abstract: During the last decades, the interest in corrosion of metals and alloys from an environmental and health perspective has increased rapidly as a consequence of stricter environmental and human exposure legislations, their extensive use as implant materials and an increasing understanding related to occupational and/or daily exposure to airborne particles. Corrosion-induced metal release, however, needs to be understood in detail and to include knowledge related to chemical speciation, i.e. the oxidation state, complexation and chemical form of released metals, parameters of high importance when considering toxicity. In this licentiate work, corrosion-induced metal runoff from roofing materials (copper, zinc, and chromium(III)-, and chromium(VI) surface treated galvanized steel) has been investigated from an environmental perspective with focus on chemical speciation of released metals (Papers I-II). From these papers it was evident that the total concentration measured in the runoff water is not sufficient for any environmental risk assessment. The environmental fate including changes in chemical speciation and hence metal precipitation has to be considered. For example, it was shown that the copper concentration decreased by three orders of magnitude already in the internal drainage system of a shopping centre with a copper roof, to a concentration lower than storm water collected from a nearby parking space (Paper I). Also, speciation measurements can explain corrosion, metal release and surface processes of chromium surface treated galvanized steel at different sites (urban and marine). Any environmental risk assessment has to be done by considering all metal species released, and compared with ecotoxic values. For example, when most chromium(VI) (the most toxic species) was released, significantly less zinc was released at the same time which decreased the overall ecotoxicity of the runoff water significantly (Paper II). When assessing environmental risks by standard laboratory tests, it is important to understand all mechanisms which are possibly influenced by individual experimental parameters and which often are different for different test substances. Some metals released, as seen in the case of iron, may precipitate with time and be pH-, solution- and buffering dependent. This behavior can lead to strongly underestimated measured metal concentrations (Paper III). When particles of metals or alloys are to be investigated (Papers III-VI), it is essential to conduct a thorough particle characterization, since the surface properties cannot be defined. In addition, the surface properties (oxide layer properties) change with varying particle size (Paper VI) and with other experimental parameters such as dispersion (Paper VI). All iron-, and chromium-based particles investigated (Papers III-VI) revealed large differences between alloy particles and pure metals. Particles of pure iron and nickel released significantly more metals compared with particles of the investigated alloys, whereas particles of pure chromium released less metals compared with the alloys. Particles of stainless steel (AISI 316L), ferro-chromium and ferro-silicon-chromium released very low amounts of metals (Papers III-VI). The released quantity increased with increased acidity (Papers III-VI) and also in the presence of complexing agents (ongoing research). The manufacturing process is of high importance, as observed for stainless steel particles when compared with a side product from stainless steel production with similar composition that released significantly more metals (Paper III). Particles of metal oxides, i.e. chromium(III)oxide and iron(II,III)oxide, released very low amounts of metals due to their thermodynamic stability. Ongoing research activities focus on the specific influence of complexing agents and proteins on the metal release process from massive sheet and particles of metals and alloys. The applicability and the possibility to use different analytical tools are investigated and elaborated for small-sized particles. A detailed understanding of the correlation between material and particle characteristics, the metal release process, the chemical speciation in interaction with proteins and/or cells, and the particle/cell interaction is essential to enable any correlation between material/particle characteristics and toxicity. The aim of this licentiate summary is – in contrast to the six included scientific papers – to explain the importance of chemical speciation for corrosion processes from a health and environmental perspective in a popular way to reach a broad non-academic audience. The summary is hence written as a guidance document for stakeholders and the regulatory community working with environmental and health risk assessment.