Environ. Sci. Technol. 2010, 44, 4322–4327
Chromium(III) and Chromium(VI) Surface Treated Galvanized Steel for Outdoor Constructions: Environmental Aspects ¨ M, YOLANDA HEDBERG, DAVID LINDSTRO AND INGER ODNEVALL WALLINDER* Division of Surface and Corrosion Science, School of Chemical Science and Engineering, Royal Institute of Technology (KTH), Drottning Kristinas v. 51, 100 44 Stockholm, Sweden
Received February 1, 2010. Revised manuscript received April 9, 2010. Accepted April 25, 2010.
The long-term degradation of chromium(III) (Zn-Cr(III)) and chromium(VI)-based (Zn-Cr(VI)) surface treatments on galvanized steel and their capacities to hinder the release of zinc induced by atmospheric corrosion at nonsheltered urban and marine exposure conditions for 2 years are investigated. Compared to bare zinc sheet, both surface treatments revealed high corrosion protection abilities and capacities to hinder the release of zinc, still evident after 2 years of exposure. The zinc barrier properties of the thinner Zn-Cr(VI) (10 nm) treatment were during the first 100 days of urban exposure slightly improved compared with Zn-Cr(III) (35 nm). However, their long-term protection capacities were inverse. Released concentrations of total chromium correspond to annual release rates less than 0.000032 (Zn-Cr(III)) and 0.00014 g Cr m-2 yr-1 (Zn-Cr(VI)) after 1 year of urban exposure. Aging by indoor storage of the surface treatments prior to outdoor exposure reduced the released Cr concentrations from the surface treatments. No Cr(VI) was released from the aged surfaces but from the freshly exposed Zn-Cr(VI). Marine exposure conditions resulted in a faster reduction of chromate to chromium(III)oxide compared with urban conditions, and a significantly lower amount of both chromium(III) and chromium(VI) released from Zn-Cr(VI) at the marine site compared with the urban site.
Introduction Chromium(VI)-based surface treatments have commonly been used to prevent white rust staining (corrosion) of zinc sheet or zinc coatings upon atmospheric storage, transport, and handling (1-3). Their capacities to temporarily hinder corrosion are related to the self-healing effect induced by hexavalent chromium (4). Since hexavalent chromium is a carcinogen, its use is strongly restricted within the European Union (5). The corrosion performance of chromium based surface treatments is commonly characterized in the literature in, for example, accelerated corrosion tests or through electrochemical investigations in bulk solution (6, 7). Their possibility to hinder corrosion-induced zinc runoff at different atmospheric conditions is significantly less investigated due to their intended use as conversion coatings or for temporary * Corresponding author phone: +46-8-7906621; fax: +46-8-208284; e-mail:
[email protected]. 4322
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 11, 2010
corrosion protection. Recent findings have elucidated their barrier capacities also at long-term atmospheric conditions (8, 9). Triggered by an increased environmental concern related to the diffuse dispersion of metals in the society, several investigations have been performed at different atmospheric conditions to generate short and long-term quantitative data on corrosion-induced zinc runoff from bare and coated zincbased materials in external buildings and roofs in relation to patina formation and surface properties (9-16), and to assess the environmental fate of dispersed zinc (9, 17). A scientifically solid base for sustainable decisions, potential legislative actions, and accurate risk assessment and management for zinc in external constructions is hence continuously generated. Zinc and chromium are both essential and potential toxic metals for water organisms, depending on, for example, their concentration, state of oxidation, and formation of complexes (18, 19). While zinc is of relatively low toxicity to humans, it can be highly toxic for water organisms (in fresh water), with a predicted no effect concentration (PNEC) of 7.8 µg L-1 (20). Active chromium(VI) is highly toxic due to a facilitated ability to penetrate cell membranes (21) and is therefore generally considered to be more toxic; however, recent studies for fresh water algae show that Cr(III) is more toxic than Cr(VI) to freshwater algae (22). The state of oxidation and the degree of complexation of chromium are, hence, essential for ecotoxicological considerations. Within the framework of environmental risk assessment in the EU, PNEC values of 3.4 µg L-1 (Cr(VI)) and 4.7 µg L-1 (Cr(III)) have been reported for surface water (23). This paper aims to: (i) illustrate the capability of chromium(III)- and chromium(VI)-based surface treatments on galvanized steel to reduce corrosion-induced release of zinc compared to bare zinc sheet at nonsheltered exposure conditions during 2 years in an urban and a marine atmosphere; (ii) correlate time dependent changes in released zinc with surface treatment degradation, corrosion rates, and patina formation; and (iii) present quantitative kinetic data on the amount of chromium and its chemical speciation as Cr(III) and/or Cr(VI) released from the two surface treatments, and illustrate differences between freshly exposed and preaged surface treatments.
Experimental Section Material. Bare zinc sheet (Zn) (0.08 wt % Cu, 0.06 wt % Ti) and galvanized steel with two different surface treatments, Cr(III)-based-35 nm thick, (Zn-Cr(III)) and Cr(VI)-based-10 nm thick (Zn-Cr(VI)), were exposed to determine zinc and chromium runoff rates (300 cm2), corrosion rates (54 cm2) and perform surface analytical studies (4 cm2). Runoff Rate Measurements. Continuous measurements were performed in an urban (Stockholm, Sweden), and a marine (Brest, France) environment. The urban site was characterized by low annual levels (
