TO UNDERSTAND THE ENVIRONMENTAL impact of materials and design choices in construction, in particular zinc used in the hot-dip galvanizing process, the International Zinc Association commissioned a pilot study to assess and quantify the life-cycle impacts of hot-dip galvanized coatings in a typical architectural application.
As some states, such as Washington, enact legislation requiring LEED ratings be used to evaluate proposals on government construction projects and more states/provinces consider doing so, hot-dip galvanizing and its use of zinc should be considered for aesthetic architectural applications and corrosion prevention.
The study, "life-cycle assessment study for hot-dip galvanized balcony system compared with painted balcony system," was completed in April 2004 by VTT Technical Research Centre of Finland. The objectives of the study were to:
- Provide a basis for future improvements in life-cycle performance of zinc products
- Establish environmental performance of hot-dip galvanized steel structure and equivalent painted structure
- Identify the relative importance of coating selection to overall life-cycle environmental impact of steel structures
VTT Research Centre of Finland had a valuable basis for the study, having developed environmental product declarations (EPDs) for building products and conducting life-cycle assessments for the galvanizing industry.
SCOPE AND DATE SOURCES
The balcony systems studied in the assessment are produced by Rannila Steel Oy, Finland, and have been in production since 1996. The structural components of this lightweight balcony system traditionally have been galvanized and then painted (duplex coating). VTT Research Centre of Finland previously had studied these balconies and developed an EPD. To compare the impacts of hot-dip galvanizing and painting individually, two coating specifications were defined to cover the required 60-year service life (see figure 1).
The environmental issues assessed were those most commonly applied in EPDs and green-building rating systems, such as LEED. The issues include the use of energy; use of natural resources; and impacts of air emissions on global-warming potential, acidification and photochemical ozone creations. These areas were estimated using established life-cycle impact category indicators from Eco-Indicator 95 method, a clef-cycle inventory data predominantly was sourced from Finnish processes and products through date for paint materials was sourced from published European databases. Recycling of the steel and zinc was considered in the assessment and allocated using a methodology set our by the International Iron and Steel Institute.
DURABILITY
The durability of the galvanized and painted coatings was estimated using ISO 14713 and ISO 12944, respectively. Corrosion rates for galvanized coatings in the study environment are reported as 0.5 to 1.0 microns per year. The 100-micron galvanized coating would not require maintenance during the 60-year service life. The painted structure would require maintenance painting every 15 years. A rather typical "standard" paint system was studied-a zinc-rich epoxy primer (40 microns in DFT), epoxy intermediate (2 coats of 80 microns in DFT) and polyurethane topcoat (40 micron DFT) chemically curing solvent-borne system. In addition to this standard paint system, a low-VOC water-based paint system also was considered in the assessment. A number of assumptions were made; most notably, the maintenance painting of the structure has the same durability and environmental profile as the original paint application. This was c conservative assumption, but was necessary because of the lack of available environmental data on in-situ maintenance painting.
PRELIMINARY RESULTS
The results of this pilot study are illustrated in Figure 2, 3, and 4 and can be summarized as:
- The choice of coating has a significant influence on the life-cycle environmental profile of the balcony structure.
- Galvanizing has significantly lower (more favorable) results across all three main life-cycle impact categories (global warming potential, acidification and photochemical ozone creation potential).
- Durability determines much of the life-cycle difference between galvanizing and paint with the burdens of maintenance painting contributing significantly to the life-cycle results of the painted balcony system.
- Appropriate allocations of the benefits of steel recycling, using the IISI model, is importance in estimating the overall life-cycle impact.
CONCLUSION
This pilot study has quantified the principal environmental impacts for a galvanized steel balcony and painted balcony. For the impact categories considered, the efficiency and durability of the galvanized balcony provided for significantly lower life-cycle environmental indicators than the painted system balcony.
These preliminary results require exploration, in particular to further define the burdens associated with maintenance painting and inclusion of life-cycle inventory data for the galvanizing process worldwide. However, because paints and maintenance painting are similar around the globe and the galvanizing process in developed countries is virtually indistinguishable from continent to continent, these refinements are unlikely to change the general results.
With the initial cost of a hot-dip galvanized coating being equal to and often less than many paint systems utilized for architectural applications and corrosion protection and the long-term cost almost always far less, hot-dip galvanized coatings also make economical sense.
