A sustainable advantage
Stainless steel is one of the most environmentally friendly metals commonly used in construction. It helps generate and save energy, provides clean air, conserves water, avoids hazardous chemicals, and limits landfill waste and contamination of the environment.
When stainless steel is properly selected and maintained with an appropriate finish, the material remains attractive for centuries. Even after years of neglect, stainless steel is often restored to its original appearance or reused in other applications.
Many of the characteristics determining a metal's "greenness" are (in)directly related to its ability to resist corrosion. Stainless steel's high scrap values and low corrosion rates ensures that the recycling rate for architectural products is very high even after a long service life. Coatings that outgas, or those adversely affecting recycling potential, are also unnecessary. Additionally, the metal's long service life maximizes other materials' performance, preventing premature failure of systems designed with stone, masonry or wood.
Interest in green construction has grown significantly, with an emphasis on evaluating entire buildings as well as individual materials. The U.S. Green Building Council's Leadership in Energy and Environmental Design rating system - as well as numerous other product evaluation methods - raise many questions indirectly related to metal choice. These include recycled content levels, potential for product reuse, likelihood of emissions, thermal comfort, durability, maintenance requirements, and various impacts on energy/water consumption, indoor air quality and indoor light.
Ideally, the designer requires a fully quantified master database that provides objective, life-cycle environmental impact assessments for all construction materials. Unfortunately, the data available for different materials is rarely directly comparable. To begin this discussion, the evaluation chart summarizes common questions and answers about the environmental friendliness of stainless steel. These points are discussed in more detail below.
Reality of recycling ratesA material's recycling potential is an important aspect of sustainable design. Reported recycling rates are based on the percentage of recycled material used in the average "heat" of metal produced. However, this method makes comparing and contrasting data difficult. For example, aluminum's published rates are very high due to the mass production of cans, many of which are recycled several times in one year. In comparison, stainless steel is used typically for 20 or 30 years before it is recycled.
Due to its value, stainless steel scrap has a very high recapture rate, but its long service life-not to mention the rapid historic growth in its production - make it impossible for stainless steel heats to have a high recycled content. As much as the stainless steel producers would like to have 100 percent scrap heats, not enough material is available.
In 2002, the International Stainless Steel Forum estimated the typical recycled content was about 60 percent. Over the last two years, however, this has probably declined somewhat because the excess Eastern European scrap inventory has now been recycled. (It is important to recognize stainless steel is 100 percent recyclable with no down-cycling, regardless of how many times it has been previously recycled.)
When evaluating metals, then, it is more meaningful to look at an average piece's probability of being recycled. Large expanses of roofing or wall panels are likely to be recycled, but a smaller item (such as a corroding threshold, gutter, flashing, or railing) might end up in a landfill when the scrap value is low. Additionally, some coatings limit or prohibit recycling, and components with significant metal mass loss due to corrosion may have negligible scrap value. These factors make stainless steel an advantageous material. Although long-term use limits its re-melting frequency, its high scrap values, coating avoidance and negligible corrosion rates ensure most of the material put into service is eventually recycled.
Corrosion and the environmentIn a 2001 study, the Federal Highway Administration and the National Association of Corrosion Engineers estimated the annual, direct economic cost of metallic corrosion in all industries and applications at $296 billion, with indirect costs at $255.4 billion. Twenty-one percent of this total figure (approximately $113.6 billion) was attributed to the indirect cost of construction material corrosion, excluding both infrastructure and industrial construction. This includes obvious failures such as roof/wall panel perforation or a structurally unsound railing. This category also comprises replacement of aesthetically unattractive, corroded components, when restoration is either impossible or fiscally illogical.
The high economic price is an indication of the significant environmental costs associated with materials failing to remain functional or aesthetically attractive over a building's life. In "green" design, the architect must consider the following questions when selecting architectural metals:
Will product replacement be needed during the probable life of the building?
How recyclable/reusable is the product?
When a coating is specified, are there concerns with off-gassing or coating loss to the environment due to wear/spalling?
Are any coatings limiting/preventing the recycling of the base metal?
How much metal enters the environment due to corrosion? Is the corrosion hazardous or aesthetically unappealing?
Considering metal loss due to corrosion, how much of the original metal is non-recyclable and will have to be replaced by mining new metal?
How much maintenance is required, and are the cleaning products potentially hazardous?
Comparative atmospheric corrosion data for different metal alloys can predict a component's service life, maintenance requirements, and metal loss to the environment.
Based on this atmospheric testing, generalized corrosion maps indicate the relative corrosiveness of different locations and help guide metal-selection decisions. (A Web source for these maps is www.corrosion-doctors.org.) Generally, areas with particularly acidic rain, high airborne particulate levels, high sulfur or nitrous oxides and ozone amounts, and increased exposure to coastal/deicing salt require more corrosion-resistant metals.
When environmental friendliness is a concern, it is critical to select materials providing a high level of corrosion protection without relying on coatings. These metals do not need replacement, nor do they shorten the life of other building materials through failure. Stainless steel is more corrosion-resistant than other common architectural metals and is unaffected by pollutants like nitric/carbonic acid and ammonia, which are commonly found in acid rain.
Potentially corrosive environments for stainless steels and other metals include sulfuric acid in acid rain, high levels of atmospheric particulate, and deicing/coastal salt (chlorides). If the right stainless steel and finish are selected and properly fabricated, installed and maintained, acid rain does not corrode.
Enhancing the environment - inside and outStainless steel is suitable for interior applications because no coatings are required and the material does not off-gas. With proper selection, ductwork made of the material can be thoroughly sanitized and remain free of corrosion perforations. Other applications include using reflective stainless steel panels to bring natural light into buildings or specifying a stainless steel termite barrier to eliminate pesticide treatments - and potentially reduce insurance costs.
Cleaning stainless steel requires no environmentally hazardous or dangerous chemicals, and the material is an important part of industrial/automotive emission reduction systems.
Roof runoff data generated in a Swedish study compared stainless steel, copper and zinc coatings on galvanized steel and zinc sheet, using rain acidity levels representative of Stockholm's relatively low pollution levels.
The primary focus was determining atmospheric corrosion's influence on roof runoff levels, "bio-availability" and "eco-toxicity." The rates of nickel and chromium were extremely low and, in many samples, below detectable limits (all samples were well under typical drinking water concentrations). The tests suggest nickel and chromium are released from stainless steel roofs at such low rates they do not cause eco-toxicity. The zinc and copper runoff levels were approximately 10,000 times higher, and in a bio-available form. Zinc and copper eco-toxicity is also a possibility as water concentrates during dry periods.
A similar Oregon study of lead runoff from roofs in low-pollution marine and inland rural sites found concentrations were between 0.7 parts per million and 3.7 ppm, compared with the U.S. Environmental Protection Agency lead drinking-water standard of zero ppm, with an action level of 0.015 ppm. In other words, stainless steel roofing's nontoxic runoff levels make it suitable for environmentally sensitive areas.
Long service lifeMaterials providing long-term performance have a much lower life-cycle cost and are more environmentally friendly because they do not contribute to landfill waste or require frequent replacement. Stainless steel is a relatively new architectural material with applications dating back only to the mid-1920s. The oldest known stainless steel roof - the Butler County courthouse in Pennsylvania - has provided trouble-free service for about 80 years.
One can easily see the benefits of long service life in side-by-side comparisons of metals in a corrosive environment. The piers in Progresso, Mexico, clearly illustrate the differences between carbon steel and stainless steel rebar use in concrete. The stainless steel rebar pier is still in service 60 years after its construction. Core tests have shown no stainless steel or concrete deterioration, and it is likely the structure will be in service for at least another 60 years, despite the corrosive marine environment. Visible in the image, too, are remnants of a second pier constructed about 30 years later using carbon steel rebar. It has not been used for some time, and in addition to the significant costs associated with lost service, all the original materials must be completely replaced at considerable environmental - and financial - cost.
Conserving natural resourcesStainless steel conserves natural resources in many ways. Less mining is required because of low corrosion rates and high real-recycling rates. In structural applications, designers can minimize material requirements by capitalizing on stainless steel's superior high-temperature performance or using higher strength alloys to reduce section size.
In roofing applications, for example, it is possible to use thinner panels and reduce heat gain and air-conditioning costs. Stainless steel sunscreens allow natural light to enter while reducing heat gain. This, too, lowers air-conditioning costs and energy consumption, and thanks to solar cells, generates clean energy.
Phoenix's City Hall is an excellent example of the cost and energy savings possible by installing polished, perforated stainless steel sunscreens over most windows during construction. The initial capital cost reduction was $285,000, thanks to reduced air-conditioning equipment capacity requirements, with forecasted annual cost savings of approximately $200,000. The effectiveness of the reflective stainless steel finish on the sunscreens remains unchanged over time because there is no corrosion.
Even after years in service, stainless steel water mains are less likely to leak due to corrosion than conventional ductile iron, and are more resistant to damage during earthquakes. While stainless steel water lines and tanks are relatively new applications in the United States, they have become commonplace in Western Europe and Japan, where soil can be very corrosive, particularly in coastal areas and where deicing salts are used.
In Zurich, Switzerland, the necessity of ensuring high water quality and minimizing water loss led to the replacement of all drinking water mains, storage and pumping facilities with a stainless steel and concrete system.
The corrosive byproducts from stone, masonry, redwood, cedar, plywood and other wood products can rapidly corrode other metals. Again, stainless steel, in the form of anchors, fasteners, and other components, will not rust when facing these by-products.
Restoration work and reuseNew York City's Chrysler and Empire State buildings are both excellent examples of the ability to restore stainless steel to its former glory. Both structures have been cleaned about every 30 years, with considerable surface deposit accumulation between restorations. A similar case is New York City's former Socony Mobil Building (now 150 E. 42nd St.), constructed in 1954 and adjoining the Chrysler Building. It was cleaned for the first time in 1995 after over 40 years of service. Photos taken during the process show the dramatic difference in appearance between "before" and "after" areas.
All three buildings were cleaned with a mild detergent and water solution containing a degreaser to remove hydrocarbon deposits. A fine, abrasive solution, which did not scratch the finish, removed more adherent surface deposits. No aggressive or environmentally hazardous materials were required, nor any products emitting hazardous fumes. This same method is used regularly on newer buildings, which receive more frequent cleanings.
Salvaging and reusing products is the most environmentally friendly source of materials available to architects. When the Pittsburgh-based architecture firm, IKM Inc., was given responsibility for modernizing and brightening the lobby and entrance of 525 William Penn Place, it found stainless steel wall and elevator panels marred by scratches and dents, and darkened by a half-century of wax, oil and grime.
IKM explored the possibility of refinishing and reusing at least some of the stainless steel. While the new lobby looks quite different, most of the stainless steel is 50 years old, having been cleaned, refinished with non-metallic abrasive pads, modified as necessary and reused. Components unsuitable for reuse were recycled.
Products manufactured from stainless steel are an excellent choice for protecting the environment and creating comfortable, attractive structures. Although independent data comparing the life-cycle environmental impact of different materials is unavailable, there is no question stainless steel will receive high ratings. However, as with all building components, performance depends on selecting an appropriate material, finish and design.
(Reprinted with permission from The Construction Specifications Institute, 99 Canal Center Plaza, Suite 300, Alexandria, VA 22314, from the August 2004 issue of The Construction Specifier. Institute member Catherine Houska is a senior development manager for Pittsburgh-based TMR Consulting and a metallurgical engineer specializing in architectural metal selection, specification and failure analysis. A past contributor to The Construction Specifier, she can be reached via e-mail at firstname.lastname@example.org. The Nickel Institute assisted in preparation of this article.)