Metal Corrosion on Buildings: Rust Costs Money
From landmark structures to street signs, our built environment is exposed to the elements. But what atmospheric influences must new buildings be protected from? And how can the «ravages of time» be slowed down when it comes to World Heritage sites? Empa researchers are analyzing the corrosion of metal building components and working on a cost-effective and sustainable approach to tackling rust.
First, the good news: Things are better today. At least when it comes to corrosion. What used to rust and fall apart back in the 1980s lasts longer today. However, the challenges of stopping large-scale corrosion remain enormous: Even today, several metric tons of steel around the world meet their end every second and—consuming precious raw materials—must be replaced. Corrosion costs an estimated three to four percent of an industrialized country’s gross domestic product, resulting in economic damage to bridges, roofs, and railways running into the billions.
One person who studies the extent and causes of corrosion and works to make the built environment sustainable is Empa researcher Ulrik Hans from the «Joining Technology and Corrosion» laboratory in Dübendorf. And because corrosion knows no boundaries, he is involved in «ICP Materials»—short for «International Co-operative Program on Effects on Materials including Historic and Cultural Monuments»—a network with more than 60 locations in Europe and beyond. In its latest report, the network highlighted the significant decline in corrosion based on long-term experiments and published the results in the journal «Corrosion and Materials Degradation.».
Switzerland: A Model Country
To investigate the causes and ensure the long-term protection of buildings that are intended to remain standing in the future, Ulrik Hans, in collaboration with the Federal Office for the Environment (FOEN), provides data on corrosion rates to the «ICP Materials» network. At each location in the network, standardized metal samples—such as weather-resistant steel or titanium-zinc, one of the most important roofing materials—are exposed to the elements. «Titanium-zinc is popular in construction because it is fully recyclable. When it comes to building preservation, it makes a significant difference whether the metal roof and gutters need to be replaced after 40 years or only after 80,» says Hans.
The site in Switzerland on Chaumont, the local mountain of the city of Neuchâtel, is essentially the prime example within the «ICP Materials» network: The «exposed» samples corrode with virtually no harmful environmental influences, but solely due to the purely natural redox reaction between metal, water, and oxygen. At the other end of the spectrum at the start of the measurements were industrial sites such as Bottrop, Germany, and Kopisty, the Czech Republic. In Kopisty, corrosion researchers were able to scrape half a kilo of rust off one square meter of steel in 1987. Today, however, the situation looks better at all sites. In industrial areas, between 100 and 150 grams of steel per square meter are lost each year. At Chaumont, only 30 grams of rust form over the same period. Zinc, which corrodes less rapidly, lost about 15 grams per square meter at industrial sites 40 years ago; today, that figure is only around 7 grams at all sites.
Identifying the Pollutants of the Future
The data show that, almost simultaneously with this encouraging development, the concentration of sulfur dioxide in the air declined—a pollutant that, when measurements began in the 1980s, fell from the sky as «acid rain.» With peak levels of 460 µg of toxic sulfur dioxide per cubic meter of air, the highest smog alert level was triggered in the Ruhr region. In areas where industrial emissions are filtered and lignite-fired heating systems have fallen out of favor, sulfur dioxide has not been a problem since then. In industrial areas, levels today are below 10 µg/m³, while on the Chaumont they have fluctuated between 0 and 1 µg/m³ for the past 25 years.
Although the correlation between reduced corrosion and decreasing sulfur dioxide pollution is obvious, the researchers« data do not always follow this correlation. »Corrosion is a complex, multifactorial process that is not yet fully understood,« says Ulrik Hans. »We now need to identify other pollutants that will become more relevant in the future.” For example, it is still not well understood to what extent high ozone levels attack modern polymer materials and how particulate matter promotes corrosion. Nitrogen oxides and volatile hydrocarbons are additional factors that, together with climate change, could affect the durability of metal infrastructure.
State-of-the-art rust predictions
With a better understanding of these relationships, suitable alloys, coatings, or nanometer-thick passive layers can be used to ensure the durability of, for example, additively manufactured or laser-structured components. Added to this is a new challenge facing today’s construction industry—one that did not play a role in the construction of Cologne Cathedral or the Acropolis: Today, care must be taken to use existing resources sparingly.
To ensure that the development processes for suitable, future-proof products proceed efficiently and sustainably, Empa researchers analyze new materials as early as the development phase. They use state-of-the-art surface analysis techniques such as Kelvin probe force microscopy and X-ray photoelectron spectroscopy (HAXPES). The only such facility in Switzerland is located in the «Joining Technology and Corrosion» laboratory at Empa. Thanks to high-energy radiation, HAXPES penetrates deep into the material, enabling precise characterization of materials and their corrosion properties. «With the resulting reliable corrosion predictions, we will be able to continue building and maintaining long-lasting structures efficiently in the future,» says Empa researcher Hans with conviction.
Corroding Cultural Heritage
Even World Heritage Sites are subject to the ravages of time. Since 2010, the «ICP Materials» network has therefore been monitoring 26 UNESCO World Heritage Sites, such as the Old Town of Bern and the St. Gallen Abbey District. In case studies, the researchers were able to demonstrate that air pollution—and, in particular, road traffic—accounts for up to 80% of the costs associated with maintaining the buildings. Risk assessments are ultimately intended to make it possible to determine where the line lies between the natural aging of a cultural monument and impending decay, so that restoration work can be carried out based on economic criteria. This involves analyzing data on material composition, metal corrosion, limestone weathering, and pollutant levels in the surrounding environment.
Corrosion poses a risk to the lifespan of the copper roofs on the towers of St. Gallen Cathedral, according to calculations by corrosion researchers. First, however, the tile roof of the church—a UNESCO World Heritage Site—must be restored next year. Corrosion has been at work here as well: the nails in the 100-meter-long roof truss have rusted. The estimated cost will be just under eight million francs.
Source: Empa
