Ingenieurwissenschaften und zugeordnete Tätigkeiten
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The Tsumeb Mine in Namibia represents one of the best-preserved mining sites in the world and is rapidly gaining cross-disciplinary interest among cultural and engineering scientists. Most of the open pit and the shaft mining equipment are still in place, including the ore processing units and the local power plant. The mining area thus deserves recognition as an industrial world heritage site, especially due to the rarity of such locations on the African continent. The Shaft #1 headgear, built in 1924, represents one of the oldest known riveted steel headgears of the Promnitz design worldwide. In contrast to similar steel structures located in the northern hemisphere, it has been exposed to a different rural semi-arid climate since it is located in the Otavi Mountain Land, characterized by semi-annual change of rainy and dry seasons. Parts of the Shaft #1 headgear have remained largely untouched for more than 70 years. Besides its outstanding heritage value, it thus also represents an interesting object for studying the composition of corrosion layers formed on mild steel surfaces when exposed to continental and industrial mining atmospheres. To find a suitable transparent corrosion prevention coating, various on-site coating samples were evaluated after 11 months of outdoor exposure, including Owatrol Oil®, which is based on natural oil and alkyd resin with strong wicking potential. The substance is frequently applied for the conservation of single components but is not yet widely used on large steel structures in the field of industrial heritage conservation. However, it represented the most stable anti-corrosion coating under the local atmospheric conditions in the on-site tests. Thus, the suitability of Owatrol Oil® as a transparent coating for corrosion protection of riveted mild steel structures in such climates was further investigated as a more recent approach for the conservation of large steel structures. Since the protective coatings are exposed to strong UV radiation in the local climate, the addition of a specific UV stabilizer mixture was also tested. For such laboratory tests, two mild steel samples were taken. The first one originated from a diagonal strut of the 1920s and the second one from a handrail mounted in the early 1960s. Using corresponding high-resolution scanning electron microscopy (HR-SEM) and energy-dispersive X-ray spectroscopy (EDX) it was found that the corrosion layers are predominantly composed of lepidocrocite and goethite. A weathering program simulating the specific environmental conditions at Tsumeb in a UV climate chamber was developed and the corrosion resistance of the mild steel surface was subsequently evaluated by potentiodynamic measurements. Such tests proved to be a fast and reliable procedure for ranking the corrosion resistance of the old mild steels. It was found that the long-term corrosion layers already provide significant protection against further corrosion in the simulated environment. However, the study also showed that this can be further improved by the application of the Owatrol Oil® as a protective coating that also seals crevices. The addition of the UV stabilizers, however, led to a significant deterioration in corrosion protection, even in comparison to that of the uncoated long-term corrosion layers on the surface. Regular overcoating seems more advisable for the long-term preservation of the Shaft #1 headgear than modifying the Owatrol Oil® Coating with the tested UV-stabilizing additives.
For centuries, churches, secular buildings and museums have been furnished with valuable works of art. Many works of art are UV- and light-sensitive. It is well known that especially UV radiation causes damage and discoloration in paintings, textiles, plastics, wood and other materials. In particular, the wavelengths between 280 and 410 nm cause color changes, embrittlement or destruction of e.g. plastics over time. Therefore, strongly UV-absorbing glasses are advantageous for architecture and are necessary for the protection of cultural assets. As restorers in all disciplines become more and more aware, the demands placed on buildings and their furnishings in terms of climate are becoming increasingly detailed and precise. The aim in each case is to preserve the valuable artwork.
For some years now, the industry has been offering the protection of cultural objects by installing special UV-protective glasses. Currently, UV protection for church buildings is realized by laminated safety glass equipped with appropriate UV-protective plastic films. Technically, this always means a second pane of glass in front of the windows, which is installed outside the building. This creates climatic gaps that are difficult to control and deterioration due to ageing effects can be expected. At the same time, this protective glazing is not invisible and has a considerable aesthetic influence on the interior and exterior appearance of the building. Meanwhile, the preservation of historical monuments accepts such aesthetic cuts on buildings in order to protect the artwork in the interior from UV light. To this day, however, the long-term durability of UV protection provided by inserted plastic films is still controversial.
To date the only available alternative on the market is a mouth-blown UV protection glass which uses a so-called overlay to provide UV protection. This shows that UV protection can also be achieved by glasses without plastic films thus realizing an exclusive inorganic protection which normally is more stable than a polymeric one. So far there are not enough studies to prove long-term durability.
The aim of this project is to provide existing glazing or new glazing to be created with a highly transparent layer that ensures this UV-protective filter function below 400 nm. The glass coating is to be applied to the glass over a large area and fired into the surface like a classic ceramic enamel paint with the same technics. In addition, it should be long-term durable in its function. It means, that the glass has to be fused at temperatures below 630 °C during the firing process, its chemical durability has to be high, its coefficient of thermal expansion has to be as close as the one of the substrate (usually float glass) and the glass has to absorb the UV-radiation within a thin thickness (thinner as 100 µm).
In this project, the long-term durability of commercial UV-protective glasses is examined. New low melting glasses containing UV-absorbing ions are being developed. Their UV-absorption as a thin layer is analyzed as well as their chemical durability and their thermal properties.
We are grateful to BMWI for the financial support in the frame of the Central Innovation Programme for SMEs (ZIM).