What do architects know about plaque and patina formation on copper, buildings and their impact on rainwater wastewater and the environment? Architect Chris Hodson, correspondent for www.copperconcept.org, asks a lead expert for direct answers.
For 15 years, Professor Ingre Odnywall Wallinder (IOW) has been involved in large-scale interdisciplinary field and laboratory research on corrosion and metal washout from copper roofs and facades conducted by the Faculty of Surface and Corrosion, Royal Institute of Technology, Stockholm.
Chris Hodson (CH): What happens when copper turns brown and then green on contact with the atmosphere?
Inegra Onewall Wallinder (IOW): All but the most precious metals such as gold and platinum oxidize and corrode to varying degrees when outdoors. We can see this in the form of rust on steel and white deposits on galvanized steel. However, oxidation of metals or alloys such as titanium and stainless steel is not visible to the naked eye. When exposed to atmospheric air, copper forms copper oxide (cuprite), which gradually takes on a darker brownish-black color. Then various basic copper sulphates and chlorides paint the surface green. The patina formula depends on atmospheric conditions, in particular the concentration of sulfur dioxide and sodium chloride are decisive. In the marine environment, the formation of basic copper chlorides imparts a bluer color to the surface. Despite these green / blue surfaces, the inner layer remains predominantly black-brown cuprite. In the absence of contamination in the air and away from the coast, plaque can retain its brown color.
CH: How does plaque affect copper surface corrosion?
IOW: The coating adheres tightly to the surface and acts as an effective barrier, significantly reducing the corrosion of the underlying copper layer. If the plaque has formed over 100 years, then the metal below will still not oxidize. But this rule does not apply in the case of easily corrosive products such as copper salts, if any.
CH: Why doesn't the plaque dissolve quickly and wash off the surface like water-soluble salts?
IOW: Firstly, the base copper compounds formed in the copper deposit have a very different chemical composition from the water-soluble copper salts. Secondly, the base compounds are part of the plaque, mainly consisting of cuprite. Thirdly, the presence of a thin film layer, combined with repeated dry and wet periods, influencing the factors of atmospheric conditions, allows partially dissolved copper released from the composition of the plaque to partially settle during drying cycles. These conditions differ significantly from laboratory conditions of bulk immersion, when there are no drying periods and dissolved copper has limited re-settling capacity.
CH: So does rainwater wash any material off the copper surface?
IOW: Some of the materials are washed off the surface of all metals. But only through the reaction of rainwater with surfaces can a certain amount of released copper dissolve. This, in principle, depends on the characteristics of the rain (intensity, amount of water, duration, acidity) and the prevailing wind directions, together with factors such as the geometry of the building, its orientation, slope and shading. Thus, the amount of materials released into the water is a very small proportion of plaque, and most of the isolated products are poorly soluble in water.
CH: What happens to the copper washed away from the building?
IOW: It has been confirmed that various materials in the vicinity of a building - including soil, concrete and limestone - effectively absorb the released copper. Interaction with these surfaces also significantly reduces copper bioaccumulation. Thus, the released copper will be trapped by the surface already in the drainage system: the effectiveness of concrete and cast iron pipes has been confirmed. In fact, over 98% of the total copper released in wastewater on concrete surfaces is bound within 20m of interaction. Some countries have already adopted sustainable wastewater management technologies, including absorbent road clothes, drains or ditches, inverted wells or sedimentation tanks, and drainage lands - rather than pipe runoff into streams and rivers. Here, studies have shown a high percentage of copper retention in the early stages when using these technologies. To summarize, we can say that in the process of binding organic matter, absorbing particles and sediment, the separated copper remains in the mineral state as part of the natural pool of copper in the earth, continuing the natural cycle of release / mineralization.
CH: Are there situations where architects need to pay close attention to drainage from a copper building?
IOW: Well, if you have designed a large copper roof that flows directly into a lake with sensitive aquatic organisms, without any prior reaction with organic matter or various surfaces, you should seek advice. Much help and advice can be obtained from the European Copper Institute, including project evaluation tools.
CH: Why do some countries still have concerns about copper in wastewater?
IOW: Most ecotoxicological studies are conducted on readily water-soluble salts to assess adverse effects on aquatic organisms, including metals in their ionic form. They have little to do with the actual situation of a copper clad building exposed to the weather, as we discussed earlier. The actual conditions of the drainage system, the rugged landscape architecture and the building environment are also very different from the conditions of the ecotoxicological tests with copper salts, where all copper is in a chemical form that can be biologically assimilated. Therefore, erroneous norms and legislation must now be corrected taking into account the real environmental situation, especially taking into account the impact on the nature of copper.
Published in the issue "Copper Architectural Forum" # 31 2011. and at www.copperconcept.org
By Chris Hodson
