Copper architecture and the environment

Over the last 15 years, Professor Inger Odnevall Wallinder (IOW) has been involved with extensive interdisciplinary
field and laboratory studies on corrosion and metal runoff from copper roofs and facades, conducted at the Division of Surface and Corrosion Science, KTH Royal Institute of Technology, Stockholm.

Chris Hodson (CH): What is happening when copper changes colour to brown and then green in the atmosphere?

IOW: All metals – except the most ‘noble’ such as gold and platinum - are oxidised and corroded to varying degrees when exposed to outdoor conditions. We can see this with rust on steel and white staining on galvanized steel. However, oxidation of metals or alloys such as titanium and stainless steel can’t generally be seen by the naked eye.

On exposed, external copper, copper oxide (cuprite) is formed initially which gives a progressively darker brown‐black appearance. Then different basic copper sulphates and chlorides make the surface green. The make-up of patina depends on prevailing environmental conditions, in particular determining concentrations of sulphur dioxide and sodium chloride. In marine environments, the formation of basic copper chlorides turn the copper surface more blue. Despite these green/blue surfaces, the inner layer remains predominantly black‐brownish cuprite. In the absence of air-borne pollution and away from the coast, the patina will stay brownish in colour.

CH: How does the patina affect corrosion of the copper surface?

IOW: The patina adheres strongly to the surface and acts as an efficient barrier significantly reducing the corrosion rate of the underlying copper metal. With copper surfaces that have patinated over 100 of years, the underlying metal has still not oxidised: this would not be the case if easily soluble corrosion products such as copper salts were present.

CH: Why doesn’t the patina dissolve rapidly and run off the surface like water-soluble salts?

IOW: Firstly, basic copper compounds developed in copper patinas are chemically very different to soluble copper salts. Secondly, the basic copper compounds are integrated within the patina, predominantly composed of cuprite. Thirdly, the thin water film conditions, combined with repeated dry and wet periods that govern atmospheric exposure conditions, enable partially-dissolved copper released from patina constituents to re‐precipitate during drying cycles. These conditions are very different from laboratory bulk immersion conditions where no dry period occurs and dissolved copper has limited possibility to re‐precipitate.

CH: So, is any material carried off the copper surface in rainwater?

IOW: All metals allow some material to be carried from their surfaces. It is only via the action of rainwater flushing the surfaces that any dissolved copper can be released. This essentially depends on rain characteristics (intensity, amount, duration, acidity) and prevailing wind directions, together with factors such as building geometry, orientation, inclination and sheltering. So the amount of material released into the water is a very small proportion of the patina and most copper products released have poor solubility anyway.

CH: What happens to any copper in water runoff from a building?

IOW: It has been shown that various material surfaces close to buildings - including soil, concrete and limestone - act as effective sinks for released copper. Interactions with these surfaces also reduce the bioavailability of the copper significantly as well. So, released copper will be retained by surfaces already in the drainage system: pipes of concrete and cast iron have proven to be very effective. Actually, more than 98% of the total amount of released copper in runoff water on concrete surfaces is retained within 20m of interaction.

Some countries have already adopted sustainable drainage techniques including permeable paving, wadis or swales, inverted wells or soakaways and wetlands – rather than piped drains into streams and rivers. Here, research has demonstrated high percentages of copper retention early on in these techniques. To summarise, through natural processes of binding to organic matter, adsorption to particles and precipitation, copper in runoff comes to rest in a mineral state as part of the earth’s natural background of copper material, continuing the natural extraction/mineralization cycle.

CH:  Are there any situations where architects need to look carefully at runoff from a copper building?

IOW: Well, if you designed a large copper roof draining directly into a lake with sensitive water organisms, without any pre-interactions with organic matter or different surfaces, you should seek advice. And there is plenty of help and advice available through the European Copper Institute, including design assessment tools.

CH: Why are there still some concerns in a few countries about copper in water runoff?

IOW: Most ecotoxicological studies are conducted on easily water-soluble salts to assess adverse effects on aquatic organisms, induced by metals in their ionic form. They bear no resemblance to the actual situation on a copper-clad building exposed to the weather, as we discussed earlier. The real conditions of drainage systems, hard landscaping and building surroundings are also very different from the artificial ecotoxicological testing with copper salts where all copper is in a bioavailable chemical form. Therefore, erroneous regulations and legislation should now be adjusted to real environmental situations, particularly consideration of the environmental fate of copper.

Published in the Copper Architecture Forum issue 31/2011.