How to avoid bi-metallic corrosion in building elements
29th April 2019
The durability of materials obviously has a bearing on the longevity of all building elements. Factors such as UV exposure, mechanical damage from wind-blown debris and freeze thaw action all take their toll on a structure. While it’s the designer’s responsibility to ensure that exposed elements of a building are fit for purpose, it is important that suppliers provide their input, especially in the often overlooked area of bi-metallic material corrosion.
Bi-metallic or galvanic corrosion occurs as a result of the flow of very small electric currents in the presence of moisture and oxygen between two dissimilar metals. This causes the more anodic of the two metals to corrode, whilst the other metal (called the ‘noble’ metal) remains unaffected. For example, steel pipe (ranked lower on the Galvanic series) with brass fittings (ranked higher) will result in the higher ranked metal (brass) causing accelerated corrosion of the steel.
However, using two or more different metals on a building’s external envelope does not necessarily mean corrosion will occur if you follow a few basic rules.
It’s worth bearing in mind that the two metals don’t need to be in contact for galvanic corrosion to occur. When a more ‘noble’ metal corrodes slightly and dissolves in rainwater and this subsequently flows over a less noble metal it will result in corrosion of the latter. This can occur where the roof or window surrounds or reveals are a different metal to the façade cladding.
Generally, the level of bimetallic corrosion in an atmospheric environment such as those that buildings are exposed to will be much lower than if the metals are immersed in water. It is, though, still an issue that needs to be taken into account at design stage when specifying metallic cladding and roofing systems.
Galvanic Series
It is possible to rank metals, including those used for building elements such as cladding and roofing, on the Galvanic Series. This lists those that are least or most susceptible to corrosive attacks. The further apart two metals are on the Galvanic Series, the greater the potential difference, and the faster the corrosion will occur on the less noble material.
When specifying a metal cladding system it is always worth checking where other materials selected for the roof, rainwater or other façade elements are positioned on the Galvanic Series. We routinely check this on all the projects we work on to ensure that there aren’t issues further down the line. In most cases, consideration has been given to this aspect, although sometimes it can be overlooked.
Square Chapel, Halifax is an example of where designers can overlook this important technical element. The original design concept had a copper wall, above a zinc roof, running off to a copper wall. Rainwater run-off from the copper (noble) onto the zinc (anodic or less noble) would have resulted in rapid galvanic corrosion and as a result, the roof had to be altered in the design phase to negative the bi-metallic corrosion issue created by the original design. Working with the design team during the specification stage we were able to alter the design, in this case changing the roofing material, to keep the external elevation materials as the architect’s design with our Proteus HR rainscreen system with a TECU Classic external veneer.
Interestingly, these two metals – zinc and copper – on their own are some of the most durable and long lasting roofing and cladding materials around. But put them together, or any other metals not closely positioned on the Galvanic Series, and rapid corrosion will be the result.
There are various ways of avoiding galvanic corrosion and the mains ones are:
- Selecting metals with similar corrosion properties
- Separating the two materials with an inert spacer
- Breaking the electrical connection by insulating the two metals from each other.
- Applying coatings to one or both materials.
- Adding corrosion inhibitor
It’s worth remembering that bimetallic corrosion is not just limited to roofing and façade elements. Anywhere in a building where two or more metallic elements are used in conjunction can result in galvanic corrosion. That includes air conditioning pipework, hot and cold water systems and structural framing connections, amongst others. With such a wide variety of potential trouble spots, it is not possible to avoid unfavourable dissimilar metal combinations, and so the best solution is to check with your supply partner
Most designers understand that the presence of different metals in any element of a building can result in localised corrosion. All metals react with their environment, although some are more reactive than others. Knowing this, and factoring it in at design stage can avoid issues after the project is handed over.