Predicting corrosion rates in metal building components
The corrosion of metal building components, such as roofs, cladding and pipe-work, is generally the result of exposure to water and oxygen in the environment, but the situation can be affected by contaminants such as airborne sea salt and fossil fuel pollution.
Problems can emerge when unexpected contaminants are present in the environment. When this happens it typically means that the environmental conditions were not properly understood when the structure was designed, or the corrosion resistance of the material was over-estimated.
In one such case encountered by BRE, a roof in a coastal location had suffered severe corrosion in a short space of time. It was constructed of profiled aluminium sheets laid at a pitch of around 5°. Glass-reinforced plastic roof lights were present on both sides of the ridge and the aluminium sheets had been installed with a lapped joint halfway down each elevation (see photo, right).
Despite being less than 15 years old, the roof had considerable corrosion. The surfaces of the sheets were generally covered with small, shallow corrosion pits. While these had had not affected the performance of the roof, at the lapped joints – which would remain wetter for longer periods – the corrosion had penetrated the full thickness of the sheet (see photo below).
The owners of this large, uninsulated roof considered the corrosion to have been accelerated by contaminants from a neighbouring industrial site, a view that was shared by the designers and suppliers of the roof. However, the neighbours contested this.
To get to the bottom of the problem, BRE inspected the roof and took samples of the corroded sheets, the corrosion products and debris from the gutters. Subsequent laboratory analysis of the debris from the gutters revealed that the contaminants on the roof would not have exacerbated the corrosion of the aluminium.
Calculating the environment’s corrosivity
The corrosiveness of an environment can be calculated using the three primary factors – the period of wetness, the deposition rate of chloride and the concentration of airborne sulphur dioxide. These factors allow the ‘corrosivity category’ for the environment to be assessed according to ISO 9223. Guide values are provided in the Standard for typical corrosion rates of iron, zinc, copper and aluminium.
However, the actual corrosion rate of metal in coastal environments can be difficult to predict. This is particularly true when features on the roof, such as overlapped joints, can hold water for long periods, thereby increasing the corrosivity in these positions.
BRE’s laboratory tests showed that the actual type of aluminium alloy that had been used to manufacture the roof sheets was susceptible to pitting corrosion and was therefore not suitable for coastal environments.
Rain water, laden with chloride from the sea, would have run over the roof and discharged into the gutters. Capillary action at the overlapped joint would tend to pull water under the upper roof sheet, holding it there as it slowly dried out and concentrating the amount of chloride in the joint. With time this had caused pitting corrosion to take place, leading to the failure of the roof panels.
Avoiding the problem
The problem could have been avoided by designing the roof to eliminate overlapped joints and other similar features, and by specifying an alloy that is resistant to coastal corrosion. Of course this would have increased the cost of the roof – there is often a trade-off between the initial cost and the service life. In order to be able to make this decision confidently, it is necessary to get the best possible estimates of corrosion rates and likely service life.