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Corrosion under control

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Avoiding corrosion between metal interfaces requires careful detailing and turning opposing forces to your advantage

It would be almost impossible to design a building without the interface between different metals - whether it is roofing materials abutting a gutter, cladding panels touching metal rails, or simply screw fixings piercing sheet cladding.

But when two different metals are connected this can cause a flow of electrons from the more reactive to the less reactive metal. In the presence of an electrolyte, such as water, the result is corrosion. The effect of such chemical interactions, referred to as 'dissimilar metals effects', can be dramatic on both the short and longterm performance and, hence, on the building overall.

With awareness, the interaction can be used positively to gain superior cladding performance. However, insufficient understanding of the likely chemical reaction of one material on another can also increase the likelihood of localised failure. Interfaces, therefore, need to be carefully considered at the initial design stage.

Avoiding contact In the selection of steel cladding it is crucial that steel is not placed in contact with less reactive metals which could cause dissimilar metals effects, and hence sacrificial corrosion of the steel.

The galvanic series (shown below left) is a useful tool in this. Metals are listed in order of their reactivity, with the most active at the top. Those listed above steel will protect it, although they must be isolated from direct contact with the steel to avoid sacrificial corrosion. Measures to isolate steel must be used for those listed in the lower half of the Galvanic series.

Isolation is generally done by the insertion of plastic foils or chlorinated rubber sheet. The experience of using bituminous materials, especially when isolating copper and steel, proved to be disastrous - in terms of staining as well as the degradation of the materials.

The Galvanic series is particularly relevant in the selection of appropriate fasteners. It is important to note that, as water is an electrolyte, metals do not necessarily have to be in direct contact with each other for dissimilar metals effects to occur, as the connection between metals can be made by the presence of linking moisture.

Negative answers Just as there are negative effects of electrolytic action between metals, the chemistry can also be made to work to advantage. An example where this interaction between metals is beneficial is where zinc is used to provide a sacrificial protection to steel, commonly known as galvanising. In this process, the zinc coating interacts with iron or steel because zinc is more electronegative (more reactive) than the host material and the zinc slowly sacrifices itself by galvanic action.

Galvanising has had many applications, from the protection of oil-rig structures and ships in aggressive marine environments, to the coating of steel strip with zinc to provide a corrosion-resistant substrate for organic coated cladding. This technique has been used by steel cladding manufacturers to provide protection to steel substrates at the cut edges and in areas where the coating may have been damaged.

While the protective layer applied to organic coated steel is often done for aesthetic reasons, it also acts as a barrier against the agents that cause corrosion - water, oxygen, salts and pollutants. Paint, for example, can be used to prevent ungalvanised steel from corrosion and is typically used on things from garden furniture to external railings to the Forth Bridge.

However, being vulnerable to damage, long-term performance is compromised, with exposed areas suffering aggressive localised corrosion.

To see the benefit of galvanising, just compare the resistance to rust in cars built more than 10 years ago - when paint was the only level of protection - to that of cars built after galvanising became common practice. Benefits are even more enhanced by using a galvanised substrate for coated steel cladding, which provides an additional barrier to corrosion agents along with sacrificial corrosion protection.

The effect of using a galvanised substrate for painted or other coated steel cladding is synergistic, in that the life of the whole system is greater than the combined life expectancy of the zinc-coated steel and painted mild steel. As the corrosion rate of zinc in the UK climate is only one-tenth to one-thirtieth the rate of steel, zinc on a galvanised steel substrate will corrode at a slower pace - in preference to the steel - through sacrificial action.

However, as zinc corrodes it leaves a gap between the substrate and coating around the exposed area. Referred to as 'under-cutting', this tends to allow water between the coating and substrate, forcing the two to separate and requiring aesthetically unappealing repairs to prevent significant detriment to cladding performance.

Under-cutting can, however, be prevented by the addition of aluminium to the zinc coating on the substrate. Under controlled manufacturing conditions, it is possible for a layer of aluminum oxide to form on the surface of the exposed area which acts as a barrier and prevents further oxidation by corrosion agents, such as air and water. This provides not only sacrificial corrosion protection though zinc, but also three levels of barrier protection from the paint, the layer of aluminium oxide and the layer of zinc.

Essentially then, metal on metal can work to advantage or disadvantage. The specifier must understand the principles of electrolytic action and prejudicial dissimilar metals to be in a better position to decide how to bring metals together, while possibly leaving them in isolation.

Dr Graeme Peacock works for the Product Development Team at Corus Colors. Contact 01244 892434

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