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ROLLED ZINC DURABILITY

1. Introduction Because of the capacity of zinc to protect itself when it comes into contact with the main components of the atmosphere, rolled zinc products used in the building industry have a very long life span. When rolled zinc comes into contact with oxygen, water or carbon dioxide in the atmosphere, a compact coating that is adhesive and not very soluble in rain water forms on the surface of the material. This coating considerably reduces the speed at which the atmosphere’s components are diffused in the zinc, and thus reduces the speed of corrosion. The durability of rolled zinc can be affected by certain atmospheric pollutants that increase the speed of corrosion. The most aggressive of these is sulphur dioxide (SO2), which is produced essentially by industrial production sites, thermal power plants and road traffic (which explains why the speed of corrosion of rolled zinc can be 4 times higher in an industrial area and twice as high in urban areas than in rural areas). Since the 1970’s, European and international legislation has been strengthening measures to fight sulphur dioxide pollution, which has led to a considerable reduction in its concentration and therefore has slowed down the speed of corrosion of rolled zinc. Today, the average speed of corrosion of rolled zinc is 1 µm per year. With an initial thickness of 0,7 mm and a level of corrosion of 1 µm/year, a simple calculation demonstrates that the estimated life span of rolled zinc is over a hundred years. The lifespan of rolled zinc increased over the last decades and will continue to increase in the years to come. 2. Zinc Corrosion mechanisms and patina formation1 The mechanism of zinc atmospheric corrosion The barrier properties of oxides formed on metallic surfaces control the way that corrosion, or other chemical attack, occurs. In the case of zinc exposed to atmospheric conditions, passive film formation (the patina) is highly dependent upon the kinetic processes that occur within local chemical environments. Upon exposure to air and moisture, zinc oxidation leads to the formation of ZnO and Zn(OH)2 species at the surface. These oxides are slowly converted to zinc hydroxycarbonates (Zn4CO3(OH)6�H2O and Zn5(CO3)2(OH)6) in the presence of carbon dioxide and atmospheric moisture, a process which occurs within minutes and hours.

1 Tim H. Muster, Ivan S. Cole : The protective nature of passivation films on zinc : surface charge, Corrosion science 46 (2004) 2319-2335

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The influence of pollutants and chloride Zinc (II) ions also coordinate with sulphate and chloride species which presence lead to the formation of species such as zinc sulphate, zinc hydroxysulfate (Zn4SO4(OH)6�nH2O), zinc hydroxychloride (Zn5 (OH)8Cl2�H2O) and zinc hydroxychlorosulfate (Zn12(OH)15(SO4)3Cl3�5H2O). The presence of zinc hydroxycarbonate as a passive layer is widely accepted as being advantageous for the longevity of zinc, a characteristic that is primarily attributed to a superior insolubility when compared to oxidation products containing simple oxides and chloride species. The sulphur dioxide (SO2) contained in the atmosphere is the main pollutant which determines the zinc corrosion rate. A linear dependence between zinc corrosion rate and the SO2 concentration in the air has been demonstrated. During the last decades, the SO2 emissions have considerably decreased

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Paris situation UK situation As a consequence, the average zinc corrosion rate has been reduced by more than 3 from 1980 until 1997 (4-5 µm/year to 1,3 µm/year ) and the corrosion rate in rural and urban environment has became more or less similar. The following graph presents the corrosion rate evolution in typical environments.

Specific environment like marine atmosphere have lead to detailed corrosion studies since the middle of the XXth century. A literature review reveals that zinc corrosion in marine atmosphere is equivalent with urban or rural atmosphere, as salt deposits are washed up by rain falls, with the exception of areas exposed to sea water showers. Oxidised species on zinc surface are mainly insoluble hydroxychloride (Zn5 (OH)8Cl2�H2O).

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The evolution of the SO2 content in Parisµg/m 3

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The evolution of the SO2 content in Parisµg/m 3

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3. Rolled Zinc durability Rolled zinc durability, when installed properly, is essentially linked with its corrosion rate, therefore with the site’s atmospheric characteristics. The use of rolled zinc for roofing has started at the beginning of XIXth century in Belgium with the development of zinc casting and rolling. It is in the 2nd half of the century that zinc roofs have become popular. Since this period, the rolled zinc use has rapidly grown. Many zinc roofs installed before the end of the XIXth century were still in place in the 80’s2, proving that despite an exposure to strong corrosive atmosphere, the durability of rolled zinc was often longer than 80 years in urban and marine environment and 100 years in rural environment. Thanks to the continuous reduction of SO2 atmospheric concentration, the rolled zinc durability has continued to increase. Considering the following updated data : � the higher corrosion rate is about 4 µm/y, � the standard zinc sheet thickness is 0,7 mm, � a zinc roof is more or less replaced once 75% of the zinc thickness has corroded, it is easy to explain than estimated life span of rolled zinc is now over a hundred years.

4. Cases study

Detailed metallurgical studies have been possible on samples taken from recently dismantled or inspected zinc roofs.

4 cases are presented hereafter :

Bray&Lu church (Fr) : 130 years Liverpool Central Library (UK) : 130 years Belgium : 80 years Paris Prefecture (Fr) : 50 years

The exact initial thickness of the zinc used for these buildings is not known but the design rules in force at the time of construction give a reliable hypothesis of this thickness. Thanks to measurement of the residual non oxidized zinc thickness, an average corrosion rate has been calculated for these building. Results are summarized in the following table. It is interesting to highlight the good correspondence between these results and the corrosion rate figures, mentioned in the paragraph 2, known from the literature and more recent corrosion studies.

2 On site examination of buildings with zinc roofs. 1956 and 1984-1985 Chambre syndicale du zinc, in : Résistance à la corrosion atmosphérique des alliages de zinc laminés, Jannuary 1986

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Building Zinc atmospheric

exposure duration (years)

Residual metallic zinc thickness (mm)

Initial thickness (mm)3

Estimated average zinc corrosion rate

(µm/y) Bray & Lu church

130 0,18 0,58 3,1

Liverpool central library

130 0,06 0,82 5,8

Belgium case 80 0,30 0,74 5,5 Paris prefecture 50 0,45 0,65 4

These studies confirm that even through long lasting exposure to stronger corrosive conditions than nowadays, zinc roofs have demonstrated a very long durability, in accordance with the figures of paragraph 3.

In conclusion, it is now well established that rolled zinc durability is nowadays : � More than 100 years in rural and urban areas � More than 80 years in marine or industrial areas

3 Gazette des Architectes du Bâtiment, Encyclopédie d’Architecture, n°14, 1863, Paris

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Bray & Lu church

� Estimated duration of zinc outdoor exposure : 130 years � Type of use : roofing � Environment : rural � Corrosivity : low

The Bray & Lu church

A zinc tile from the Bray&Lu Church

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Microstructure (reflected light) The thickness of the corrosion layer at the front surface (facing the outdoor environment) is very irregular resulting in a rather rough surface. The thickness of the corrosion layer can

vary between ±55µm and ±175µm but most of the time the thickness is between 55 and 100µm.

The corrosion layer at the back is thinner, namely about 60µm. Sporadically, the bottom layer shows a spot with a rather thick corrosion layer of ±170µm.

According to EDS microanalysis the corrosion layer is build up as follows starting from the upper outside:

√ a mix of Zn metal and Zn oxide

√ the main middle part is Zn sulphate

√ a rather thin irregular layer that is a mix of Zn oxide, Zn chloride and Zn metal

The thickness of these layers is very irregular and depends on the total thickness of the corrosion layer. Some Pb metal precipitates are clearly present in the Zn matrix. The minimal residual thickness of non corroded zinc varies from ±180µm to more than 400 µm.

Back side

Front side

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Liverpool Central Library

� Estimated duration of zinc outdoor exposure : 130 years � Type of use : roofing � Environment : mix of urban, industrial and marine � Corrosivity : high

Central Library roof zinc cladding

A tile from the Liverpool central Library

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Microstructure (reflected light)

The thickness of the corrosion layer at the front surface is extremely irregular and rather excessively large and therefore resulting in a very rough surface. The thickness of the

corrosion layer can vary between ±215µm and ±900µm but most of the time the thickness of the upper corrosion layer is between 250 and 450µm. At some spots, the corrosion reaches nearly the lower corrosion layer at the bottom surface.

The corrosion layer at the back is much thinner and more regular and has everywhere a

thickness of ± 65 to ±95µm. According to EDS microanalysis the corrosion layer is build as follows starting from the upper outside:

√ Zn sulphate in which some islands with a mix of ZnO and Zn metal are seen

√ the main middle part is Zn sulphate

√ a rather thin layer in which ZnO is dispersed in Zn the matrix

No Cl has here been detected.

The thickness of these layers is also very irregular and depends on the total thickness of the corrosion layer. Some Pb metal precipitates are clearly present in the Zn matrix. The minimal residual thickness of non corroded zinc is ±60µm.

Upper side

Bottom side

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Belgium

� Estimated duration of zinc outdoor exposure : 80 years � Type of use : roofing � Environment : urban � Corrosivity : medium

The analysed zinc samples

Microstructure (reflected light)

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Microstructure (reflected light) The thickness of the upper corrosion layer is irregular resulting in a relatively rough surface. The

thickness of the corrosion layer can vary between ±70µm and ±300µm.

The thickness of the bottom corrosion layer is very regular and is ±70µm. The bottom side has a very smooth and regular surface. The minimal residual zinc thickness between upper and bottom corrosion layer was measured at several places and gives an average of about 270µm.

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Paris Prefecture � Estimated duration of zinc outdoor exposure : 50 years � Type of use : roofing � Environment : urban � Corrosivity : medium The zinc samples have been taken from the existing roof thanks to the refurbishment of the slates roof.

Prefecture of Paris

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Microstructure (reflected light)

The thickness of the upper corrosion layer is rather regular resulting in a surface that is relatively smooth. Sometimes a pit of ±100µm deep, showing no corrosion layer, can be seen at the upper

surface. The thickness of the corrosion layer can vary between ±20µm and ±80µm with a mean thickness of 53µm (mean of 7 measurements).

The thickness of the bottom corrosion layer is very regular. It can vary between ±40µm and ±70µm with a mean thickness of 58µm (6 measurements). The bottom side seems to have a very smooth and regular surface. The minimal residual zinc thickness between upper and bottom corrosion layer was measured at several places and gives an average of about 450µm.