Patina makes exposed rolled zinc applications very durable.Rolled zinc products in building applications have a very long life due to the self-protecting property of zinc. In Europe, there are numerous examples of zinc roofs renovated after about 100 years of continuous service.
The reasons for such a long service time are now well understood:
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The metallic zinc surface reacts with atmospheric oxygen (O2) in the presence of water (H2O) to form zinc hydroxide (Zn(OH)2).
The zinc hydroxide reacts with atmospheric carbon dioxide (CO2) to form zinc hydroxicarbonate (2ZnCO3.3Zn(OH)2) which is the main component of the patina.
The patina forms a layer which is compact, adherent, insoluble in rainwater, and which will hinder any further exchanges between oxygen and zinc, thereby controlling the corrosion rate of rolled zinc and maintaining it at a low level.
Nevertheless, the durability of zinc can be reduced by some acid pollutants, which increase the corrosion rate. The main pollutant is the sulphur dioxide (SO2). Sulphur dioxide reacts with the patina to form zinc sulphate (ZnSO3 + ZnSO4), which is soluble in water and is washed away by rain.
Sulphur dioxide is produced by some industrial plants, oil-fired central heating installations and road traffic. As a result, the corrosion rate is greater in an urban or industrialised area than in a rural environment.
Fortunately, since the 1970's, SO2 pollution of the atmosphere has been considered as a major environmental problem. Therefore, European and other legislators have strengthened legislation against this type of pollution. This has resulted in a general reduction of the concentration of SO2 in the atmosphere and an even more drastic reduction in particularly polluted sites such as cities or industrial areas.
As a result, the corrosion rate of rolled zinc was greatly reduced during the second half of the 20th-century.
Today, the corrosion rate of rolled zinc is generally of 1(1) µm/year. With a typical initial thickness of 0,7 mm and a main corrosion rate of 1(1) µm/an, we see that the life expectancy for rolled zinc is above a hundred years. The life expectancy of rolled zinc has increased over the last decades and will continue to increase over the years to come.