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THE ZINC MILLENNIUM MAP PROJECT (1998-2000)
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Background
The first atmospheric corrosion study of zinc was conducted back in 1967 and since that time, results have been up-dated periodically. In 1982, a colour coded map of England and Wales showing the background atmospheric corrosion rate of zinc was first produced by the Ministry of Agriculture, Fisheries and Foods (MAFF) in conjunction with Mr Tom Shaw (consultant). Subsequent maps incorporating both Scotland and Northern Ireland were produced in 1986 and 1991 by Agricultural Development Advisory Service (ADAS) again with the assistance of Mr Shaw.
By 1997 there was evidence to show that sulphur dioxide H2O(SO2) levels in the atmosphere had fallen considerably since 1991 when the last map was produced. As SO2concentration in the air is the main determinant of zinc corrosion, in atmospheric exposure, it was likely that the performance of a galvanized coating was being underestimated.
Therefore, aware of the value of up-to-date atmospheric corrosion data for both specifiers and end-users of galvanized steel, Galvanizers Association chose to fund a new study of background atmospheric corrosion rates of zinc to cover the UK and (for the first time) the Republic of Ireland in 1998-2000. By bringing together ADAS as Project Organisers and Mr Shaw as Corrosion Consultant, Galvanizers Association utilised all the available expertise from previous projects in the planning and running of the Zinc Millennium Map Project.
Details of the study are outlined below:
Methodology
Exposure Sites
It was essential that as large a number of sites as possible was used in production of the map. Equally important was the fact that the sites offer good coverage of the UK and Ireland.
Private sites used in previous work for exposure of zinc cans were utilised along with meteorological centres around the country. Orange Plc. kindly offered their telecommunication towers as fixed sites in order that they may obtain specific data for company installations. Further sites were provided by Association members and their contacts.
The expansion of the project to cover the Republic of Ireland meant that a new set of sites were required. These were largely provided by Electricity Supply Board (ESB) who offered to expose zinc cans at various sites. Additional sites within Ireland were also provided by Association members.
Test Samples
The zinc cans used in the project were sourced by Mr T Shaw, each having a unique identifying number. Each of the cans were cleaned and weighed in accordance with a modified version of ASTM Method G1-72 developed by Mr T Shaw which had been utilised in previous projects by ADAS and MAFF.
Following weighing each can was despatched with detailed instructions on exposure of the sample and a brief questionnaire in order to gather additional information such as exact date of exposure, national grid reference, environment type (marine, urban etc.) and height above ground level.
Each of the cans was positioned on a rubber bung and attached to a bracket to be fixed to a wall. They were then exposed for a period of two years in order to permit two climatic cycles of weathering. After these two years had passed the samples were removed and sent for laboratory testing.
On return each can was weighed. Cleaning of the cans was conducted in accordance with the modified version of ASTM Method G1-72 before final weighing. Calculation of the weight loss for each can was then determined and a weight loss per unit area calculated on the basis that only the external surface of the cans were exposed.
Results
In general, sulphur dioxide (SO2) is the primary determinant of the atmospheric corrosion rate of zinc, other considerations such as climatic conditions, air salinity etc. being important secondary factors. Results of testing in Stockholm, Sweden between 1978-92 shows that the average atmospheric corrosion rate of zinc decreased as SO2 levels fell during the period.
Since production of the previous map in 1991, SO2 emissions as measured by National Atmospheric Emissions Inventory - DTR have fallen across the UK by over 50%. It follows that given this drop in SO2 levels, a significant reduction in the corrosion rate of zinc would have taken place.
The results of the Zinc Millennium Map Project indicate that the average annual atmospheric corrosion rate of zinc has fallen across the UK. All of the samples returned showed a reduction in the corrosion rate indicated by the ADAS map of 1991.
Results for selected towns and cities indicate a reduction in all regions of the UK, with an average fall in the annual corrosion rate of over 70%.
Further evidence of the fall that has taken place is given by selecting sites at random which would be considered to be industrial, urban and rural environments. The downward trend in the background atmospheric corrosion rate for each category is illustrated by the chart 'Atmospheric Corrrosion rate in different environments'. This shows that in the period 1986-2000 the average atmospheric corrosion rate for all three environments has fallen by over 75%.
Conclusions
1. The average annual atmospheric corrosion rate for zinc has fallen since 1991. A typical reduction of greater than 60% having taken place across most of the UK.
2. Sites previously considered to be a 'polluted industrial inland environment' may now be more correctly classified as an 'urban inland environment'.
3. The reduction in the atmospheric corrosion rate of zinc means that a long maintenance free coating life in excess of 50 years may now often be achieved (even in urban environments) by a standard 85 µm thick galvanized coating.
4. A heavier galvanized coating, often formed on structural steel sections, may achieve a coating life in excess of 100 years in many environments.
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