The Zinc Millennium Map provides potential cost savings

Citation

(2001), "The Zinc Millennium Map provides potential cost savings", Anti-Corrosion Methods and Materials, Vol. 48 No. 6. https://doi.org/10.1108/acmm.2001.12848faf.001

The Zinc Millennium Map provides potential cost savings

The Zinc Millennium Map provides potential cost savings

Keywords: Galvanizers Association, Coatings, Galvanizing.

A galvanized steel bridge built in 1990 with a life expectancy of 38 years can reportedly now be expected to last a further 55 years.

This potential for significant financial savings on construction and infrastructure projects has been highlighted by a recently completed study to monitor atmospheric corrosion rates of zinc conducted throughout the length and breadth of the UK and Ireland.

The results of the Zinc Millennium Map Project are said to demonstrate how the reduction in S02 levels since 1991 has helped to increase the life expectancy of hot dip galvanized coatings. While it is well known that levels of S02 have been falling for several decades, these new findings have a particular significance for those with responsibility for sanctioning construction projects where the use of galvanized steel is an option.

It is the S02 in the atmosphere that is the primary determinant of the corrosion rate of zinc coatings on steel structures. The new figures obtained in this major study will support the increasingly common requirement to consider whole life costs and that means looking at longevity and maintenance requirements as well as initial investment.

In this respect the Zinc Millennium Map has particular relevance to authorities spending public money on all manner of things from major infrastructure projects to steel framed car parks, sports stadiums, fencing and railing and even street lights and furniture.

Specifiers can reportedly obtain a clearer guide than was previously possible from BS EN ISO 14713:1999 on the "corrosivity category" of any given site. Whereas previously, urban areas might have been classified as C4, reference to the Millennium Map would indicate that in most instances they are now C3.

Also, previously, a specifier might have concluded that it would be impossible to guarantee a coating life of, say 30 years, using a standard 85µm thick galvanized coating, so making it necessary to provide additional corrosion protection. With many results for built-up areas now being less than 2µm per year, specifiers should now be able to consider the use of a galvanized coating as the sole means of corrosion

Additionally, for structural steel sections where a coating thickness is said to be often well in excess of 100µm, a significantly longer coating life may be achieved. In urban areas where a car park may be constructed for instance, a 140µm coating would reportedly give a 100 year life span and even in the most industrially polluted areas a coating life of 70 years can be achieved. This is state to go far beyond alternative methods of protection and negates the need for regular and costly maintenance.

Atmospheric corrosion measurements on a national scale were first conducted in 1967 to provide the electricity supply industry with the basis for cost benefit analysis of galvanizing transmission towers. The data was updated over the years up to 1991 by specialist corrosion consultant Tom Shaw, MAFF and ADAS (Agricultural Development Advisory, Service). However by 1997 there was evidence of further significant falls in atmospheric S02 and this prompted the UK Galvanizers Association to fund the Zinc Millennium Map project, bringing together all of the previously used expertise and extending the measurements to the Republic of Ireland for the first time. Around 1,000 sample sites were identified for this comprehensive study and the resulting Zinc Millennium Map can be used as a practical planning and specification tool.

Since the first zinc atmospheric corrosion survey back in 1967, results have been updated periodically and a colour coded map of England and Wales showing the background zinc atmospheric corrosion rate was first produced in 1982 by the Ministry of Agriculture, Fisheries and Foods (MAFF) in conjunction with Mr Tom Shaw. This has since been updated, with more recent maps produced by Agricultural Development Advisory Service (ADAS).

However, by 1997 there was evidence to show that sulphur dioxide levels had fallen considerably since 1991 and 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, as mentioned, chose to fund a project 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, the Galvanizers Association utilised all the available expertise from previous projects in the planning and running of the Zinc Millennium Map Project.

Exposure sites

It was essential that as large a number of sites as possible were used in production of the map. Equally important was the fact that the sites offer good coverage of the UK and the Republic of Ireland.

Private sites used in previous work were utilised along with meteorological centres around the country. Orange plc, the mobile phone company, kindly offered their telecommunication towers as fixed sites in order that they may obtain specific data for company installations. Further sites were also 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 various sites. Additional sites within Ireland were also provided by Association members and their contacts.

Test samples

Zinc cans were used as the test samples and each was given a unique identifying number. The cans were cleaned and weighed in accordance with the system developed by Mr Shaw. Then each can was sealed in a dry polythene bag before being packaged with a bracket and accessories for exposure on-site. The cans were 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 can was positioned on a rubber bung which was tightened up by means of a screw so only the exterior of the can was exposed. The bung was attached to a bracket fixed to a wall, ensuring that the can was not in contact with the bracket.

Each sample was exposed for a period of two years in order to permit two climatic cycles of weathering. Towards the end of the two year period letters were forwarded to each site with detailed instructions on sample removal.

Samples were removed on or as near as possible to the second anniversary of their installation. On their return each can was cleaned and weighed. 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 was 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 secondary factors. This is borne out by previous maps produced by MAFF and ADAS in 1986 and 1991 where the areas of highest corrosion rate correlated with areas of high pollution. Additionally, results of testing in Stockholm, Sweden between 1978-92 showed that the average atmospheric corrosion rate of zinc decreased as the sulphur dioxide reduced during the period.

Since production of the previous map in 1991, sulphur dioxide emissions as measured by National Atmospheric Emissions Inventory – DTR have fallen across the UK by over 50 per cent. It would be expected that, given this drop in sulphur dioxide level, 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 1991 ADAS map.

Results for selected towns and cities are reported to indicate a reduction in all regions of the UK, with an average fall in the annual corrosion rate of over 70 per cent.

Further evidence of the fall that has taken place is said to have been given by selecting sites at random which would be considered to be in industrial, urban and rural environments. The downward trend in the background atmospheric corrosion rate for each category is illustrated by Figure 1. This shows that in the period 1986-2000 the average atmospheric corrosion rate for all three environments has fallen by over 75 per cent.

Figure 1 Change in the average background atmospheric corrosion rate for industrial, urban and rural environments during 1986-2000

Conclusions

Sulphur dioxide is the main determinant of the atmospheric corrosion rate for zinc. Sulphur dioxide emissions in the UK, as measured by National Atmospheric Emissions Inventory – DTR, have fallen year on year since 1991 with a total reduction of over 50 per cent.

The average annual atmospheric corrosion rate for zinc has fallen since 1991. A typical reduction of greater than 60 per cent having taken place across most of the UK.

Sites previously considered to be a "polluted industrial inland environment" may now be more correctly classified as an "urban inland environment".

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 city environments) by a standard 85mm thick galvanized coating.

The heavier galvanized coating formed on structural steel sections may achieve a coating life in excess of 100 years in many environments.

Details available from: Galvanizers Association.Tel: +44 (0)121 355 8838; Fax: +44 (0) 121 355 8727;E-mail: ga@hdg.org.uk; Web site:hdg.org.uk