Current Australian and overseas experience demonstrates that in some environments, steel transmission line towers will be well protected against corrosion for seventy years by hot dip galvanizing. Only a few of the steel lattice structures in Australia are this old but overall the performance of galvanized coatings in Australia has been excellent where good maintenance practice has been followed.
The Electricity Trust of South Australia recently completed the construction of a new 70km line to reinforce supply to Whyalla (110km north west of Adelaide across Spencer Gulf) and the far west of South Australia. Given the corrosion performance of other transmission towers in the area and the load capacity of the line, the new line structures could have a viable life of 100 years.
However, two of the towers are located on piles in the waters of Spencer Gulf, which provides a much more corrosive environment. These towers are 110 metres in height, 17 metres wide at the base and each weight 60 tonnes. Their size and location make both access and the work environment for future maintenance very difficult.
Graham Brown, Transmission Structures Engineer with ETSA says "The extremely high cost of maintenance and replacement of these towers encouraged ETSA engineers to work closely with GAA to identify a method of enhancing the performance of the standard zinc coating." Since an increase in life expectancy would be beneficial, a duplex system comprising galvanizing plus a two-coat epoxy paint system was recommended by GAA to provide the required additional maintenance-free life.
Painting was carried out away from the site to minimise contamination of the galvanized steel surface by airborne salts, and any damage to paint coatings during transport was repaired on site before erection. The selected paint colour matches the colour of weathered zinc, so few people would be aware of the extra protection provided.
Close co-operation between all those involved in providing the duplex system at the construction stage has provided benefits expected to be far greater than later maintenance painting over hard-to-clean corroded steel.