Hot dip galvanized steel is among the maintenance-free materials whose corrosion resistance can be expected to last over fifty years. In reality, however, instances of structures whose steel has corroded in around twenty years have been found (Fig. 1). For train track steel structure supports made of hot dip galvanized steel, the Railway Technical Research Institute (RTRI) has developed criteria for determining whether measures for preservation of over sixty years are required, and for preservation methods by coating.
      Standards have been established for the site administrator to determine whether preservation measures are required, based on the age of the structure and on observations of rust conditions in each region, utilizing characteristics such as different corrosion speeds between members and regions, and changes in rust elements as corrosion progresses (Table 1).
      Paint preservation methods for corroded hot dip galvanized steel were also examined. Firstly, the paint was selected and then a paint property test was performed. The test showed that the durability of a coat of paint applied to a surface with a remaining plating layer is upwards of double that applied after steel corrosion started. It was also shown that paint with a slower deterioration speed leads to better effects on structure durability improvement. Additionally, it was shown that the effects of the double coating finish method combining urethane resin paints (with sufficiently slow deterioration speed) and epoxy resin paints with high environment isolation are remarkable. Under this method, if the coating applied before the hot dip galvanized layer wears thin and steel corrosion starts, durability of the paint layer after repair can be expected to be more than twenty years.
      The above technology can also be applied to hot dip galvanized steel bridges.