More than 140 years have already passed since the opening of the first railway in Japan. During that period, a lot of railway structures were constructed. Many of them have been used for more than 100 years and are still in service. In many cases, it is necessary to continue to use those existing structures with appropriate maintenance because the replacement is difficult while in service.This paper firstly describes an outline of current status of the existing railway structures, especially steel bridges, and maintenance of them. Secondly, it introduces recent topics of research and development regarding the maintenance of the steel bridges, such as monitoring, renewal and seismic retrofit.

Corrosion kinetics of a stainless steel in nitric acid at elevated temperature has been investigated with special emphasis placed on its correlation with reduction rate of Cr (VI) in the solution. Extra High Purity (EHP) stainless steel, which is immune to intergranular corrosion in trans-passive potential range in nitric acid solution, has been employed to examine the dominant factor of corrosion rate. The results indicate that dissolution amount of the alloy agrees well with the stoichiometric value of dissolution based on the reduction amount of Cr (VI). This fact means that the dissolution kinetics of the stainless steel is simply dominated by reduction rate of Cr (VI) in the boiling nitric acid including Cr(VI). Reduction rate constant of Cr (VI) follows the identical Arrhenius equation including all the data with and without boiling, although oxidation rate constant of Cr (III) to Cr (VI) is strongly affected by boiling. A dominant factor of corrosion rate has been cleared based on the reduction rate constant of Cr (VI) and the oxidation rate constant of Cr (III).

SUS304 stainless steels with the metal/glass crevice were tested potentiostatically in artificial seawater. The propagation behavior of crevice corrosion during the test was observed by the original apparatus. Geometric analysis of the crevice corrosion propagation behavior with time was carried out by using crevice corrosion shape parameters that were newly defined in this paper. The main results were as follows:
1） The corrosion depth of initiation site was about 20μm, and the place of maximum corrosion depth was located near crevice edge in any applied potential. The maximum corrosion depth was deeper under the higher applied potential. The corrosion depth of the dark gray area was below about 5μm. From these results, crevice corrosion mainly occurred at the interface between passive area and corroded area until the tip of corrosion reached to the edge of crevice. This means the dark gray part was only the trace of corrosion with corrosion products.
2）The shape of corrosion area changed to a quasi-elliptical shape from a apparently round one with crevice corrosion propagation. This tendency became stronger at higher potential and/or as the corrosion area got close to the crevice edge.
3）The major axis length (a) was divided into two parts at the initiation site, a_edge for the edge of crevice and a_mp for the center of crevice. a_edge became large exponentially as higher applied potential and as approaching the edge of crevice. However, a_edge became slightly small with time, and the potential dependence was small.
4）It was considered that the expanding speed of the crevice corrosion was determined by the potential of the corrosion tip. The current density at the tip was larger as the tip close to the crevice edge by increasing of the potential of the tip. As the result of this phenomenon, the expanding speed was faster as the tip position close to the crevice edge.