As a result of corrosion surveys, port steel structures have been damaged by splash zone corrosion at splash zone and by concentrated corrosion both at low tide level and in sea water. Corrosion damages could lead to serious reduction in strength and, in the extreme, to collapse and to unexpectedly expensive maintenance work possibly requiring total closures and consequential loss of revenue. An organic or inorganic covering method has been applied to the steel at splash zone and cathodic protection to the steel below sea water level. When the PW method is applied to port structures, there is the strategy of designing to a low first cost followed by more substantial maintenance in accordance with the relationship between discount factors and time for different rates. The strategy would be accompanied by increased risks of unexpected maintenance requirements and lower levels of safety. When maintenance consists of repair and shutdown, shutdown costs can be higher than repair costs and tend to dominate calculation of discounted maintenance costs. Accordingly, there will be scope to design for changes in the balance between construction and maintenance costs from the economical and technical points of view. This paper will look at the balance between first and maintenance costs, the application of the PW method to a steel pipe pile model and some problems in the application of the PW method to port structures.
The objectives of this study are to clarify the dissolution behavior of the ferritic phase (α phase) and the austenitic phase (γ phase) of a duplex stainless steel (DSS), and to propose a corrosion mechanism of the DSS through dissolution behavior of either the α phase or the γ phase at corrosion potential, respectively. Just a single phase specimen (α or γ phase specimen) was prepared by using selective dissolution of the DSS. The α phase showed less-noble corrosion potential compared to the γ phase, and passivated at less-noble potential. When the polarization curves of each phase are summed up, the obtained curve showed good agreement with the DSS polarization curve. The α phase of the DSS dissolved preferentially at corrosion potential. Based on these results, a preferential dissolution mechanism model of a DSS is proposed using internal polarization curves of each phase.
Corrosion rate of steel is controlled by oxygen diffusion through in a 10-100 μm electrolyte film. If the electrolyte thickness decreased to less than 10 μm, oxygen dissolution rate controls the corrosion rate. We developed a numerical analysis model for galvanic corrosion under a thin electrolyte film in consideration of oxygen dissolution rate and diffusion rate. In this model, oxygen salting-out effect, thermal dependence of electric conductivity and oxygen solubility were also considered. NaCl solution salt spray test (SST) and cyclic corrosion test (CCT) were conducted on Fe/Zn galvanic electrode and galvanic current was measured to compare with the numerical results. The galvanic current showed its maximum value during wetting and drying process. The galvanic current calculated by this model agreed with the SST and CCT results. In addition, the electrolyte thickness in the CCT was estimated by numerical analysis model, it is indicated that galvanic current becomes maximum value when the electrolyte thickness is 3-10 μm.