2021 Volume 107 Issue 12 Pages 1047-1056
Steels are widely used for structural material of bridges. Some of bridges have been used for more than half century, and degradation of bridges become severe problems, due to the atmospheric corrosion of metallic materials. Especially, atmospheric corrosion of steel is caused by cycles of wet and dry conditions, and accelerated by evaporation of raindrops containing Cl- ions. In this study, corrosion of pure iron and steels is investigated under wet-dry cycling condition with NaCl solution by scanning electron microscopy (SEM) and 3D-optical microscopy (3D-OM). Initially 20 mm3 of 0.02 M-NaCl solution was dropped on pure Fe, SM490Y, and SMA490AW specimens, and then, droplets of water were dropped at the same position of the specimens at 9 min intervals for 150 cycles.
All specimens were covered with white, black, and reddish corrosion products at the water-dropped position, and the reddish ones became major with increasing water-dropping cycles. SEM images after corrosion product removal showed pitting corrosion on all specimens, and the corrosion was more severe at the edge areas of water-dropped position than at the center areas. 3D-OM obtained after 150 cycles showed that the deepest pit produced at the edge areas were in the order of Fe >> SM490Y = SMA490AW, and that the total volume loss at the edge areas by corrosion were in the order of Fe > SMA490AW > SM490Y. The corrosion mechanism can be explained by higher rates of O2 supply at the edge areas and denser corrosion products on steel than on Fe.
