Corrosion Fatigue Behavior of Low-Carbon Structural Steel in NaCl Solution

Abstract

Corrosion fatigue is one of the environmental degradations of metallic materials and it has become a great concern as the cause of failure of infrastructures such as bridges and offshore structures. Although it is well-known that corrosion can enhance the degradation of fatigue life, the detailed mechanism of corrosion fatigue is still unclear. Therefore, it is needed to clarify the effects of corrosion on corrosion fatigue life and on the behavior of crack initiation and propagation under cyclic deformation. In this study, corrosion fatigue behavior of low-carbon structural steel is investigated by 4-point bending tests in aqueous NaCl. The material used in this study was low-carbon structral steel (JIS SM490). The chemical composition of the steel is listed in Table 1. The material was heat-treated at 845 OC for 45 min and then quenched in an oil bath. Furthermore, the material was tempered at 550 OC for 60 min and then cooled in a water bath. The mechanical properties of the heat-treated material were evaluated to be 442 MPa of yield stress and 614 MPa of tensile stress. The heat-treated material was machined into a V-notched 4-point bending specimens in the size of 8 mm wide, 100 mm long, and 4 mm thick. The V-shaped notch of 1 mm in depth was introduced at the center of the specimens. The 4-point bending tests were performed in 1 M and 0.1 M NaCl solutions. The applied stress waveform was triangular at the stress ratio of 0.18 and the frequency of 0.167 Hz. The maximum stress applied to the specimens was changed in the range from 500 MPa to 600 MPa. In this study, fatigue life of the steel was investigated as a function of applied maximum stresses. It was found from the results that fatigue life of the steel was reduced in aqueous NaCl compared with that in air. In addition, the fatigue life in aqueous NaCl was improved by applying a cathodic potential to the steel. These results indicate that anodic dissolution of iron can enhance corrosion fatigue damage of the steel in NaCl environment. In addition, although corrosion fatigue life evaluated in 1 M NaCl was almost the same as that in 0.1 M NaCl, crack initiation was accelerated in 1 M NaCl compared with that in 0.1 M NaCl. This suggests that crack propagation is retarded due to the blunting of crack tips in the higher concentration of NaCl. Figure 1