Corrosion Fatigue Mechanism and Its Electrochemical Evaluation in Moist Environments

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This research project was proposed to simulate the corrosion fatigue behavior in wet and dry or moist environments and to establish an evaluation method based on electrochemistry. In this project, anodic dissolution current at a crack tip or change in electrode potential induced by mechanical strain were measured and those wave forms were analyzed by Fourie analysis. Our preliminary experiments suggested that the content and magnitude of higher harmonics component in the current or potential responses have good relation with the initiation and growth of the fatigue cracks. The main purpose of this project is that to establish a procedure to detect and evaluate the crack initiation and growth rate from the electrochemical data.Harmonics analysis of the crack current which synchronized to the strain wave revealed that the initiation of crack was detected clearly. A model of crack growth was proposed and checked again for the simulated current by the harmonics analysis. These simulations … More performed for dipped condition supported that opening of the crack at the tip started when the strain reached to some threshold level and decay of the anodic current occurred exponentially.Under a wet and dry cycle condition, the crack and surroundings were covered very thin water layer. Before the crack initiation, the potential waveform in drying condition was less than 0.5mV and this potential change caused by charge and discharge of the electrical double layer. After the crack initiation, the amplitude of the potential increased up to several mV and opening of the crack leaded a decrease in potential so that the sinusoidal waveform was truncated around the maximum strain. The harmonics analysis on the potential waveforom indicated that the second harmonics component increased by the crack, so this can be used as a indication of crack initiation for the monitoring of corrosion fatigue. In case of corrosion fatigue test by four points bending, very thin water layer formed for long time and anodic reaction was suppressed by deposition of corrosion products, therefore acceleration of the crack growth rate did not observed. Less