Estimation of Corrosion Fatigue Damage from the Current Response at the Crack tip

Abstract

In the previous paper, the corrosion fatigue test of commercial pure iron had been performed in 5 mol/m³-chloride-containinig sodium borate and boric acid buffer solution of pH 8.39. The response current to the applied cyclical elastic deformation in two different strain waveforms, sinusoidal and trapezoidal was successfully simulated using two parameters, a decay factor and frequency of the applied strain waveform, where the decay factor is a parameter indicating the repassivation ability. The response current was found to be due mainly to charging or discharging current of electrical double layer before crack initiation and the current (damage current) due to iron dissolution and repassivation from the crack tips superimposed on the charging current after crack initiation. In this study, the effects of the decay factor and the frequency on the corrosion fatigue damage have been investigated.
The simulation results showed that the amount of charge of the damage current increases with decreasing decay factor, and with increasing frequency under a constant period. The decrease in the decay factor implies the promotion of the anodic dissolution at the crack tip. In other words, the chemical process (corrosion) contributes largely to the crack propagation. On the other hand, since the increase in the frequency reduces the time factor for the progress of corrosion reaction in each strain cycle, the mechanical process becomes more important factor for the crack propagation in high frequency corrosion fatigue test.
From the simulation results of the total current due both to the double layer charging or discharging and anodic dissolution (damage current), it was found that the increases in the ratio of the damage current increases the higher harmonic components in the response current and decreases the phase shift between the applied strain and the response current. Accordingly, the degree of corrosion fatigue damage can be detected by the monitoring of these harmonic components and phase shift.