腐食疲労の除荷過程における電流応答に及ぼすき裂閉口の影響

Abstract

Response current to strain for iron electrode polarized at a potential of about 200 mV less than the pitting potential was measured in a sodium borate and boric acid buffer solution of pH 8.39 containing 5 mol/m3 chloride ions. The different types of strain waveforms (triangular, trapezoidal and negative saw teeth) were employed to clarify the current decay behaviors in the unloading cycle. Rapid current decay during the unloading cycle was observed for sinusoidal and triangular strains. For the response current induced by the trapezoidal strain, it was observed that current decay came to an end during the holding period at the maximum strain. Current flow was not observed during the unloading cycle after the current decay finished. It was detected from the negative saw teeth strain waveforms that current decay came to an end during the unloading cycle was under way. It suggested that there was an existence of certain strain value where the current decay stopped during the unloading cycle. The response current in the negative saw teeth waveform was calculated, assuming that the current decay was caused by not only the repassivation process at the crack tip but also the reduction of active fresh surface area which was introduced in the preceding loading cycle. The calculated results agreed well with the experimental results. Moreover, response current in the two cyclic strain waveforms was calculated. One was a triangular strain waveform with strain rate kept constant, the other was a sinusoidal strain waveform with strain rate changing continuously. It was confirmed that the changing in the fresh surface area influenced to the current response induced by the cyclic strain.