Crack detection by harmonics analysis of the polarization current during corrosion fatigue of iron

Corrosion fatigue tests of a commercial iron with a hole were carried out in a borate buffer solution containing 5 mol/m³ NaCl at a constant passive potential. Polarization current behavior induced by strain cycles has been investigated in order to detect crack initiation from the hole and used for the estimation of corrosion fatigue damages. The amplitude of the polarization current synchronized with the strain cycle and the phase shift between the polarization current and the strain were calculated from the polarization current behavior. The second harmonics component of the polarization current was also calculated. In the early stage of the corrosion fatigue test, the phase shift was almost constant and the second harmonics component was not detected. However, the phase shift decreased and the second harmonics component of the polarization current increased with increasing fatigue cycle. It is considered that changes of the phase shift and the second harmonics component of the polarization current result from crack initiation and the early stage of crack propagation from the hole.