Ultrasonically induced cavitation studies of electrochemical passivity and transport mechanisms ii. experimental

Focused ultrasound was used to investigate processes that influence depassivation and repassivation phenomena on pure Fe and on cast iron in 2NH2SO4at 1.58 MHz and at intensities up to 7.8 kW/cm2. At sufficiently high intensities, ultrasound was found to affect significantly the time of passivation and to hinder repassivation completely. The critical acoustic focal intensity, either required for depassivation or to prevent repassivation, was found to vary exponentially with potential. Once the oxide film had formed on the metal, the acoustic focal intensity needed to break down the film depended logarithmically on the time of passivation. The mass transfer coefficient in the focal region was found to be proportional to the acoustic focal intensity to the one-third power.