Ultrasonically induced cavitation studies of electrochemical passivity and transport mechanisms

Focused ultrasound was used to investigate processes that influence dcpassivation and repassivation phenomena on pure Fe and on cast iron in 2N H2SO4 at 1.58 MHz and at intensities up to 7.8 kW/cm 2. At sufficiently high intensities, ultra-sound was found to affect significantly thc time of passivation and to hinder rcpassivation completely. The critical acous-tic focal intensity, either required for depassivation or to prevent repassivation, was found to vary exponentially with po-tential. 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. Metals that are protected from corrosion by spontaneous fbrmation of surface films may be susceptible to attack in the presence of ultrasonic avitation if the protective sur-face film is damaged at a rate that exceeds its ability spon taneously to repair. The purpose of this investigation (Part II) is to clarify the critical conditions for breakdown by ultrasonic cavitation f Fe and of cast iron in H2SO4