Experimental and Theoretical Investigation of Localized Ultrasonically-Induced Cavitation Effects on Electrochemical Passivity (Corrosion, Mass Transfer, Depassivation)

276 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1985.Focused ultrasound was used to study electrochemical passivity, and the processes controlling the depassivation and repassivation phenomena were quantitatively analyzed. Experimental investigations were coupled with theoretical models to enable the prediction and evaluation of mass transfer rates and fluid pressures near the metal surface.Pure iron and cast iron in 2N H(,2)SO(,4) were studied. A curved lead zirconate titanate piezoelectric transducer with a fundamental frequency of 1.58 MHz generated acoustic focal intensities ranging up to 7800 W/cm('2). The high intensities produced and collapsed cavitation bubbles near the metal surface creating large stresses within the metal oxides which caused film breakdown. Ultrasound was also used to increase the time of passivation and to hinder repassivation completely. The acoustic focal intensity was related exponentially to the potential for both depassivation and repassivation studies. Once the oxide film had formed on the metal, the acoustic focal intensity needed to breakdown the film was logarithmically dependent on the time of passivation.Mass transfer studies, involving a platinum microelectrode and utilizing the limiting current method, were performed in order to observe the influence of ultrasound on the mass transfer rate. The mass transfer coefficient was found to be proportional to the acoustic focal intensity to the one-third power. Further studies led to the development of a mass transfer probe which was used to measure the energy distribution in an ultrasonic field in the presence of cavitation.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD