Ultrasonically Induced Cavitation Studies of Electrochemical Passivity and Transport Mechanisms: I . Theoretical

Abstract

High intensity ultrasound causes the formation of cavitation bubbles which collapse in a manner that, in the presence of a solid surface, form high velocity fluid microjets directed toward the surface. The intense fluid agitation at surfaces in a focused ultrasound field enhances transport of momentum, heat, and mass and influences the behavior and integrity of surface films on metal electrodes. A mathematical model is proposed to determine the reaction and transport between liquid microjets and a reactive solid surface. The conditions were estimated under which oxide depassivation and repassivation occur as a function of ultrasonic intensity, surface film thickness, and fluid microjet surface coverage. The model was based on the concept that cavitation induces sufficient momentum and mass transfer rates (water hammer pressures) at a surface to create oxide film stresses leading to depassivation. The model was used in a companion paper (1) to evaluate experimental data on the corrosion behavior of iron in sulfuric acid.Collapse of cavitation bubbles near solid surfaces causes the formation of local fluid microjets which create intense pressure fields and vigorous local fluid motion. For passive metals immersed in corrosive solutions, such cavitation phenomena can influence passive films as well as depassivation and repassivation events. In Part I of the present investigation, a mathematical model of cavitation-induced effects on passivity and transport processes near surfaces is developed. In Part II (1), experimental data are reported and evaluated with use of the model.

Electrochemical and Photographic Detection of Cavitation Phenomena within a Variable Frequency Acoustic Field

“…19 Perusich and Alkire investigated the effect of a focused ultrasonic field on mass transport to an electrode. 20, 21 Dewald and Peterson employed a 20 kHz ultrasonic source and constructed a hydrodynamically modulated electrode by the use of a pulsed ultrasonic field. 22 Zhang and Coury demonstrated that high pseudo-steady-state mass transfer with impressive mass transfer rates could be achieved by positioning an electrode within the plume of an operating ultrasonic horn.…”

Combined pulsing currents and agitation for electrodeposition

“…This effect can contribute significantly towards improvement of mass transfer. [43][44][45][46][47][48][49] However, micro-jetting can also cause pitting and erosion to the electrode surface during electrodeposition. 43,50 Additionally, there remains a concern regarding US agitation, which itself is generated by cavitation, which may increase the number of gas bubbles.…”

Aplicação do ultra-som em sistemas eletroquímicos: considerações teóricas e experimentais

“…A aplicação do ultrasom a uma solução pode aumentar drasticamente o transporte do material eletroativo à superfície eletródica. Muitos autores têm se dedicado a desenvolver descrições quantitativas e à identificação dos processos físicos envolvidos 18,19,[61][62][63][64] . Na Figura 2a estão apresentados voltamogramas cíclicos típicos da reação do par reversí-vel Fe(CN) 6 4-/3-sobre um eletrodo de DDB em meio de KCl 0,1 mol L -1 na ausência (I) e na presença do ultra-som (II).…”

“…Os efeitos da aplicação do ultra-som no transporte de massa são complexos e têm sido propostos teoricamente 18 , empiricamente [19][20][21] e por modelos aproximados de simulação 22,23 . Importantes aspectos da sonoeletroquímica foram relatados por Walter e Phull 24 mostrando a quantificação de processos sonoeletroquímicos.…”