Effects of ionic migration on the concentrations and mass transfer rate in the diffusion layer of dissolving metals

Abstract

The transport processes occurring within the diffusion layer of dissolving anodes are analysed with the help of a mathematical model which takes into account mass transfer by both diffusion and ionic migration in the presence of a supporting electrolyte. The steady-state transport equations are solved for the ionic concentrations and potential difference as a function of distance within the diffusion layer, metal-ion charge and diffusivity, supporting electrolyte concentration and metal dissolution rate. Upon normalization of the variables, a dimensionless group\((I = ix/zFc^0 D_{M^{z + } } )\) is obtained. This group includes the transport properties of the system and shows the inter-relationship between them. The anodic dissolution of Cu in HClO4 was chosen to test some of the predictions of the system. The measured metalion concentrations were much less, while the potential gradient was much higher, than predicted. This is explained on the basis of ionic interactions which operate at higher concentrations. It is shown, both theoretically and experimentally, that in this strong acid medium the concentration of hydrogen ions decreases in the diffusion layer of a dissolving anode due to ionic migration of the hydrogen ions in accord with the prevailing potential gradient.