Thorough electrochemical kinetic and energy balance models clarifying the mechanisms of normal and abnormal growth of porous anodic alumina films

Abstract

Thorough kinetic and energy balance models of growth of porous anodic alumina were formulated. Insight into film growth mechanisms showed that all local processes interact with those in the rest surface and the processes rates and parameters vary along Al surface, imperceptibly in normal and perceptibly in abnormal film growth. Al was anodised at 0.51 and 1.02M H2SO4, 0–25°C and anodic potential 23.5–25V. The normal growth or abnormal one presenting island-like and strong burning were discerned and related with peculiar chronoamperometric plots. Island-like burning emerges at random places in the two Al faces above threshold conditions after a current density boundary in the second transient stage. Among this stage and quasi-steady state the plots show intense maxima and poor reproducibility. More afar from them this burning near a side is transformed to strong at corresponding positions in Al faces at this stage after a higher current density boundary. Plots rise fast coming from fast rise of current in burnt area due to avalanche-like hastened processes. Burning is a joint kinetic, Joule heating, thermochemical and heat transfer local effect yielding higher local current density and real anodising temperature. It emerges after the self-regulated mechanism of normal film growth collapses when local starting parameters, like thickness of barrier layer and/or attached electrolyte layer, etc. distributed in Al surface exceed limiting values which vary with conditions, experimental details and position. New methods to avoid burning can be elaborated that are vital for proper Al anodising.