Experimental study and modelling of heat transfer during anodizing in a wall-jet set-up

Abstract

The anodizing of aluminium is an electrochemical surface treatment yielding the formation of an alumina film, the characteristics of the formed oxide strongly depending on the considered anodizing conditions. Heat transfer has an important influence on the anodizing process, which can be explained by considering the production of heat near the aluminium anode, combined with the significant influence of the local electrode temperature on the process of oxide formation. The influences of temperature and heat transfer on the growth of the anodic oxide film during anodizing of high purity Al are studied on a laboratory scale in a wall-jet electrode reactor. The impinging jet configuration of the reactor creates a non-uniformly accessible electrode with variable convection as a function of the radial position on the anode. The influence of the resulting non-uniform heat transfer on the local temperature of the electrode is monitored by local temperature measurements on the backside of the aluminium anode, whereas its influence on local film growth is evaluated by means of FEG-SEM surface and cross sectional analyses. A comparison between the simulated and experimentally acquired data is presented. The controlled and known electrolyte flow in the wall-jet reactor enable numerical simulations of the convection which supply additional information on the encountered conditions of heat transfer. The anodizing process itself is simulated using a model based on the high field theory.

Anodizing of aluminium under applied electrode temperature: Process evaluation and elimination of burning at high current densities

“…This study considers anodizing experiments performed under conditions of controlled convection in an electrochemical reactor with a wall-jet geometry. Similar to the presented results, variations in the local electrode temperatures were considered in [22]. However, in the latter case they were deliberately evoked during the anodizing process by changing the electrolyte convection (and thus influencing the convective heat transfer).…”

“…For the potential evolutions, presented in Fig. 7(a), these standard deviations vary between 0.2 V and 0.25 V. An estimation of the corresponding fluctuations of the actual electrode temperature, inducing these potential variations, can be obtained by considering a previous study by the authors [22]. This study considers anodizing experiments performed under conditions of controlled convection in an electrochemical reactor with a wall-jet geometry.…”

Anodizing—The pore makes the difference

“…[ 103,106 ] Thermocouples, on the back side or embedded in the anodized aluminum sample, were used to measure the heat generation during the anodizing in operando . [ 73,103,107,108 ]…”

“…[103,106] Thermocouples, on the back side or embedded in the anodized aluminum sample, were used to measure the heat generation during the anodizing in operando. [73,103,107,108] In the aforementioned joint project between Fraunhofer IKTS Dresden and DECHEMA-Forschungsinstitut the pore size of a modified phosphoric acid anodizing (MPAA) process could be varied by a combination of parameters approximately between 40 and 140 nm. Thereby, the determination of the pore parameters (size, number) is based on the evaluation of scanning electron microscopic images.…”