Spatial and Temporal Instabilities in a Technical Process, Electropolishing of Brass

Electropolishing is a technical process to obtain smooth and shiny surfaces and a means of electrochemical metal removal, where the workpiece is the anode. The operation point of electropolishing is the transpassive region of the cyclovoltamogram, where two competing processes occur: The dissolution of metal (Cu and Zn) and the hydrolysis of water. Due to the hydrolysis of water, oxygen is formed at the anode causing gas bubbles to rise.Within the scope of this work, the behaviour of brass CuZn37 in electrolytes containing phosphoric acid and alcohol was studied. The anodic dissolution of brass in the transpassive region created a complex temporal behaviour. Depending on the electrolyte mixture chosen and the current density employed, different dynamical regimes could be realised. Time series and cyclovoltamograms revealed the appearance of an N-NDR or N-HNDR. Therefore, this technical process could be categorized according to the classification of dynamical electrochemical systems.When electropolishing was performed for a time period that was longer than an hour, stationary spatial pattern formation occurred on the electrode. These patterns were in the range of micrometers and could be characterised by employing methods of fractal geometry. The spatial structure formation typical of electropolishing processes is formed by an overlay of two competing phenomena. On the one hand, there is an isotropic structuring due to the formation and the growth of polarized oxygen bubbles at the anode. On the other hand, an anisotropic structuring occurs when the gas bubbles release from the metal surface and rise in front of it. Enriched electrolyte of a higher density submerging in front of the anode interacts with these gas bubbles. The interplay of all these processes leads to the typical streaming (Gasbahnen). Therefore, the timing and the mode of interaction between the gas bubbles and the metal surface are critical parameters for the final structure of the metal surfaces