Hydrodynamic effects on the macromorphology of electrodeposited zinc and flow visualization: The effect of neutral salts and electric field

Abstract

The macromorphological features of electrodeposited zinc that result from hydrodynamic flow conditions on a rotating-disk electrode (RDE) and along a flat-plate electrode (FPE) in a channel flow cell have been investigated as to the effect of neutral salts, their complexing contributions to the growing surface morphology and in particular their influence on the electric field otherwise counterbalancing the flow field. All complex hydrodynamic characteristics of the re-attaching flow and the presence of streamwise-directed counter-rotating Taylor vortices behind the critical region of the stagnation and separation are clearly outlined and marked by the macromorphology of three-dimensional relief imprints. The macromorphological features of relief equiangular spirals (RDE) and ridged striations (FPE) in zinc electrodeposition represent mainly the competition between the flow field and the electric field strength, and depend on the average current density, the rotation speed (RDE) or linear flow velocity (FPE), the deposition time, the ionic zinc content, the electrolyte composition, the pH value and the local hydrodynamic flow pattern. The counterbalancing effect of a rather more homogenized electric field on and in the vicinity of the electrode surface, as provided and maintained by higher neutral salt concentrations, in particular at low zinc contents, has been clearly outlined and resulted in more compact flatter deposits. The flow field contribution is strongly suppressed, but is not eliminated, so that the macromorphology of electrodeposited zinc is characterized by a proportionally lower number density of corrugated striations. The ridged spirals (RDE) and corrugated striations (FPE) still persist and grow in more compact, flatter deposits alternating with depression channels and exhibit some features characteristic of a more homogeneous electric field.