M. Aroyo et N. Tzonev, The influence of kinetic & hydrodynamical factors on the structure & properties of conversion coatings, PLAT SURF F, 86(2), 1999, pp. 58-61
For each crystallization process, including the formation of conversion coa
tings, there must he a supersaturated solution, so that any steps taken to
increase the number of nuclei will favor formation of fine-grained coatings
. The greater the supersaturation, the greater the number of crystal nuclei
and vice versa, Because the extent of such supersaturation is proportional
to the concentration gradient of species, the thinner the diffusion layer,
the larger the rate of spontaneous formation of new nuclei. This paper sum
marizes the results from a series of experiments that were aimed at decreas
ing the thickness of the diffusion layer by the combined application of pul
sed cathodic polarization (up to 150 mV) on a rotating disk electrode and t
he addition of a hydrodynamically active (HDA) agent to the forming solutio
n, The combination of these two factors results in accelerating the partial
reactions of oxidation and reduction. By varying the pulse frequency of ap
plied cathodic polarization, we can regulate and control both processes. Th
e resulting leveling power dependence on pulse frequency helped define the
optimal pulse frequency for the formation of conversion coatings with homog
eneous and fine-grained structure. It coincides with the value of pulse fre
quency of the maximum leveling power.