The influence of kinetic & hydrodynamical factors on the structure & properties of conversion coatings

For each crystallization process, including the formation of conversion coatings, there must be 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 summarizes the results from a series of experiments that were aimed at decreasing the thickness of the diffusion layer by the combined application of pulsed cathodic polarization (up to 150 mV) on a rotating disk electrode and the addition of a hydrodynamically active (HDA) agent to the forming solution. The combination of these two factors results in accelerating the partial reactions of oxidation and reduction. By varying the pulse frequency of applied cathodic polarization, we can regulate and control both processes. The resulting leveling power dependence on pulse frequency helped define the optimal pulse frequency for the formation of conversion coatings with homogeneous and fine-grained structure. It coincides with the value of pulse frequency of the maximum leveling power.