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The several types of sonication methods were applied to access the different morphologies of ZnO nanostructures on the surface of mild steel. To achieve this goal, a sonictor equipped with the probe extender was used as a high intensity ultrasonic apparatus for direct sonication. Furthermore, an ultrasonic bath (low intensity) and a cup-horn system (high intensity) were applied for indirect sonication. To find the effect of the acoustic waves on the ZnO morphology, the micrographs of obtained surfaces were compared to the sample prepared by the conventional method using scanning electron microscopy (SEM). In this work, the beneficial effects of sonication were subjected on the breaking down the agglomerates to smaller size particles, metal surface activation, and on the facile approach to nanostructures synthesis. The influence of the resulting ZnO structures over the corrosion protection of the electroless Ni-P alloy coatings was evaluated by the potentiodynamic polarization technique (Tafel extrapolation).

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Considering the current interest in the preparation of modified surfaces with fixed nanostructures, in this research, the hot water treatment was chosen as a simple and efficient procedure for the direct growth of ZnO nanostructures (ZnO NSs) on the metallic surface. To improve the method and reduce the cost of experiments, the commercial galvanized steel (steel containing a thin layer of zinc) was used as the base substrate. The use of the galvanized steel as an inexpensive and available metal surface has advantages compared to the pure Zn foil (used in other researches). In addition, in the present work, the effective pretreatment of the base substrate surface by applying the chemical etching under the low intensity ultrasonic irradiation facilitated the reaction progress (Zn → ZnO) in a shorter time and relatively low temperature in comparison to many reports. To find the effect of acoustic energy on the final ZnO morphology, the hot water treatment was carried out under conventional and sonochemical conditions. Finally, as a practical application, resulting surfaces were coated with electroless Ni-P deposits and the effect of as-grown ZnO NSs (as the sublayer) on improvement in the corrosion protection of binary Ni-P deposits was evaluated by the electrochemical method.