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Abstract
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According to the PEO coatings remarkable properties, this surface process is extensively used in order to modify the surface and improve the properties of valve metals and their alloys. However, few articles about PEO treatment on niobium have been published. This study reviews effects of electrolyte composition and different voltages on the surface behaviors of formed PEO coatings on niobium. The PEO treatment of niobium in silicate electrolytes showed this procedure enables the accommodation of Si in the oxide coatings. The PEO procedure indicated that the oxide layers that were produced before the breakdown of 0.1 M H3PO4 solution are amorphous, whereas after the breakdown they become fairly crystalline and mostly consisted of Nb2O5. The PEO procedure in Ca- and P-containing electrolytes showed that the voltage has an important effect on the sparks nature that were produced within the growth of oxide and the properties of the coatings highly depend on the kind of sparks. Also, corrosion behavior of the PEO coatings in these electrolytes do not only depend on the coating thickness but also its structure is controlled by the spark regime. Finally, the PEO procedure of Nb in silicate-, phosphate- and aluminate-based solutions indicated that the corrosion resistance declines in the order of phosphate-> aluminate-> silicate-based solutions.
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