Study on the impression mechanism of caustic soda on the morphology, thickness and chemical composition of PEO coatings produced on aluminum in silicate electrolytes

Plasma electrolytic oxidation (PEO) is a plasma assisted electrochemical treatment which can provide or is usedto change the coating of some metals to an oxide ceramic layer on the surface of some metals. In PEO technology, spark or arc plasma micro-discharges in an aqueous solution are utilized to ionize gaseous media in the solution to form complex compounds on the metal surface through the plasma chemical interactions. This work deals with the study on the effect and impression mechanism of caustic soda on the microstructure and thickness of the silica-alumina ceramic coatings formed on the aluminum substrate by the plasma electrolytic technique. Morphology, surface roughness profile and surface elemental analysis were studied by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). Also, the phase composition of the coatings was detected using X-ray diffraction (XRD). The results showed that increasing the NaOH content of the electrolyte causes thickness reduction and increasing of the coating micro cracks. The results of surface roughness profiles, reported as mean value of the surface roughness (Ra), showed that Ra increases from 0.73 to 1.08 m by increasing the NaOH content of the electrolyte up to 20g/l However, Ra decreased in higher concentrations. Also, reducing of the silica phases and increasing of the amorphous phases were observed in more concentrated electrolytes. Moreover, the results revealed that by increasing the NaOH content of the electrolyte, the coating porosity decreases and the distribution of the Al, Si and O elements becomes more homogeneous in the coating cross section.