A re-understanding of the breakdown theory from the study of the plasma electrolytic oxidation of a carbon steel - A non-valve metal

Plasma electrolytic oxidation (PEO) is normally not applicable to non-valve metals. However, in this study, PEO of Q345 carbon steel, a non-valve metal, has been carried out under pulsed unipolar regimes in aluminate based electrolytes with the addition of sodium dihydrogen phosphate to understand its coating formation and plasma generation mechanisms. Real time imaging, optical emission spectroscopy (OES), gas collection, scanning electron microscopy (SEM) assisted with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) have been adopted to characterize the PEO processes. It has been found that the cell potential-time responses and plasma initiation are affected by the surface roughness of the carbon steel and the electrolyte agitation, which are not encountered in PEO of valve metals. The addition of sodium dihydrogen phosphate is critical to the establishment of stable plasma discharges and formation of thick PEO coatings on carbon steel. It has been found that the formation of a thin layer of aluminum phosphates (AlPO4 and AlPO4 center dot 2H(2)O) or alumina aluminum phosphate (AAP) at the initial PEO stage plays an important role for the plasma generation and hence PEO coating formation on carbon steel. The variations in the cell potential-time responses are associated with the stability of the aluminum phosphates or AAP layer on the carbon steel. The findings in this paper indicate that the presence of a dielectric layer on the metal surface is a prerequisite for the occurrence of plasma breakdown. Hence, the previous models that dielectric breakdown in PEO is preceded by the breakdown of a bubble layer need to be reconsidered. (c) 2018 Elsevier Ltd. All rights reserved.